DBMETHYLDITHIOP‘HOSPHATE COMPLEXES

0F

NIOBIUMGV) TETRAHALIDES

Dissertation for the Degree of Ph. D.
MECHEGAN STATE'UNWERSITY
RGGER NOLAN McGINNlS

1974‘ ' -

 

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thesis entitled
DIMETHYLDITHIOPHOSPHATE COMPLEXES
OF
NIOBIUM(IV) TETRAHALIDES
presented by
Roger Nolan McGinnis
has been accepted towards fulfillment
of the requirements for
Ph.D. degree in Chemistry

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ABSTRACT
DIMETHYLDITHIOPHOSPHATE COMPLEXES OF NIOBIUM(IV) TETRAHALIDES
By

Roger Nolan McGinnis

Eight-coordinate Nb(Dmtp)4 was isolated from the reaction of sodium
dimethyldithiophosphate (NaDmtp) with niobium tetrahalides.The infrared
spectrum indicates that Dmtp-.is bidentate. The electronic spectrum
exhibits three d-d transitions and supports a D2d dodecahedral config-
uration for the complex. The esr spectra confirm this geometry with the
parameters <g> = 1.9494, gll a 1.8971, gL - 1.9756, and < a > = 0.01354 cm-¥
Using the molecular orbital theory developed for D2d complexes, metal-
ligand bonding parameters were obtained which indicate strong mixing of
metal and ligand orbitals.

Complexes of the type Nb2(Dmtp)4X4 (X = C1,Br,I) were obtained from
the reaction of the niobium tetrahalides with two moles of NaDmtp. The
infrared spectra indicate that Dmtp- is bidentate in all three complexes.
Three d-d transitions were observed in the electronic spectra. In addition,
a band due to a double excitation was observed at 390 nm. indicating a
metal-metal interaction. The complexes were proposed to be halogen bridged
dimers with D2h symmetry (02v about each niobium atom). The magnetic
moments decrease in the order I > Br > C1. The bromide and chloride com—
plexes are diamagnetic while the iodide complex exhibits antiferromagnetism
with a singlet-triplet separation of -140 cm"1 and a room temperature
magnetic moment of 2.32 Bohr Magnetons. Additional confirmation of an

electron exchange-coupled dimer was obtained by the observation of the

triplet state esr spectrum of Nb2(Dmtp)414 with both the AMS 9 :_1 and

Roger Nolan McGinnis

the AMs a :_2 transitions present. The esr parameters are gll - 2.0663,
31 - 1.9361, All - 0.00681 cm'l, and Al - 0.00345 cm’l. The zero field
splitting is 0.06318 cm-1 which corresponds to a niobium-niobium separa—

tion of 3.53 K. This complex represents the first reported case of electron

exchange-coupling in niobium(IV).

DIMETHYLDITHIOPHOSPHATE COMPLEXES
OF

NIOBIUM(IV) TETRAHALIDES

By

Roger Nolan McGinnis

A DISSERTATION

Submitted to
Michigan State University
in partial fulfillment of the requirements

for the degree of

DOCTOR OF PHILOSOPHY

Department of Chemistry

1974

To Eva

ii

ACKNOWLEDGMENTS

The author wishes to express his appreciation to Professor James B.
Hamilton for his guidance and assistance during the course of this study.

Appreciation is also extended to Mr. Kirby Kirksey, whose suggestions
and discussions were invaluble.

The author is also deeply grateful to his wife, Eva, for her support,

encouragement, and help in the preparation of this thesis.

iii

II.

III.

IV.

VI.

VII.

TABLE OF CONTENTS

Introduction

Review of Previous Work

A. Eight-coordinate Niobium(IV) Complexes
B. Dimeric Niobium(IV) Syste

C. Exchange-coupled Dimeric d Complexes

Purpose of This Work

Experimental
A. Materials
B. Preparation of Niobium Halides

Preparation of sodium dimethyldithiophosphate
Analytical Determinations

Molecular Weight Determinations

Conductance Measurements

Electron Spin Resonance Spectra

Nuclear Magnetic Resonance Spectra

Electronic Spectra

Vibrational Spectra

Magnetic Susceptibility Measurements
Syntheses

O

ripacqrqrncarntncyca

Results and Discussion

A. Preparation and Properties of Complexes
B. Vibrational Spectra
C. NMR Spectra of Nb2(Dmtp)4X4 (X - Cl,Br)

D. Electronic Spectra

,3. Electron Spin Resonance Spectra

F. Magnetic Susceptibility Measurements
Summary and Conclusions

Bibliography

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Table

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LIST OF TABLES
1. Infrared Spectral Data for Dmtp- (4000-600 cmfl)
2. Infrared Spectral Data for Nb(Dmtp)4 (4000-600 cmfl)
3. Infrared Spectral Data for Nb2(Dmtp)4X4 (4000-600 cmfl)
4. Far Infrared Spectral Data for Dmtp'"1 (600-50 curl)
5. Far Infrared Spectral Data for Nb(Dmtp)4 (600-50 cmfl)

6. Far Infrared Spectral Data of Nb2(Dmtp)4X4 Complexes
(600-—50 cur-1)

7. Proton Magnetic Resonance Data
8. Electronic Spectral Data for Nb(Dmtp)4 in CH2C12

9. Electronic Spectral Data of Nb2(Dmtp)4X4 Complexes in

10. Ear Spectral Parameters of Nb(Dmtp)4
11. Ear Parameters for NbZCDmtp)4I4
12. Diamagnetic Core Corrections

13. Magnetic Data for Nb2(Dmtp)4X4 Complexes

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Figure
Figure
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Figure
Figure
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LIST OF FIGURES

Apparatus for determination of electronic spectra

2. Electronic Spectrum of Nb(Dmtp)4 in CH2C12 solution

3. The crystal field splitting for DZd and D4d symmetries

4. Electronic spectrum of Nb2(Dmtp)4C14 in CH2C12 solution

5. Electronic spectrum of Nb2(Dmtp)4Br4 in CH2C12 solution

6. Electronic spectrum of Nb2(Dmtp)4I4 in CH2C12 solution

7. The crystal field splitting diagram for DZh symmetry

8. Preposed structure for Nb2(Dmtp)4

9. Ear spectrum of Nb(Dmtp)4 in CH2012 at ambient
temperature

10. Ear spectrum of solid Nb(Dmtp)4 at ambient temperature

11. Ear spectrum of Nb(Dmtp)4 in CH2C12 at 77°K

12. Ear spectrum of Nb2(Dmtp)4I4 in CH2C12 at ambient
temperature

13. Ear spectrum of Nb2(Dmtp)414 in CH2C12 at 77°K

. Experimental and calculated magnetic susceptibility

Magnetic susceptibility of Nb2(Dmtp)4X X - Cl,Br)

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I. Introduction

The niobium tetrahalides are polymeric, diamagnetic solids whose
basic unit can be represented as Nb2X8. The tetrahalides consist
of an infinite chain of distorted octahedra sharing opposite edges

and containing alternating metal-metal bonds.1'3

The niobium tetrahalides react with donor ligands to form paramagnetic
six and eight coordinate complexes in which the metal-metal bond is
cleaved. In some cases the metal-metal bond is retained giving diamag-
netic or very weakly paramagnetic six—coordinate adducts. In addition
to adduct formation,substitution of halogens can occur to give complexes
of the type NbX4.nLn (n=l-4) with either retention or cleavage of the
metal-metal bond. Six and eight are the predominate coordination numbers
of niobium(IV) although a number of complexes are seven-coordinate if
one considers the metal-metal bond. The six-coordinate complexes are
octahedral. The eight coordinate complexes are dodecahedral except for
the dipivaloylmethane complex which is square antiprismatic.4

Metal—metal bonding is common for niobium(IV) due to the relatively
expanded d orbitals which allow overlap to occur and the presence of
an unpaired d electron. The fact that metal-metal bonding does occur
accounts for the prominence of six4xmrdinate diamagnetic compounds.

The choice of ligands also influences the stereochemistry of
niobium(IV). Small donor atoms decrease the steric repulsions and tend
to favor the higher coordination numbers. In addition, a metal to ligand
pi bonding capability favors a high coordination number. These two

requirements appear to be mutually exclusive. For instance, the eight-

coordinate complexes Nb[SzCN(C2H5)2]4 and NbX4(diarsine)2 have been pre—
pared but the corresponding oxygen and nitrogen complexes do not form.
Nitrogen, halogen and phosphorus donors have been found to give only

six coordinate addition compounds while with arsenic donors only eight-
coordinate addition complexes have been obtained. Oxygen and sulfur
donor ligands are more versatile, both forming six coordinate addition
and substitution complexes. In addition, oxygen donors form eight-
coordinate substitution complexes while sulfur donors form both eight-

coordinate addition and substitution complexes.

II. Review of Previous Work

A. Eight Coordinate Niobium(IV) CompleXes
Several comprehensive reviews have been published concerning
the chemistry of niobium(IV).5"7 This review will be primarily concerned
with the eight-coordinate,paramagnetic niobium(IV) complexes; dimeric,
diamagnetic niobium(IV) complexes; and other binuclear d1 systems.

Clark and coworkers8 obtained eight-coordinate complexes with the
bidentate ligand o-phenylenebis(dimethylarsine)(or diarsine). Complexes
of the type NbX4'2diarsine were prepared by heating NbX4, NbX5, or NbOX3
with diarsine in a sealed evacuated tube. The complexes are isomorphous
with the dodecahedral titanium(IV), zirconium(IV), and hafnium(IV) halide
diarsine complexes.9 The diffuse reflectance spectra exhibit four d—d
transitions as expected for dodecahedral complexes. The magnetic moment
of 1.69 Bohr Magnetons, calculated from the esr spectrum of the chloride
complex, is consistent with a single d electron in an orbitally non-degen-
erate ground state.

Deutscher and Kepert prepared additional eight-coordinate complexes
using 4-methyldiarsine and 4-ethyldiarsine.10 These compounds exhibit
visible spectra similar to those of the diarsine complexes and were
assumed to be dodecahedral. The spectrum of NbC14(Etdiars)2 exhibits
maxima at v = 9.75 kK, Emax a 10; v = 12.8 kK, Emax = 26; and v = 17.5 kK,
Emax = 6. Solutions of NbBr4(Etdiars)2 exhibit maxima at 12.72 and
15.92kK and the iodide complex gives maxima at 10.1 and 12.7 kK. The
effective magnetic moments are 1.63, 1.67 and 1.80 B.M. for the chloride,
bromide, and iodide complexes, respectively, and are not unexpected for

a single d electron in a nondegenerate ground state. The high moment of

the iodide complex was attributed to a large temperature independent
paramagnetic contribution.

A number of eight-coordinate substitution complexes have been
prepared with beta-diketonates. Deutscher and Kepert have prepared
niobium(IV) complexes of acetylacetone (Acac), benzoyltrifluoroacetone
(Bta), thenoyltrifluoroacetone (Tta), and dibenzoylmethane (Dbm).11: 12
In addition, complexes of 8-hydroxyquinoline (0x) and tropolone (T) were
prepared. The complexes were prepared by reaction of the apprOpriate
niobium tetrahalide with the free ligand in an acetonitrile—triethylamine
solution. In all cases the infrared spectra indicate that the ligands
are bidentate and equivalent. The d—d transitions were not obse ved in
the diffuse reflectance or solution spectra. The room temperature
magnetic moments vary from 1.43 — 1.66 B.M. except for NbT4 which has
a magnetic moment of 0.74 B.M. Powder X-ray patterns indicate Nb(Acac)4
is not isomorphous with the square antiprismatic Zr(Acac)4;13 Nb(Tta)4
is not isomorphous with square antiprismatic Zr(Tta)4;14 and Nb(Dbm)4
is not isomorphous with the square antiprismatic Th(Dbm)4.15 The ear
spectra of Nb(Acac)4 and Nb(Dbm)4 consist of a broad asymmetric signal
with the general shape expected for gl.> gll, The values of gL are
1.95 and 1.98 for Nb(Acac)4 and Nb(Dbm)4, respectively. It has been
demonstrated that 31.) g|| for a dodecahedral complex and that the
reverse is true for a square antiprismatic complex.16"18 All of the
niobium complexes were assumed to be dodecahedral since 81.) g||
and they are not isomorphous with known square antiprismatic complexes.

Podolsky prepared the tetrakisdipivaloylmethane (me) complex of

niobium(IV) by reaction of niobium tetrachloride with the free ligand

in acetonitrile.19 Only two d-d transitions are observed in the
visible spectrum at 14.2 kK., Emax = 280 and at 15.3 kK, Emax - 636.
The ear spectrum exhibits ten lines with <Ig> = 1.95 and a hyperfine
splitting of 110 gauss. The anisotropic constants are found to be:

gL a 1.928, gll = 1.997, AL 8 141 gauss, and All = 53 gauss. On the
basis of observing only two d-d transitions and 3]] > gi, the structure
of Nb(me)4 was proposed to be a D4 square antiprism. This was later
confirmed by X-ray structural analysis4 and represents the first reported
case of a square antiprismatic niobium(IV) complex.

Bidentate sulfur donor ligands have also been found to form eight
coordinate niobium(VI) complexes. A number of workers have prepared the
dieth3rldithiocarbamates (Detc).20’22 The ir spectrum indicates the
ligand is bidentate. A band observed at 360 cm"1 was assigned as a
metal-sulfur stretching mode. One d-d band is observed in the visible
spectrum at 363 mu with Emax = 48. The magnetic moment 13 1.57 B.M.
indicating a single unpaired electron. No esr data have been reported.

Hamilton and McCarley prepared eight-coordinate sulfur donor complexes
with 1,2—dimethylthioethane(Dth).23 The compounds are paramagnetic
adducts with composition NbX4(Dth)2. The far infrared spectra indicate
that the ligand is bidentate and metal—halogen and metal-sulfur stretching
frequencies were assigned. The solid state electronic spectra are similar
to those reported by Clark for NbX4(Diars)2.8 The magnetic moments for
the chloride, bromide and iodide complexes were determined to be 1.60,
1.61 and 1.28 B.M. respectively. An esr spectrum was observed only for
the solid chloride complex. The spectrum consists of a broad asymmetric

band with <g> = 1.92, g1 = 1.98, and gll= 1.80. The complexes were proposed

to be dodecahedral on the basis of the solid state ehxironic spectra and
the ear spectrum of the chloride. This was later confirmed by Wilson24
who obtained the solution esr spectrum and found gl_> gII as expected

for a dodecahedral complex.

B. Dimeric Niobium(IV) Systems
In addition to the mononuclear, paramagnetic, niobium(IV) complexes,
a number of dimeric niobium(IV) compounds have been prepared. These
complexes all have small magnetic susceptibilities indicating retention
of the metal-metal bond. If any halogen atoms are present, two of them
usually bridge the metal atoms.

Wentworth and Brubaker have prepared two diamagnetic niobium(IV)
ethoxide complexes.25 The electronic spectrum of [NbCl(002H5)3Py]2
(Py-pyridine) exhibits a single peak at 365 mu with nonreproducible
extinction coefficients. The complex is diamagnetic with a corrected
susceptibility of -1150 x 10‘6 c.g.s. units. 0n the basis of the
nonlability of the chlorine atoms, the molecular weight data, and the low
susceptibility, the complex was assumed to be dimeric with bridging
chlorine atoms and a direct metal-metal bond. When the dimer was treated
with sodium ethoxide,Nb(0C2H5)4 was obtained. This compound is diamagnetic
with a corrected susceptibility of ~100 x 10"6 c.g.s. units. A single
band is observed in the electronic spectrum at 380 mu.

Brown and Newton prepared NbCla ' N(C2H5)3 by refluxing a mixture of
NbC14 and triethylamine.26 This complex is diamagnetic and has a single
broad peak in the visible spectrum at 380 mu similar to those reported for
NbCl4, NbBr4, and those reported by Wentworth and Brubaker. This band

was attributed to a niobium-niobium bond.

Machin and Sullivan prepared a number of compounds which were
thought to be dimeric.22 From the reaction of niobium tetrachloride
with potassium thiocyanate they obtained Nb(NCS)3C1. The magnetic
susceptibility is 40 x 10"6 c.g.s. units indicating a structure similar
to that of NbC14. A band is observed in the diffuse relectance spectrum
at 18,000 cm’l. When potassium cyanate was allowed to react with niobium
tetrachloride, Nb(CNO)3C1 was obtained. The magnetic susceptibility for
this compound is also low, be being 90 x 10‘6 c.g.s. units. Four bands
were observed in the diffuse reflectance spectrum but assignments were
not made. When less than four moles of sodium diethyldithiocarba-
mate react with niobium tetrabromide, Nb2(Detc)5Br3 is obtained.

This complex was a weak 1:1 electrolyte in nitromethane and was

postulated as [Nb2(Detc)5Br2]Br. The magnetic susceptibility is markedly
field dependent. Peaks are observed in the diffuse reflectance spectrum
at 23,500 cm-1 and 19,500 cm-1 with a shoulder at 16,000 cm-l.

Fowles, Tidmarsh and Walton27 prepared NbC14'S(CH3)2 by the direct
reaction of NbC14 and dimethylsulfide. The complex was reported to be
antiferromagnetic with a room temperature magnetic moment of 0.44 B.M.

Two bands were observed in the electronic spectrum at 11.2 and 16.0 kK.

and were assigned as d-d transitions. The magnetic moment and far infrared
spectrum of NbClqoS(CH3)2 suggest that this species is structurally similar
to the related d1 antiferromagnetic titanium(III) derivatives TiCl3-ZS(CH3)2
and TiCl3o28C4Hg.28

Hamilton and McCarley also investigated the reaction of dimethylsulfide
with NbCl4 and NbBr4.29 Complexes of the type NbX4[S(CH3)2] were obtained

by allowing the appropriate niobium tetrahalide to react with dimethylsulfide

in benzene. The monoadducts are weakly paramagnetic with room temperature
magnetic moments of 0.36 and 0.50 B.M. for the chloride and bromide com-
plexes, respectively. While no evidence was found for an antiferromagnetic
interaction as proposed by Fowles and coworkers, the same structure that

Fowles proposed was suggested.

C. Exchange-coupled Dimeric dl Complexes
While Fowles and coworkers found NbC14-8(CH3)2 to be antiferro-
magnetic, Hamilton and McCarley found that it was diamagnetic. There
have been no other reports of exchange interactions in niobium(IV) pairs.
However, a number of complexes containing other d1 metal ions exhibiting
electron exchange have been prepared.

Electron exchange interactions can be detected in two ways. An
important indication of intramolecular exchange coupling in complexes
which contain more than one transition metal ion is the characteristic
deviation of the magnetic susceptibility of these compounds from the
Curie—Weiss Law. When an isotropic interaction occurs between two
paramagnetic metal ions, each with a single unpaired electron, the ex-
change interaction term JSl'Sz is required in the spin Hamiltonian. J
is defined such that a negative value corresponds to an antiferromagnetic
interaction in which the ground state is a singlet and the triplet state
is J energy units above the ground state. The magnetic susceptibility
of such a system, as first formulated by Bleaney and Bowers,30 is in

accordance with the relationship:

XM = _g3_%1_§§_ [1 + 1/3 exp(J/kT)]"1 + NCl (1)

where Na.is the temperature independent paramagnetism and the other con-

stants have their normal meaning.

Another indication of exchange coupling in dimeric systems is the
characteristic esr spectrum. The esr of a triplet state system consists
of 2nI + 1 lines with a hyperfine splitting A/2 where A is the normal
splitting for the individual ions with a nuclear spin, I. In addition
to the normal AMS 8':.1 transitions, a "forbidden" AMS 8;: 2 half-field
transition may be observed at g ca. 4 arising from the magnetic dipole—
dipole interactions of the paramagnetic metal ions in a dimeric species.
This half-field transition is usually only observed in the frozen
solution spectrum and its presence is definitive evidence for an exchange
coupled system.31

Mos t c as es of exchange coupling have been reported for copper(II)
dimers,which by the hole formalism can be treated as d1 ions,and several
reviews are avaflable.32933 While there are fewer examples of exchange—
coupled dimers among metals of the left side of the transition series a
number of complexes have been prepared.

Martin and Winter investigated the magnetic behavior of [CpZTiCl]2.34
The complex is antiferromagnetic with a Neél temperature of 170°K
indicating singlet and triplet states in thermal equilibrium. Assuming
a titanium-titanium separation of ca. 3.5 A, the singlet-triplet
separation, J, was calculated to be -387 cm‘l.

Coutts, Martin,and Wailes35 investigated the magnetic susceptibilities
0f[(Cp)2TiX2Ti(Cp)2] (X=F,Cl,Br, and 1). Characteristic singlet-triplet
behavior was observed with the strength of the titanium-titanium inter—
action increasing in the order F <i01 = I < Br. The values of the singlet—
triplet separation, J, are -62, -159 to —186, -276, and -168 to -179 em-1

for the F, Cl, Br, and I complexes, respectively.

10

Fowles, Lester.and Walton28 prepared thioether complexes of
the type [TiX3°2L]2 where X = Cl, Br, and I and L = dimethylsulfide or
tetrahydrothiophene. For X = Cl the complexes are strongly antiferro-
magnetic with Neél temperatures of ca. 320°K,while for X = Br or I Curie-
Weiss behavior is exhibited. From the magnetic and far infrared data they
concluded that the chloride complex is most likely a dimer with a direct
metal-metal bond but they did not rule out the possibility of bridging
chlorine atoms.

Smith, Lund, and Pilbrow36 studied the esr spectra of a number of
dimeric titanium(III) hydroxycarboxylic acids. Both the AMS 8 :11 transi-
tion at g ca. 2 and the "forbidden" AMS =‘:_2 transition were observed
giving definitive proof of exchange coupled dimers. Magnetic parameters
were evaluated by computer simulation of the spectra. The metal-metal
separation varies from 4.7 A for the mandelic acid complex to 8.6 A for
the 8-hydroxyquinoline complex with most of the complexes having a separa-
tion of ca. 6.5 A.

Carr, Boyd, and Smith37 prepared the binuclear tetrakis(aminomethy1)-
methane complex of titanium(III) chloride. The triplet state esr spectrum
was observed with both the AMS = 1 and AMs = 2 transitions present.
Computer generated spectra were compared with the experimental solution
esr spectra and the titaniumrtitanium separation was found to be 5.6K.

Only one exchange coupled molybdenum complex has been reported.38
Huang and Height prepared a binuclear molybdenum(V) oxoglutathione
complex. They obtained a solution esr spectrum containing eleven lines
using isotropically enriched 95Mo, indicating electron exchange coupling

in the dimeric complex. Under the imperfect assumption of axial symmetry

11

the magnetic parameters were found to be < g > a 1.962, gll - 1.966,
gi - 1.960, < a > - 0.0029 em-l, A 0.0047 curl, and B - 0.0020 cm-l.
The zero field splitting,D, is 0.0083 cm.‘1 which corresponds to a
metal-metal separation of ca. 6.0 K. The low field AMS =':_2 transi-
tion was not observed.

Carr, Boyd, and Smith39 reinvestigated this system by computer
simulation of the ear spectrum.They found that the symmetry is less
than axial and the magnetic parameters which best fit the spectrum
are g“ = 1.97, gi = 1.96, R - 5.0 A.

In marked contrast to the few complexes prepared with other early
transition metals a large number of exchange coupled vanadyl complexes
have been prepared.0ne of the earliest reports concerned the prepara-
tion of vanadyl complexes of tridentate Schiff bases. Ginsberg, Koubek,
and Williams40 investigated the magnetic susceptibilities of a series
of 5-substituted N-(2-hydroxypheny1)salicylideneimine complexes of
V02+.The compounds are strongly antiferromagnetic with Neel tempera-
tures ranging from 80 to 120°K.The singlet-triplet separation varies
from -90 to -218 cm."1 with most of the complexes having J ca. 120 cmfl.
The results were interpreted to be due to a direct metal-metal inter-
action between unpaired spins of the dxy orbitals of vanadium in con—
trast to the analogous copper complex where superexchange occurs
through bridging oxygen atoms.

The most work on exchange coupled vanadyl compounds has been devoted
to the study of anionic.cx- hydroxycarboxylates, particularly the tartrate
complexes. Initially there was a great deal of controversy as to whether

exchange coupling actually took place. Various authors offered differing

12

emplanathmufi

1‘45. Tapscott and Belford41 initially investigated the
properties of vanadyl dl-tartrate in aqueous solutions. At pH greater
than 7 they found a dimeric anionic complex was formed which contained
two vanadyl ions bridged by two tetranegative tartrate groups. The
solution esr spectrum exhibits fifteen lines indicative of exchange
coupling with < g > = 1.98 and a hyperfine splitting of 40 gauss (about
half the splitting of monomeric vanadyl d-tartrate). The magnetic moments
rare normal indicating the energy difference between the singlet and
triplet states is small. The vanadium-vanadium distance was proposed

to be ca. 4.0 A as determined by molecular models and later confirnmfl
by X—ray crystallography.

Dunhill and Smith42 investigated both the vanadyl citrate and
tartrate systems. They could not confirm the tartrate was dimeric from
ear measurements since no hyperfine structure was observed in the low
temperature spectrum and there was no linear variation of intensities.
The vanadyl citrate complex is dimeric. The "forbidden" AMS - i_2
transition consisting of fifteen lines was observed at g ca. 4.05.

The g = 2 portion of the spectrum was complicated and could not be
unambiguously interpreted.

Dunhill and Symonsl‘3 reinvestigated the vanadyl tartrate system because
triplet state esr spectra like those reported earlier by Tapscott and
Belford are normally not observed in solution. They observed the
fifteen lines at g ca. 2 but in addition, they found a set of very
weak high and low field satillite peaks which support the conclusion
that a triplet state is involved.

Belford and coworkers44 reinvestigated the tartrate system and a

13

number of other (x-hydroxycarboxylates. Contrary to Dunhill and
Smith's results, the half-field, AMs = i 2, transitions were clearly
visible giving definitive evidence for dimer formation. The zero
field splitting was calculated to be ca. 0.033 cmrl.

James and Luckhurst45by computer simulation of the esr spectrum,
found that the tartrate complex is rigid in solution and the metal-
metal separation is essentially the same as for the solid confirming
Belford's work.

Recently, Smith and coworkers46 investigated a series of other
vanadyl (z-hydroxycarboxylates by computer simulation of the esr spectra.
With l—hydroxycyclohexanecarboxylate they found a dimeric vanadyl
complex was formed which gave a fifteen line esr spectrum with both
the g = 2 and the g B 4 transitions present, The zero field splitting
was calculated to be 0.055 cm."1 corresponding to a vanadyl separation

of ca. 3.6 A which is consistent with the analogous copper compound.

III. Purpose of This Work

In view of the number of electron exchange-Coupled complexes
which have been prepared with (11 ions it is SUprising that no such
niobium(IV) complex has been reported.Exchange-coupled complexes of
vanadium(IV) and titanium(III) have been prepared with bidentate
oxygen donor ligands. Niobium(IV) reacts with most oxygen donor ligands
to give paramagnetic complexes in which the metal-metal bond is
completely cleaved. This reactivity of niobium(IV) to oxygen and
water and its ability to abstract oxygen from donor solvents seriously
limits the number of ligands which could be used to prepare dimeric
electron exchange coupled complexes. Metal-metal bonding occurs much
more often in niobium(IV) complexes with sulfur donor ligands.The
complexes in which metal-metal bonding occurs have been prepared
with neutral sulfur donor ligands and are diamagnetic. In an attempt
to prepare exchange coupled niobium(IV) complexes 3 different type
of sulfur donor ligand was used.The uninegative, bidentate ligand,
sodium dimethyldithiophosphate (NaDmtp) has been found to form a large
number of substitution complexes with other metals of varying size.47
It was hoped that by replacing only the nonbridging halogen atoms in
NbX4 that an exchange-coupled complex could be prepared in order to

gain insight into the metal-metal bonding which characterizes much

of the chemistry of niobium(IV).

14

IV. Experimental

Due to the sensitivity of the compounds to oxygen and water all the
compounds were handled under high vacuum or in a Vacuum Atmospheres
Corporation nitrogen filled drybox containing less than 1 ppm water and

oxygen.

A. Materials
High purity (99.9%) niobium metal and niobium pentachloride

were purchased from Alpha Inorganics. Phosphorus pentasulfide was ob-
tained from Matheson, Coleman, and Bell and was used as received.
Analytical grade methanol was obtained from J.T. Baker Chemical Co. and
was dried by refluxing over sodium methoxide. Analytical grade methy-
lene chloride was purchased from J.T. Baker Chemical Co. and was dried
by refluxing over calcium hydride. It was distilled under nitrogen

atmosphere and stored over molecular sieves.

B. Preparation of Niobium Halides
Niobium pentabromide, niobium pentaiodide, and the three

tetrahalides were prepared by using procedures previously described.48.49

C. Preparation of sodium dimethyldithiophosphate
An excess of dry methanol was allowed to react with phosphorus
pentasulfide according to the equation:
P285 + 4CH3OH + 2H82P(0CH3)2 + H28 (2)
The crude dimethydithiOphosphoric acid was purified by distillation
under a vacuum of 5 torr over a temperature range of 80 — 85°C and was
obtained as a viscous, colorless liquid. The acid was dissolved in an
equal Volume of anhydrous ether and neutralized with excess anhydrous

sodium carbonate. The sodium dimethyldithiOphosphate(NaDmtp) was dissolved

15

16

in anhydrous spectrograde acetone and the solution was filtered. The
solvent was removed by pumping under dynamic vacuum at room temperature

and the salt was stored in the drybox.

D. Analytical Determinations
Microanalyses were performed by Galbraith Laboratories, Inc.,

Knoxville, Tennessee.

Preliminary analyses were performed to determine halogen. Samples
of the complexes were added to aqueous ammonia and heated until solution
was complete. The samples were cooled and acidified with dilute nitric
acid. The solution was filtered and the filtrate was analyzed for halogen
ion by potentiometric titration with standard silver nitrate solution.
A Beckman expanded scale pH meter was used in conjunction with a silver
indicating and a saturated calomel reference electrode. Niobium was not

determined due to the presence of phosphorus.

E. Molecular Weight Determinations
The molecular weight of the complexes was determined cryoscopically
in dry benzene. Recrystallized benzil was used as the calibrating solute
in the determination of the molal freezing point depression constant of
benzene (5.38°C mfl). Freezing point depressions were measured in the
concentration range 0.01 to 0.08 m with a Beckman differential thermo-
meter graduated at 0.01° intervals. Temperature readings were estimated

to :_0.001° with the aid of a magnifying thermometer reader.

F. Conductance Measurements
Molar conductivities were measured with a Wayne—Kerr Model B221

universal bridge and a Freas type solution cell with bright platinum

17

electrodes. The cell constant was determined to be 0.2175 cm‘1 at 25°C
by using a standard KCl solution. Both purified nitromethane and methy—

lene Chloride were used for molar conductance measurements.

G. Electron Spin Resonance Spectra
All esr spectra were obtained on solutions and powders by use
of a Varian Model E—4 spectrometer with an operating frequency range of
8.8 to 9.6 GHz and equipped with a field dial-regulated magnet. Low
temperature spectra were obtained by use of a liquid nitrogen insert
dewar or a Varian Model V4540 variable temperature controller. Samples
were sealed in quartz tubes under a nitrogen atmosphere. The magnetic

field was calibrated using strong pitch (g=2.0028).

H. Nuclear Magnetic Resonance Spectra
A11 nmr spectra were obtained by use of a Varian Model A56/60D

spectrometer operated at 60 MHz.

1. Electronic Spectra
Solution spectra were recorded using a Cary Model 17 spectrophoto-
meter. Cylindrical fused silica cells, 1.0 cm long and adapted for use
at low pressure (Figure 1), were used.48 Saturated solutions were loaded
in the dry box. The cell assembly was then evacuauxito ca 10’5 torr.
After sealing off the cell assembly, solvent and/or solutions of various
concentrations could be distilled through a medium porosity frit into the

cell.

 

 

18

TO HIGH VACUUM MANIFOLD

STOPCOCK

BALL- SOCKET JOINT

SEAL OFF HERE AFTER
LOADING

 

  
  

 

 

 

 

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MIXING
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SPECTROSCOPY

CELL

Figure 1. (Taken from Reference 48) Apparatus for determination of

electronic spectra.

19

J. Vibrational Spectra
Solid state infrared spectra were obtained by use of a

Perkin—Elmer 457 (4000-250 cm‘l) spectrophotometer. Samples were
prepared in the drybox and were mounted as Nujol mulls between cesium
iodide plates. Mulls were prepared immediately before measuring the
spectra.

Far infrared spectra were obtained by using a Block Engineering
Company Model—FTSl6 (3900-20cm‘1) far infrared Fourier—transform

spectrophotometer. High density polyethylene was used for windows.

K. Magnetic Susceptibility Measurements
Magnetic susceptibility measurements were obtained using an

Alpha electronic Faraday balance. An Alpha model 4600 magnet provided
with variable gap, four-inch pole pieces and constant H°dH/dZ pole caps
was used with an Alpha model 3002-1 current regulated power supply.
Force measurements were obtained with an Alpha electrobalance to detect
weight change between field-on and field-off conditions. The electro-
balance, suspension wire, and sample container were protected from air
currents and moisture condensation by a vacuum enclosure.

The sample temperature was maintained using an Alpha model 3013
temperature regulator which was calibrated with a platinum resistance
thermometer. The sample container was machined from teflon rod to form
a cylindrical bucket 10mm. in length and 7 mm. in diameter. A threaded
cap on the bucket permitted a tight seal to prevent decomposition due
to exposure to air. The samples were loaded into bucket in the drybox.

The compound (NH4)2Ni(SO4)2-6H20 was used as a magnetic suscepti-

bility standard. Simmons50 has indicated that nickel ammonium sulfate

20

hexahydrate is one of the most satisfactory of several common standards.
With this material consistently reproducible results were obtained. The
temperature dependence of the magnetic susceptibility of this standard

follows the relationship:

Xg = (3065/T + 0.5) x 10-6 emu/g. (3)

L. Syntheses
1. Tetrakisghmethyldithiophosphaujniobium(IV)

A 3-5 g sample of NbX4 (X;Cl, Br, I), excess NaDmtp, and a
magnetic stirring bar were introduced into a round bottom flask. The
flask was evacuated to ca. 10'5 torr and 50-80 m1 of methylene chloride
was vacuum distilled into the flask. The flask was isolated from the
vacuum system and the mixture was stirred for 4—5 days at room tempera-
ture. A yellow solution and a yelknrbrown precipitate were obtained.

The precipitate was recovered by filtration, extracted with methlyene
chloride, and filtered again to remove sodium halide. A yellow, cry-
stalline solid was recovered from the filtrate by removal of methylene
chloride in vacuo. This product was then washed with dry pentane and
dried in vacuo.

Anal. Calcd. for Nb(Dmtp)4: Nb, 12.9; C, 13.3; H, 3.35; S, 35.5.

Found: Nb, 12.9; C, 13.2; H, 3.28; S, 35.1. Molecular weight in

benzene: Calcd., 721; Found, 660. M.P., 128° C with decomp.

2. Dichlorojp-dichlorotetrakis(dimethyldithioPhosphato)diniobium(IV)

A mixture of 1.20 g. of NbCl4 and 1.85 g. of NaDmtp were
placed in a round bottom flask in the same manner as above and stirred
for 7-8 days. The solution and solid obtained were both dark green-brown.

The complex was recovered as described above.

21

Anal. Calcd. for Nb2C14(Dmtp)4: Nb, 19.5; C, 10.0; H, 2.5;
S, 26.8; CI, 14.9. Found: Nb, 19.6; C, 10.2; H, 2.3;
S, 22.6; CI, 14.5. Molecular weight in benzene: Calcd., 956;
Found: 935. Molar conductance in nitromethane: 45 ohm"1
cm‘1 M"1
3. Dibromo-u-dibromotetrakis(dimethyldithiophosphato)diniobium(IV)
The bromide complex was prepared in exactly the same manner
as the chloride complex. A mixture of 2.12 g of NbBr4 and 1.85 g of
NaDmtp was allowed to react. The resulting solution and solid were
both a deep red-brown.
Anal. Calcd. for szBr4(Dmtp)4: Nb, 16.4; C, 8.4; H, 2.1; S, 22.6;
Br, 28.2. Found: Nb, 16.3; C, 8.4; H, 2.1; S, 22.5; Br, 27.8.
Molecular weight in benzene: Calcd., 1134; Found 1165. Molar
conductance in nitromethane: 50 ohm"1 cm"1 M‘1
4. Diiodo-u-diiodotetrakis(dimethyldithiophosphato)diniobium(IV)
The iodide complex was also prepared in the same manner
as the chloride complex. A mixture of 3.08 g of NbIl. and 1.85 g of
NaDmtp was allowed to react. The resulting solution and solid were a
deep red-brown color.
Anal. Calcd. for Nb214(Dmtp)4: Nb, 14.1; C, 7.25; H, 1.81;
S, 19.4; I, 38.4. Found: Nb, 14.5; C, 7.52; H, 1.88; S, 20.2;
I, 38.6. Molecular weight in benzene: Calcd., 1322; Found,

1396. Molar conductance in nitromethane: 39.6 ohm“1 cm"2 M’l.

V. Results and Discussion
A. Preparation and Properties of Complexes
The reaction of the niobium(IV) halides with excess sodium
o,o-dimethyldithiophosphate in toluene or dichloromethane proceeds

according to equation 4.
NbX4 + 4NaDmtp + Nb(Dmtp)4 + 4NaX (4)

The complex was isolated as a light yellow powder which is quite soluble
in benzene, toluene, and dichloromethane. The complex is air and water
sensitive as indicated by a color change and the distinctive odor of the
dithiohposphoric acid on exposure to such conditions. The complex melts
with decomposition over the range of 123-128°C.

If only two moles of sodium dimethyldithiophosphate are present

reaction occurs according to equation 5.
2NbX4 + 4NaDmtp + Nb2X4(Dmtp)4 (5)

The complexes are obtained as deep red powders which .are only slightly
soluble in toluene and dichloromethane. These complexes are also air
and moisture sensitive. They each react with additional sodium

dimethyldithiophosphate to give Nb(Dmtp)4 according to equation 6.
Nb2X4(Dmtp)4 + 4NaDmtp + 2Nb(Dmtp)4 + 4NaX (6)

This also indicates that the oxidation state of niobium remains +4
throughout. While Machin and Sullivan22 reported the diethyldithiocarbamate
complex, Nb2(Detc)5Br3, no evidence was found for a corresponding dithio-

phosphate compound.

22

23

B. Vibrational Spectra

Infrared Spectra (4000-6006m‘1)

 

Dimethyldithiophosphort acid has been studied in the infrared
region by Mclvor, Grant, and Hubley51 who assigned the vibrations by com-
parison of the spectrum with those of other substituted phosphates. The
results from this study of NaDmtp are presented in Table 1. Assignments
were made by comparing the infrared spectra of the free acid and the
sodium salts of dimethyldithiophosphate and diethyldithiophosphate.
Infrared spectral data of the complexes are presented in Tables 2 and 3.

In complexes containing Dmtp, bonds due to phosphorous-sulfur
stretching modes are expected to be most sensitive to coordination via
sulfur. Casey, MacKay, and Martin52 suggested that Dmtpl’ may be depicted

by the resonance forms in equation 7.

s s
(CH 0) P 11’ (CH 0) g/
3' 2"' . - ** 3 2 “-

\ ._

\

§

(7)

S S

(A) (B)

Two v(P-S) modes are observed in NaDmtp at 560 and 740 cm‘l. On coordina-
tion both are shifted to higher energy to ca. 600 and 790 cm"1 which is
expected as the bond order of the phosphorous-sulfur bond is increased,

as shown in the resonance forms.

24

Table 1

Infrared Spectral Data for Dmtp- (4000—600 cm-l)

v(cmf1)
Assignment HDmtp NaDmtp
McIvor et al51 This work
V(C-H) 3070 m 2980 m(sh) 2920 m
2935 s
v(S-H) 2500 2450 s(b)
6(CH3) 1460 m 1445 s 1430 m
v(P-OCH3) 1203 m
v(CH3-O) 1185 m 1175 s 1170 m
v(P-OCH3) 1030 s(b) 1010 s(b) 1015 s(b)
Cll3 rock 630 m
v(SP-S) 658 s(b) 650 s(b)
unassigned bands 2830 m 3350 m(b)
1830 w 1605 m(b)
1310 w 1145 w
945 m 1100 w
815 m 850 m 655 w

s,strong; m,moderate; w,weak; sh,shou1der; b,broad

25

Table 2

Infrared Spectral Data for Nb(Dmtp)4 (4000-600 cm-l)

v(cm-1)
Assignment This work
v(P-OCH3) 1198 m
V(0-CH3) 1165 s
V(P-OCH3) 1015 s(b)
V(P-S) 788 m(b)
unassigned bands 1308 m
1253 s
1148 m(sh)
965 s(b)
885 w
680 w
650 w

s,strong; m,moderate; w,weak; b,broad; sh,shou1der

26

Table 3

Infrared Spectral Data for Nb2(Dmtp)4X4 (4000-600 cm-l)

Assignment
v(P-OCH3)
v(O-CH3)
v(P-OCH3)
v(P-S)
CH3 rock

unassigned bands

v<cm'1)

C1
1200 w
1163 m
1012 m(b)
790 m(b)
632 w(sh)
1310 m
1255 s
1150 w(sh)
965 s(b)
930 m(sh)
885 w
765 w

680 w
650 w

Br
1198 w
1163 m(b)
1015 m(b)
788 m(b)
630 w(sh)
1308 m
1253 s
1148 w(sh)
960 s(vb)
885 w(sh)
840 w

765 w(sh)
680 w

I
1198 w
1163 m
1008 s(vb)
790 s(b)
635 m
1308 m
1253 s
1150 w(sh)
965 m(sh)
885 w(sh)

680 w
650 w

s,strong; m,moderate; w,weak; b,broad; sh,shou1der; v,very

27

Far infrared spectra (600-50cmfl)

The far infrared spectra were recorded for the free acid and
its sodium salt and are presented in Table 4. The far infrared spectral
data for Nb(Dmtp)4 are presented in Table 5. The niobium-sulfur bands
were assigned by comparison of the spectra of the complex with that of

53 Two strong, broad bands

NaDmtp and other dithiophosphate complexes.
which are observed at 385 and 345 cm'1 but are not present in NaDmtp are
assigned as y(Nb-S) modes.

The far infrared spectra for Nb2(Dmtp)4X4 complexes are presented
in Table 6. The two strong, broad bands observed at ca. 350 and 390cm-1
are assigned as v(Nb—S). These bands are in the range typically found
for eight-coordinate sulfur donor complexes of niobium(IV), 320-270
cm-1. 20-22, 53

Halogen sensitive bands are also found when the spectra of the com-
plexes are compared. In Nb2(Dmtp)4C14 the band at 302 and its shoulders
at 313 and 273 cm’1 have been assigned as v(Nb-C1) modes. This band is
in the range (299—310 cm-;) typically found for v(Nb-C1) in eight-co-
ordinate niobium complexes.23: 54 The shape of the band, its relative
intensity,and its absence in NaDmtp and the other complexes support this
assignment.

By using the ratio v(Nb-Br)/v(Nb-C1) = 0.76 that was found for the
monodentate thioether adducts of NbX429, one expects to find a v(Nb-Br)
mode ca. 230 cm'l. A band not present in NaDmtp, Nb(Dmtp)4 nor in the
chloride and iodide complexes is found at 235 cm'1 with a shoulder at
202 cm"1 and is assigned as v(nb-Br). The ratio v(Nb-I)/v(Nb-Cl) calculated
as 0.5629 predicts a v(Nb-I) mode ca 169cm“1. A band is observed at 173 cm-1

with shoulders at 183 and 158 cm"1 and is assigned as v(Nb-I).

28

Table 4

Far infrared Spectral Data for Dmtp- (600-50 cm-l)

V(cm-1)

HDmtp NaDmtp

McIvor at 0271 This work

v(P-S) 560 8

Assignment

v(P-SH) 525 s(b) 525 m
493 s(b) 490 m

5

C(P-O-CH3) 423 m 428

unassigned bands 390 w 528
365 w(sh) 508
340 m 435

395
375
367
328
315
305
295
240 s(b)
220
200 w
182 w
172 w

€ 8 € 5 3 3 8 fl 8 3

t

s,strong; m,moderate; w,weak; b,broad; sh,shou1der

29

Table 5

Far Infrared Spectral Data for Nb(Dmtp)4 (600-50 cmfl)

1)

v(cm-
Assignment
v(P-S) 595
v(Nb-S) 385

unassigned bands 515
502
433
425
390
375
360
295
273
255
235
225
190
178
170

S

s, 345 s

8333

m(b)
w(sh)
m

w
s(b)
w

m

w

m

m

w

s,stronp; m,moderate; w,weak; b,broad; sh,shou1der

Table 6

30

Far Infrared Spectral Data of Nb2(Dmtp)4X4 Complexes (600-50 cm-l)

Assignment
v(P-S)

v(Nb-S)

v(Nb-X)

unassigned bands

v(cm-1)

X a C

600 s

390 s
350 m

1

313
302
273

512
485
465
455
440
435
425
412
400
370
330
287
255
237
220
208
187
177
168
160

m(b)
w(sh)

s(b)

8 8 3 m 8 5

600

388
353

235
202

512
470
435
430
405
375
365
345
325
310
300
283
278
255
220
205
185
170
140

Br

s(b)

W

x.
600

387
350

183
173
158

520
510
490
485
468
452
438
420
400
372
362
320
295
280
275
265
255
243
225
203
183
170

8 8 8 8 O 8 8 3 8 9 8 3 3

s,strong; m,moderate; w,weak; b,broad; sh,shou1der; v,very

31

C. NMR Spectra of Nb2(Dmtp)4X4 (X a Cl, Br)

The proton magnetic resonance data are presented in Table 7.

Table 7

Proton Magnetic Resonance Data

Compound Chemical Shift (r) JPOCH3 (Hz)
(CH3O)2PSZNa 6.40 14.8
Nb2(Dmtp)4C14 6.40 15.0
6.21 15 0
Nb2(Dmtp)4Br4 6.30 15.0
6.16 15 0

The spectra exhibit doublet structure from 31F splitting of the methyl
peak. Two differern: methyl resonances are observed, indicative of
nonequivalent methyl groups. The chemical shifts and POCH3 coupling
constants are similar for the chloride and bromide complexes and are

not greatly different from those of sodium dimethyldithiophosphate. This
implies that transmission of metal d orbital effects via sulfur and
phosphorus outer d orbitals is negligible55 and that the orbital hybridi—
zation employed by phosphorus is essentially constant. No spectrum was
observed for the iodide complex due to its paramagnetism, which will be

discussed later.

D. Electronic Spectra
Nb(Dmtp)é
Visible and near infrared spectra were studied by using the tech-
nique described in the experimental section. These studies were carried
out with dichloromethane solutions of the complexes. The spectrum and

and wavelengthmaxima are given in Figure 2 and Table 8 respectively for NbCDntp)4.

32

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33

Table 8

Electronic Spectral Data for Nb(Dmtp)4 in CH2C12

A (nm) e
890 sh (b) 0.96
765 sh (b) 1.92
610 sh (b) 2.18
260 (b)

c = 2.30 x 10"2 M

The qualitative features of the spectrum include three weak broad bands

at 890, 765 and 610 nm. In addition a strong broad band is observed at

260 nm. This intense band in the ultraviolet region is due to ligand

charge transfer. This assignment was confirmed by observing an identical

band at 260 nm for a dichloromethane solution of the isomorphous Zr(Dmtp)4.
There are three possibilities for assigning the bands in the visible

and near ir regions: The bands may be due to (1) d-d transitions,

(2) metal to ligand charge transfer, and (3) ligand to metal charge

transfer. In view of the low intensities of the bands at 890, 765 and

610 nm (e = 0.96, 1.92 and 2.18 M"1cm'l respectively) they can confidently

be assigned as arising from d-d transitions. For an eight-coordinate tran-

sition metal complex two common symmetries are observed: the D2d triangular

dodecahedron and the D4d square antiprism. Figure 3 shows the crystal

field splitting diagrams for a Dzd dodecahedron and a Dad square antiprismb’ss

In the case of D4d symmetry, two d-d transitions are predicted while for

D2d symmetry three transitions are predicted. Since three bands were ob-

served which are assigned as d-d transitions a D2d dodecahedral symmetry is

Dzd
._........dxy

d12

 

..__.dx2_y2

Figure 3. The crystal field

34

 

splitting for D

2d

andD4d

 

symmetries

35

suggested for Nb(Dmtp)4. Further confirmation of this symmetry will be
discussed when the ear data are presented.
Nb2(Dmtp)#X#:complexes
The solution spectra andwavelength maxima are given in Figures

4,5, and 6 and Table 9 respectively for the Nb2(Dmtp)4X4 complexes. The
solution spectra consist of five bands for the chloride compound and six
bands for the bromide and iodide compounds. All the observed bands are
broad and there is a large uncertainty in their maxima. The ligand charge
transfer band is observed in all three complexes ca. 38 kK. All the com—
pounds exhibit peaks corresponding to those found in the spectrum of
Nb(Dmtp)4 with the exception of the chloride which lacks the band at
11.2 kK. In addition, all of the complexes have two peaks ca. 22.0 kK
and 26.0-28.0 kK which are not present in the spectrum of Nb(Dmtp)4.

Hansen and Ballhausen57 observed bands at 14.0 and 27.0 kK in the
electronic spectrum of dimeric copper acetate monohydrate. The band at
14.0 kK closely resembled the band observed at 14.0 kK in the spectrum
of copper sulfate. A theoretical treatment of the spectra using a coupled
chromOphore model indicated the band at 27.0 kK, which is not present
in the spectra of monomeric copper(II) complexes, arises from a double
excitation. Their calculations predicted a number of such double excita-
tions ranging in energy from 23.0 to 30.0 kK. Brown and Newton26 attrib-
uted a band at 26.0 kK to a niobium-niobium bond in the spectrum of
diamagnetic triethylamine adducts of NbC14 and NbBré. In light of Hansen's
calculations this band can be assigned as a double excitation and while
it is not evidence for a direct metal~meta1 bond, it does indicate a sig-

nificant metal—metal interaction.

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Table 9

Electronic Spectral Data of Nb2(Dmtp)4X4 Complexes in CH2012

Complex 1(nm) s(cm“1 "-1)
X = C1 750 3.25
555 7.48
460 25.7
355 62.3
260 713.0
X - Br 895 4.05
770 9.18
610 22.5
455 70.3
390 76.2
270 733.0
X a I 890 4.90
740 8.23
595 19.6
450 54.7
380 65.2

260 721.0

40

For a compound in which electron coupling occurs and the ground
state splitting is small enough that both the singlet and triplet
ground levels are populated at room temperature, the observed room
temperature spectrum will contain both singlet-singlet and triplet-
triplet transitions. These transitions are not expected to occur at
equal energies57 and therefore, the observed spectra are broad. In
addition, Fowles and coworkers58 found that in the acetonitrile adducts
of the niobium tetrahalides halogen(n) + niobium(d) transitions occur
throughout the visible region and as a result, the d-d transitions can
be partially masked.

The band observed from 26,000-28,000 cm-1 can now confidently be
assigned as due to a doubly excited transition. The bands at 11,000;
13,000; and 16,000 cm—1 are most likely due to d—d transitions on the
basis of extinction coefficients and their presence in the spectrum of
Nb(Dmtp)4. The assignment of the peak at 22,000 cm”1 is unclear.It is
unlikely that it arises from a d-d transition due to its intensity. How-
ever, it could arise from either a doubly excited transition or halo-
gen(n) + niobium(d) charge transfer.

The dimeric Nb2(Dmtp)4X4 posseses so many nonequivalent ligands that
an exact structural determination based on chemical and physical pr0per-
ties alone is virtually impossible. However, if a number of assumptions
are made a good approximation of the structure is possible.

The relative nonlability of the bridging halides in NbX425 suggests

that the dimeric complexes contain bridging halogen atoms rather than

41

bridging dithiophosphate ligands or a direct metal-metal bonded complex.
The infrared spectrum bears out this assumption. Secondly, the fact that
the dimeric complexes react slowly with additional NaDmtp to form the
paramagnetic Nb(Dmtp)4 indicates that both metal atoms are in the +4
oxidation state rather than a Nb3+— Nb5+ pair.With these two assumptions
the number of possible isomers is reduced to four.0f these four possible
isomers only one, the D2h isomer, will give three d-d transitions in

its electronic spectrum. The crystal field splitting diagram for D

2h

symmetry is presented in Figure 7.

 

 

....__. dxy
dx2_ y2
—dxz dyz
..._.d,2

Figure 7. The crystal field splitting diagram for D2h symmetry

Considering the atoms bonded directly to niobium the most likely struc-
ture has D2h point symmetry (C2v about each niobium atom) and is shown

in Figure 8. The niobium atoms in this structure are seven-coordinate

(excluding the metal-metal bond). While the dithiophosphate ligands are

chemically equivalent, the methyl groups on each dithiophosphate are not.

42

oxeaouaavmpz now ousuosuum venomoum .w ousuwm

 

 

 

e
m m o
o o
/a\/ \k\0 m
\ m x m —
m
x n /I IIIIIIII n2 x
\ // \\ I;
\ ll \\ z,
m\ Ill \\\ m
1 x. . _
e\m /a
mo\\ _/om

43

E. Electron Spin Resonance Spectra
Nb(Dmtp)i
Esr studies were performed as described in the Experimental
section.The spectra are presented in Figures 9, 10, and 11. Since the
hyperfine splittings are on the order of 150 gauss the high field approx-
imation cannot be rigorously applied and second order corrections should
be employed. The pertubation of the Zeeman transition resulting from

the hyperfine interactions was corrected by means of the following

 

 

equations59
hv = gBHo (8)
f 1c' H-H+>+—<—a—:2[I(I+1) 2] (9
or so ropic g O— m <amI 2H0 -mI )
A2 2
for 8!! Ho = Hm + AllmI + .l [ 1(1 + 1) - mI ] (10)
2H
0
f H = H + A + (A7] + 3i ) [ 1(1 + l) - 2 ] (ll)
“81 o m Pr “‘1
4Ho

where Hm is the magnetic field position of the ear line due to the com-
ponent mI of the nuclear spin 1, v is the klystron frequency, and <a>,
All, and Al are the hyperfine splitting constants. The calculations are
necessarily reiterative and were carried out by desk calculator. Normally
three iterations were sufficient. The hyperfine splitting constants were
determined from the positions of the fifth and sixth, fourth and seventh,

third and eighth, second and ninth, and first and tenth lines where

resolution permitted. The separation of the hyperfine components in gauss

44

ousuwuoosofi ucoHafi< up

0 Aunvu
9.:

N

H0

N

:0 ca «Aousovpz mo assuuonm umm .m assuam

 

 

 

 

 

 

 

 

 

45

menumuonaoe usoans< um «Anuscvnz pwaom mo souuuomm pom .oH ousaam

 

.OOON_

46

Me um um

N

am

N

:0 a« eaoueavpz mo Esuuuoom new .HH ouswam

 

 

 

 

 

JIafll

0::

 

 

 

 

 

 

 

 

47

is related to the energy splitting in cm.1 between adjacent hyperfine

levels as follows :
-1 -5
A(cm ) = g x 4.6686 x 10 x A(gauss) (12)

The experimental esr parameters are listed in Table 10 with the corrections

due to second order effects.

Table 10

Ear Spectral Parameters of Nb(Dmtp)4

* * t
(3) 8" 8i <a> AH Al
solid
(ambient temperature) 1.9780
solution
(experimental) 1.9791 1,8756 2.0308 136.9 216.9 92.0
(corrected) 1.9494 1.8971 1.9756 135.4

-4 ~1
* vaerfine splittings are given in units of 10 cm .

In dichloromethane glass at 77°K the ear spectrum may be described

by the spin Hamiltonian with axial symmetry60:

H - H +
gIIB zSz gL (Hxsx + Hysy) + AIISZIZ + Al(SxIx + Sny) (13)

where S = 1/2, I(93 Nb;100%) = 9/2. At room temperature in liquid solution,

the anisotropies are averaged to zero, and the Hamiltonian becomes :
H = < 8 >BH'S + <ia >I°S (14)

Theoretical calculations of ear parameters for a D2d dodecahedron

16,17
a

and a D4d square antiprism.ahow that g' > g|| for a dodecahedron nd

48

gll > 21 for a square antiprismlé’la. For Nb(Dth)4: Bl ‘ 1-9755 and

gII = 1.8971 indicating dodecahedral symmetry in support of the electronic
spectrum. The g values for the solid and solution esr spectra are essen-
tially the same, 1.9780 and 1.9791 respectively, indicating the stereo-

chemistry remains the same.

Data from the ear studies can now be used in conjunction with the
electronic spectral assignments to determine the applicability of an
ionic model to the present system. It can be demonstrated that while the

dodecahedral model has D symmetry, it can be considered as arising from

2d

the distortion of a cube.If a metal atom is at the center, the net effect

is a tetragonal distortion. For a de-yz ground state the gyromagnetic

ratios are given by Equation 15.17

81 2%
= 2.0023 --———- = 2.0023 --——- 15
8| I AE3 81 AEZ ( )

where A is the free ion spin-orbit coupling constant , AE3 - (2B2- 281),

2 4+
as

and AB = (2E3- B1). Taking the spin-orbit coupling constant for Nb

3
748 cm-1, the calculated values of g" and gl are 1.6374 and 1.8881

respectively. These values are much lower than the experimental quantities
and this indicates the inadequecy of the ionic model. It is possible to

qualitatively assess the amount of covalent bonding via Equation 16}7

81 2 2 2 2 2
g = 2.0023 - ——-°L-Y—-— g - 2.0023 - 4114— (16)
II E3 1. E2

2 2
The parameters a , B , and 72 are associated with the B1, E, and 82

molecular orbitals formed by linear combinations of metal and ligand

 

up .- r- ".‘.].l‘_r1

49

orbitals of appropriate symmetry. The range of possible values for each
of the three parameters is 1.0(ionic bond) to 0.50(covalent bond). Agree-

ment with the experimental g values is found forcrzy2 - 0.288 and
2 2

08 - 0.234. For pure covalent bonding in the ground and excited states
(Eyz - (382 = 0.0625. Thus, it appears that the niobium d orbitals are .‘

strongly mixed with ligand orbitals in the formation of Nb(Dmtp)4.
Further confirmation of the bonding is obtained using Equation 17

developed by MIcGarvey}6

 

E 11;”

AII = P[-K + (gll - 2.0023) + 3/7 (gi - 2.0023)]
(17)
Al = P[-K +2/7 + 11/14 (gL - 2.0023)]
where P = g gNB B< r-3 > and is defined as positive for 93Nb which has
e e ave
a positive nuclear moment, <‘ru3 >ave is the reciprocal cube of the

average radial distance of the outer electrons from the nucleus, and K
is the isotropic contribution to the hyperfine constant due to polariza-
tion of the inner electron spin density by the unpaired d electron.
Agreement with experimental data is found for values of K - 0.9665 and
P = 131.1 10-4cm-1. Comparing these values with those for a Nb4+ free
ion, K = 1.0(pure d orbital) and P = 192 10-4cm-1,61 the smaller experi-
mental value for P indicates that the unpaired electron is more delocalized,
hence more covalent in bonding orbitals.

Since the molecular orbital calculations indicated mixing of metal
and ligand orbitals, an attempt was made to observe superhyperfine

1
structure arising from 3 P splitting in the esr spectrum. No phosphorous-

niobium shf splitting is observed in the room temperature or frozen

50

solution esr spectra.The powder esr spectrum of Nb(Dmtp)4 diluted into
an isomorphous Zr(Dmtp)4 matrix shows extensive 31P-93Nb superhyper-

fine splitting giving additional evidence to support the delocalization

of the lone electron.

Nb2(Dmtp)414

 

The ear spectra of Nb2(Dmtp)4I4 are presented in Figures 12 and
13. The solid and frozen solution spectra are identical. The spectrum of
a triplet-state system consists of 2nI + 1 lines with a hyperfine splitting
of A/2 where n is the number of metal atoms present with nuclear spin,I,
and A is the normal hyperfine splitting for a single metal ion. In addition
to the normal AM8 = 1.1 transitions, a "forbidden" AMS -Li 2, half-field
transition may be observed at g ca- 4 arising from the magnetic dipole-
dipole interactions of the paramagnetic metal ions in a dimeric species.
The "forbidden" AMS - :_2 transitions arise when the magnetic field is
off the symmetry axis of the molecule by an angle, 6. The intensity and
resolution of these transitions are normally much less than for the
AMS s‘:_l transitions. This half field transition is normally only
observed in the frozen solution spectrum and its presence is considered
definitive evidence for an exchange-coupled system?1 When two neighboring
niobium(IV) ions interact, as occurs when dimeric complexes are formed,

the Hamiltonian for the pair may be written:
H = H1 + H2 + Hint (18)
where H1 and H2 are each of the form :

H1 2 e 8H stz + gi (Hxsx + Hysy) + Al ISzIz + 21(stx + sny) (19)

51

ousuouonaou. use“? up

 

 

 

 

 

 

 

 

1
.
~-—-.—‘_
w -—

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

N

.8

N

no as «Heaeueevuaz we eauaeoem hem .NH «passe

_o Ocu—

52

llII'FLl-IIILI

Me: um «Houmo a.“ «aways—Bug m0 533% you .2 meow:

_Ooo«_

 

 

A

a

 

 

 

53

Hint’ representing the interaction energy between the two ions, has the

form :

(20)

2 2 2
Hmt - D[sz - 1/3 3(8 + 1)] + s(sx - sy) — J81 32

where S = S1 + 82 if the ions are in sites of the same symmetry and D and
E are the zero field splitting parameters. In a complex which contains
an even number of electrons the degeneracy of the ground state may be
removed in accordance with the Jahn-Teller effect. For a system which
exhibits axial symmetry, it has been found that D>>E and, in fact, if the
x and y symmetry axes are equivalent E = 0. Therefore, E can be consid-
ered a measure of the deviation from axial symmetry. Exchange between a
pair of paramagnetic ions may be represented as a cosine coupling
between the effective spins, S1 and 82. The exchange integral, J, rep-
resents the energy separation between the singlet and triplet states.
When J is negative the ground state is a singlet and the complex is anti-
ferromagnetic. The value of the singlet-triplet separation can be deter-
mined from both ear and magnetic susceptibility measurements. Since J
can be determined more accurately by the latter method, it will be
discussed with the magnetic susceptibility data.

Lacking single crystal esr data, the values of the zero field
splitting constants have been determined in several ways including computer

36’37’39’42’46; use of Bleaney.8 equat10n56 relating

microwave frequency, gll, All, Al, and D38’44; and by making the approx-

38,44

simulation of spectra

imation that the low and high field parallel lines are separated by 2D

Assuming that the complex has axial magnetic symmetry, an approximate

54

value for D can be obtained from the separation of the outermost pair of

parallel lines in the frozen solution spectrum by :

_ e 21
H19 H1 20 ( )

60
The value of D was also determined through Bleany's equation for the

field direction along the symmetry axis :

A2 2
+ 1_ [1(1 + 1) - MI + (2M8 - 1)MS] (22)

w
0

mo = 8"BH + 2D(MS - 1/2) + AIIMI

 

where I = 9; M = 1,0; MI = +9, +8, +7,... 0; and mo is the radiation
3 ._ ._ ._

frequency. The resultant esr parameters are presented in Table 11.

The values obtained for the zero field splitting vary between 0.06328 cm-1

and 0.01876 cm—1 depending upon the method of calculation.

Since the D value provides a reasonable measurement of the inter-
metal distances which may give structural information for the paramagnetic
species, the assumption of axial symmetry is made for this system so
that D and the metal-metal distance can be obtained without exceedingly
30phisticated computation. It is clear that accurate hyperfine and zero
field splitting parameters cannot safely be extracted from these spectra
without an analysis of considerable sophistication including a theoretical
simulation of the frozen solution spectrum with general non-coincident
axes for all of the magnetic interaction parameters. Nevertheless, the
results of the ear studies on the copper62 and vanadyl44 tartrates suggest
that the error in the derived esr parameters under the axial symmetry

approximation is not very large. If D is attributed to the magnetic dipolar

55

Table 11

Ear Parameters for Nb2(Dmtp)4I4

8AM =.: 2 4.1320
< 8 > 1.9795
*
< a > 45.3
*
All 68.1
*
A 34.5
gII 2.0663
g1. 1.9361
g
D (Bleaney's equation) 187.6

R(0 = 0) 4.76 K

*
D (magnetic field approx.) 631.8
R(e - 0) 3.53 K

* hyperfine splittings are given in units of 10-4cm-1

56

interaction between two electron spins, it is expressed as63

2
D a 3/4 3282 < 1 - 3cos 6 > (23)

r3 max
12

 

where r12 is the interelectronic distance and 0 is the angle between the

r vector and the magnetic field direction. Assuming that 6 equals the

12

angle between the niobium-niobium axis and the magnetic field and

1/<riz> = l/R3, R being the niobium-niobium distance, one obtains :

1/3

1 (24)

 

(K) = [ 0.325g2 (,1 - 3 eoszel

R
calc _1
D(cm )

Taking Hll to be along the Nb-Nb axis, 2.3. 0 = 0°, one calculates a
niobium-niobium distance of 3.53 - 4.76.A. This can be compared with a

Nb-Nb distance of 3.36 A in a - NbIA.

F. Magnetic Susceptibility Measurements

An important indication of electron exchange-coupling in complexes
which contain more than one transition metal ion is the characteristic
deviation of the magnetic susceptibility of these compounds from the
Curie-Weiss law. When an isotropic interaction occurs between two para-
magnetic metal ions, each with a single unpaired electron, the exchange
interaction term, JSl-Sz, is required in the spin Hamiltonian. J is defined
such that a negative value corresponds to an antiferromagnetic inter-
action in which the ground state is a singlet and the triplet state
is J energy units above the ground state. The susceptibility of Nb2(Dmtp)414

passes through a maximum at 140°K and decreases rapidly as the temper-

ature is lowered. Above 140°K the susceptibility decreases gradually but

57

does not obey the Curie-Weiss law.

The close resemblence of this magnetic behaviour to that established
for binuclear copper(II) acetate64 and bis-n-cyclopentadienyltitanium(III)
chloride34 also suggests that isolated pairs of metal atoms interact to
form a lower singlet state(S - 0, diamagnetic) and a slightly higher
triplet state(S - 1, paramagnetic). As the temperature is lowered the
singlet state becomes more populated at the expense of the triplet
state and the susceptibility and magnetic moment decrease. The applica—
bility of this hypothesis can be evaluated by comparing the experimental
results with the xM(T) curve derived from Equation 25 which is appropriate

for a singlet-triplet model64:

2 2
g fi_§_§_ _ ’1
XM 3H [ 1 + 1/3 exp( J/kT) ] + Xd + No (25)
- I 23 -1
where N - Avogadro s number,6.023 x 10 mole
B = Bohr Magneton, 0.9273 x 10'.20 erg/gauss
g = electron gyromagnetic ratio(from esr data)

k - Boltzman constant

'-3
II

absolute temperature,°K

xd a diamagnetic susceptibility, emu/mole

N
a

Van Vleck temperature independent paramagnetism, emu/mole
An estimate of the singlet-triplet separation can be obtained using the

relationship34:

J = - 1.247 kTN (26)

where TN is the temperature of maximum susceptibility. In practice,one

58

plots a family of curves and chooses a value of J which gives the best
fit with the experimental data. At temperatures much greater than the
Neel temperature, one predicts exchange-coupled complexes to exhibit
simple paramagnetism in accordance with the Curie-Weiss law. A plot of

XM versus l/(T + 0) should yield a straight line with an intercept on the
XM axis equal to the sum of the temperature independent paramagnetism
and the diamagnetic susceptibility. Knowing the values of xd for specific

65

atoms, ions, and molecules from tables , the value of Na can be

calculated. The diamagnetic core corrections are listed in Table 12.

Table 12
Diamagnetic Core Corrections

Ions (10.6 emu/mole)

d
Nb4+ -14

61‘ -23.4
1' -50.6
Dmtp- -78.8

The compounds under investigation all decompose before Curie-Weiss

behavior is observed.

The experimental susceptibility of Nb2(Dmtp)414 is compared with
the susceptibility calculated from Equation 25 in Figure 14. While the
shapes of the calculated and experimental curves are qualitatively the

same, their magnitudes differ. This is not totally unexpected since a

59

«Hquusavmnz mo hufiHfioHuooomsm oauoswma poumasoamu poo Housoawuooxm .qH ousmfim

c... ._.

 

 

   

oow can con 00—
e . - q d . . m «T
1000.
.. 7 1 x
a W
e lcoon m.
09
I a
w
n
.89.. w/
0
al
1
po.o_:u_ou 0 1000s
BEoEtooxo o

 

 

60

number of assumptions are implicit in Equation 25 which are not completely
valid for this system. Assumptions which the ear data indicate may not
be valid are (1) the ligand fields are strong enough to quench the
orbital angular momentum and leave only the spin angular momentum free.
This is unlikely in view of the large spin-orbit coupling constant for
niobium(IV) and (2) the unpaired electrons are localized on metal atoms.
In addition, other thermally accessible excited states may be available.
A magnetic moment which has been calculated from the slope of the
Curie—Weiss plot has significance only if it has been established that
0 arises from magnetic exchange. When the origin of Bis unknown or a
complex does not obey the Curie-Weiss law, the common procedure is to
calculate an effective magnetic moment at a specified temperature. The

expression for the effective magnetic moment is given by Equation 27.

neff = 2.828 [(xM- xd)T 11/2 (27)

Since values of “eff are given for the majority of paramagnetic compounds
these will be reported here.The magnetic moment can also be calculated
from the gyromagnetic ratio obtained from the ear studies. Equation 28
gives the relationship between the magnetic moment and the 3 factor for

a specific total spin quantum number, S.

u=gmo+1n“2 no

Electron spin resonance measurements detect only temperature dependent
paramagnetism whereas bulk susceptibility measurements detect the total

magnetism of the material.

61

Both the chloride and bromide complexes are diamagnetic and their
susceptibilities are essentially independent of temperature. This is con-
sistent with the proposed structure of these compounds. In a halogen
bridged complex the niobium atoms are much closer together with the
smaller chloride or bromide bridges than in the case of iodine. Thus,
since the metal atoms are closer together a stronger interaction of the
unpaired electrons is expected for the case of chlorine and bromine
causing the singlet-triplet separation to be much greater. These com-
pounds might be expected to exhibit antiferromagnetic behavior at
higher temperatures but they decompose before such behavior is
observed. The susceptibility plots for the chloride and bromide complexes

are presented in Figure 15 and the magnetic data for all the complexes

are listed in Table 13. The magnetic susceptibilities increase in the
order I > Br > Cl which is expected as the metal-metal separation decreases

with smaller bridging ions.

Table 13

Magnetic Data for Nb2(Dmtp)4X4 Complexes

* s *
compound xM(300°K) Xd (xM — xd) neff(300°K)
X = Cl - 23.0 - 436.8 413.8 1.00
X = Br — 19.0 - 481.6 462.6 1.05
X = I 1700 - 545.6 2245.6 2.32

* units are given in 10..6 emu/mole

62

Aum.so u so exeaeuaaveaz we seaeaeaaeeeaaa assesses .mH enemas

 

 

 

 

x6 hr
own com own com on— oo—
_ . q A I. A J.T.
3 LJ 3 a 3 U
n1 mm Hm r4 .t
c c c o c c 6 l emu
_
x
w”
X
lawn. It
09
a
m
ens w/
0
mm
..m H... X O
C” x 0 loop

 

 

VI. Summary and Conclusions

Eight-coordinate Nb(Dmtp)4 was isolated from the reaction of niobium
tetrahalides with NaDmtp. The infrared spectrum indicates that Dmtp_ acts
as a bidentate ligand. The electronic spectrum exhibits three d—d transi-

tions and supports a D dodecahedral configuration for the complex. The

2d
esr spectra confirm this geometry with the parameters < g > - 1.9494,
gH -= 1.8971, gl - 1.9756, and < a > - 0.01354 cn’l. Using the molecular
orbital theory developed for D2d complexes, metal-ligand bonding parameters
were obtained which indicate strong mixing of metal and ligand orbitals.
Complexes of the type Nb2(Dmtp)4X4 were obtained from the reaction of
the niobium tetrahalides with two moles of NaDmtp. The infrared spectra
indicate that Dmtp- is bidentate in all three complexes. Three d-d transi-
tions are observed in the electronic spectra. In addition, a band due to
a double excitation is Observed at 390 nm‘ indicating a metal-metal
interaction. The complexes are. proposed to be halogen bridged dimers
with D2h symmetry (sz about each niobium atom). The magnetic moments
decrease in the order I > Br > C1. The bromide and chloride complexes are
diamagnetic while the iodide exhibits antiferromagnetism with a singlet-
triplet separation of -l40 cm"1 and a room temperature magnetic moment
of 2.32 Bohr Magnetons.Additiona1 confirmation of an electron exchange-
coupled dimer was obtained by the observation of the triplet state esr
spectrum of Nb2(Dmtp)4I4 with both the AMS-;:_1 and AMé'.i.2 transitions
present. The ear parameters are gll - 2.0663, 31.. 1.9361, AII - 0.00681
cm-l, and Al.- 0.00345 cm-l. The zero field splitting is 0.06318 cm.—1

which corresponds to a niobium-niobium separation of 3.53 A. This complex

represents the first reported case of electron exchange-coupled niobium (IV).

63

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