SPECTROPHOTOMETRIC AND KINETIC
STUDY OF MOLYBDENYL THIOCYANATE
COMPLEXES IN METHANOL

Thesis for the Degree of M. S.
MICHIGAN STATE UNIVERSITY
CHRISTOPHER CHIKE EZZEH
1968

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ABSTRACT

SPECTROPHOTOMETRIC AND KINETIC STUDY
OF

MOLYBDENYL THIOCYANATE COMPLEXES IN METHANOL
By

Christopher Chike Ezzeh

This thesis discusses the formation of molybdenyl
thiocyanate complexes by the reaction of diammonium
oxypentachloromolybdate(V) with the thiocyanate anion in

methanol through the reaction

MOOCl5 + n SCN ——» MOOC15_n(SCN)n + nCl (1)

In the visible region of the spectrum the reactant complex
has two broad peaks at 712 mu and 450 my and two well de—
fined peaks at 257 mu and 309 me in the ultraviolet region.
On the addition of thiocyanate a new peak with a maximum at
“60 mg is formed. The peaks at 712 mu and 309 mp remain
broad and diffuse but the peak at 257 mu shows a very pro-
nounced increase in absorbance with an increase in the

thiocyanate ion concentration.

It was noted that mixing the original complex and
thiocyanate resulted in the rapid formation of an absorbing

Species followed by a slower build-up of another species.

The continuous variation method showed that the species
formed shortly after mixing is a 1:2 complex but that this
soon gives way to a 1:4 species which is the principal ab—

sorbing complex at long times.

The presence of a 1:2 complex was also indicated by
the determination of the association constants for the
complexes. By using the method of Hume, association

constants were obtained for the following presumed equili-

bria:
— — K]. —'e .—
MoOCluMeOH + 2 SCN ——+ MoOCl2(MeOH)(SCN)2 + 2 Cl (2)
and
- - K2 _ _
MoOC12(MeOH)(SCN)2 + 2 SCN -——+MoO(MeOH)(SCN)u + 2 Cl (3)

K1 and K2 are 2.“ i 0.26 x 102 and 2.72 t 0.21 x 10“.
There was no evidence for the 1:5 complex at relatively low
thiocyanate concentrations and it was assumed that the
axial chloride anion in the original salt was replaced by a

molecule of the solvent in solution.

A stopped-flow apparatus was used to determine the rate
of formation of the complexes. This revealed, for ratios of
molybdenum to thiocyanate less than 1:2, a very rapid build
up in absorbance. The pseudo-first-order rate constant,
obtained by a method similar to that used in treating the

formation of the ferric thiocyanate complex (51, 52), was

plotted against the ligand concentration. This plot gave

a minimum in the region between the ratios of 1:2 and 1:3.

Studies with a rapid-scanning monochromater showed that
during the first part of the reaction a very rapid formation
of a complex (presumably 1:1) occurs followed by a slower
build up in absorbance and a shift of the maximum to longer
wavelengths. At higher concentrations the build up of the
initial complex could not be followed and it was presumed
that the 1:1 complex formed very rapidly, prior to the

formation of the 1:2 complex.

Both the electron paramagnetic resonance spectrum and
the optical spectrum indicated the absence of dimerization
of molybdenum(V) in the thiocyanate system. The esr spec-
trum shows, at thiocyanate to molybdenum ratios of four to
one and higher, nine hyperfine lines indicating an inter-

action with four nitrogen nuclei.

An overall reaction scheme compatible with these

results is

 

 

M00015 + MeOH :2: MoOCluMeOH- + 01‘ (u)
- fast - -
MoOCluMeOH + SCN » MoOCl3(SCN)(MeOH) + Cl (5)
MoOCI3(SCN)(MeOH)‘ + SCN‘ :EZ MoOCl3(SCN): (6)
, = k _ _
M00013ISCN)2 rearrangement Mooc12(SCN)2MeOH + Cl (7)

MoOCI2(SCN)2Me0H‘ + 2SCN élgfl MoO(SCN)uMeOH- + 201' (8)

» MoO<SCN)5 + MeOH (9)

 

MoO(SCN)uMeOH— + SCN-

Steps (6) and (7) are included to account for the ionic
strength-dependence of overall rate as well as the concen-

tration dependence of the apparent rate constant.

SPECTROPHOTOMETRIC AND KINETIC STUDY
OF

MOLYBDENYL THIOCYANATE COMPLEXES IN METHANOL

By

Christopher Chike Ezzeh

A THESIS

Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of

MASTER OF SCIENCE

Department of Chemistry

1968

Mr.

To my Parents

& Mrs.

Johnson W. Ezzeh

Acknowledgement

The Author wishes to express his sincere appreciation to
Professor James L. Dye for his continued interest, counsel and
encouragement during the course of this investigation. His
gratitude goes also to Messrs E. Hansen, V. Nicely, M. DeBacker

and E. Minich for their individual help.

TABLE OF CONTENTS

I. INTRODUCTION

II. HISTORICAL

A. Valence state of molybdenum in the complex

B. The interaction between molybdenum (V) and
the thiocyanate anion

C. Ultraviolet and visible spectra of
diammonium-oxypentachloromolybdate (V)
and molybdenum thiocyanate complexes

D. Dimerization of [MoOClS=] in solution

E. Hydrolysis of diammonium oxypentachloro-
molybdate (V)

F. Evaluation of molybdenum-to-thiocyanate
ratios in the complexes

III. EXPERIMENTAL

A. Preparation and purification of materials

B. Instrumental

IV. RESULTS AND DISCUSSION

A. Association constants for the complexes

B. Evidence against dimer formation

PAGE

l2

13

16

18

18
19

22

22

30

V.

VI.

SUMMARY

APPENDIX

A. Theoretical

REFERENCES

PAGE

56

58
58

62

LIST OF TABLES
TABLE PAGE

1. Absorption Bands of [M00012] ..' ........ . ..... .. 7

2. Data for the Determination of the Association
Constant for the Highest Complex ............. 25

3. Data flor the Determination of the Association

Constant for the Lowest Complex .............. 27
A. Values Of Kpseudo for Different Thiocyanate

Concentrations ......... ............... . ...... AA
5. Calculated Values of k and K ..... ............ 55

l

LIST OF FIGURES

FIGURE PAGE
1. Ultraviolet spectra of 0.003M (NHu)2MoOCl5 ..... 9
2. Determination of the highest complex with two
species present and one absorbing ..... . ...... 2A
3. Determination of the formation constant for the
lowest complex with two species present and
both absorbing ........................... ..... 26

A. Continuous variation plot one minute after mixing .. 29
5. Continuous variation plot 195 minutes after mixing 31

6. Shift in wavelength with time at high thiocyanate

concentration 000000 oooooo 000000 ...... .o.....'.... 32

7. Beer's law plot for the complex at moderate
concentrations ................... ............... 3A

8. Beer's law plot for the complex at high
thiocyanate concentration .... ................ .... 35

9. Beer's law plot for the l:A complex ........ ...... . 36

10. Log plot of Beer's law for the complex at.

moderate concentrations ..... ....... ............. 37

11. Frozen glass epr Spectrum of the complex at high

ilgand Cancentration ..... ..... .................. 38
12. Frozen glass epr Spectrum of 1:2 complex .... ..... . A2
13. Plot of K vs ligand Concentration ...... ..... A3

psuedo

FIGURE

1A.

15.

16.

17.

18.

Rapid—Scanning monochromater traces

Variation of 1n K with the square root

pseudo

of ionic strength ............................

Variation of K with high ligand

pseudo

concentrations ................... .............

Plot of 1
K
obs

 

Oscilloscope traces from the stopped flow A-..

apparatus ....................................

vs ligand concentration' ...........

PAGE

“7

INTRODUCTION

The nature of the thiocyanates of molybdenum(V) cannot be
easily explained. Different formulations have been proposed for
these species depending on the media used. Previous workers have
left unresolved several questions about the nature of the chemical
reactions such as the stability and nature of the complexes, rate
of ligand exchange and relation of exchange rate to the stability
of the ligand in the complex, the extent of solvent association
within the inner coordination sphere and the importance of the
molybdenum-oxygen double bond in determining the kinetics of the
substitutional processes.

In 60% acetone-water mixtures Perrin (1) reported that three
species exist in equilibrium. These complexes, which have molybdenum
to thiocyanate ratios of 1:1, 1:2, and 1:3, have different colors.
The presumed 1:1 complex is colorless, the 1:2 complex, dominant at
low thiocyanate concentration, is yellow, while the 1:3 complex is
amber,r Hiskey and Meloche (2) by using AN hydrochloric acid in
water reported the existence of a 1:3 complex. Babko (3) claimed
that 1:5 and 1:3 complexes were formed and suggested that under
appropriate conditions a colorless molybdenum thiocyanate anion and
cation occur in solution, while Kolling (5) isolated a 1:5 complex
from chloroform extracts. In addition to these, Wilson and McFarland
(A) were able to extract molybdenum oxytetrathiocyanate from organic
solvents. The complexity of these solutions is very evident from

the above.

The present investigation was carried out by'using methanol as
the solvent in an attempt to determine the nature of the complexes
formed in this medium and to study the kinetics of their formation.
During the course of this investigation it became necessary to study
the peak shape of the initial complexes formed at different thio-
cyanate concentrations for a given molybdenyl concentration. The
kinetic study revealed that the interaction of molybdenum(V) and
thiocyanate can be broken down into at least three steps. These are
the rapid and sucessive formation of two light absorbing species of low
thiocyanate-to-molybdenum ratios and the subsequent build-up of
another absorbing species over long periods of time. In order to
study the rapid formation of these species,stopped-flow-techniques
were used. This thesis gives a description of the methods used and
the results obtained and includes a proposed mechanism to explain

the results.

II
Historical

Small amounts of molybdenum present as molybdate, can be
detected by reduction with stannous chloride in the presence of an
alkali thiocyanate. The red color formed by this reaction has been
used in a number of colorimetric procedures. The earliest observation
of this reaction was by Braun and Skey (6). The reaction has generally
been carried out in approximately one molar acid and at high thiocyanate
ion concentrations.

The nature of the red coloration has proved to be difficult to
determine and has resulted in several conflicting descriptions. Some
authors thought that molybdenum was in the +3 oxidation state (7,8)
while others thought it was in the +5 state. Subsequently the complex
has been formulated as MoO(SCN)3, MoO(0H)2(SCN)3 and MoO(SCN)5=. Some
of these complexes possess varying degrees of hydration depending on
the conditions of their production. 0n the basis of simple analysis,
Krauskopf and Schwartz (9) reported the extraction of Mo(SCN)3 from
ether. Much confusion existed about these complexes until 1928 when
Wardlaw, et a1 (10) reinvestigated the problem and formulated the
complex on the basis of Werner's Theory as R2EM002(CNS)3] or
R2[M00(SCN)5] in which R is the univalent cation.

In 19A0 Hiskey and Meloche (2) reviewed earlier formulations of
these complexes. Their investigation attempted to throw more light

on three issues:

d
a

I
.1
v
,_ .V I‘

K

I MoOCluMeOH_ + 2 SCN- ——$— MoOClé(Me0H)(SCN)2' + 2 Cl- (2)
and . '
_ _ K2 - -
MoOCl (MeOH)(SCN) + 2 SCN ——A~ MoO(MeOH)(SCN) + 2 Cl (3)
2 2 <r—— A

K1 and K2 are 2.A t 0.26 x 102 and 2.72 t 0.21 x 10“. There was
no evidence for the 1:5 complex at relatively low thiocyanate concen-
trations and it was assumed that the axial chloride anion in the

original salt was replaced by a molecule of the solvent in solution.

A stopped-flow apparatus was used to determine the'rate of formation
of the complexes. This revealed, for ratios of molybdenum to thiocyanate
less than 1:2, a very rapid build up in absorbance.\ The pseudo-first-
order rate constant, obtained by a method similar to that used in
treating the formation of the ferric thiocyanate complex (51, 52), was
plotted against the ligand conce'htration. This plot gave a minimum

in the region between the ratios of 1:2 and 1:3.

Studies with a rapid-scanning monochromater showed that during the
first part of the reaction a very rapid formation of a complex
(presumably 1:1) occurs followed by a slower build up in absorbance and
a shift of the maximum to longer wavelengths. At higher concentrations
the build up of the initial complex could not be followed and it was
presumed that the 1:1 complex formed very rapidly, prior to the format-
ion of the 1:2 complex. «_ ‘

Both the electron paramagnetic resonance spectrum and the optiCal

spectrum indicated the absence of dimerization of molybdenum (V) in

the thiocyanate system. The esr spectrum shows, at thiocyanate to

nwlybdenum ratios of four to one and higher, nine hyperfine lines

indicating an interaction with four nitrogen nuclei.

An overall reaction scheme compatible with these reSults is

 

M0001= + MeOH'———+ MoOCl MeOH‘ + 01' (u)
5 +—-— A

MoOCluMeOH + SCN‘ —§§§E+ MoOCl3(SCN)(MEOH)- + C1- (5)

. K _
MoOC13(SCN)(Me0H) + SCN ————+ M00013(SCN)2 (6)
= k _ _

M00013(SCN)2 rearrangement, MoOCl2(SCN)2Me0H + Cl (7)
I

MoOC12(SCN)2MeOH' + 2SCN ElQA MoO(SCN)uMeOH- + 201‘ (8)

MoO(SCN)uMe0H_ + SCN- :———+ MoO(SCN); + MeOH (9)

Steps (6) and (7) are included to account for the ionic strength-
dependence of overall rate as well as the concentration-dependence of

the apparent rate constant.

(1) whether the complex formed in the usual analytical procedure
was identical with that formed between molybdenum (V) and
thiocyanate and whether molybdenum (III) formed a red complex,

(2) whether the pronounced color changes, which have been observed
in molybdenum (V) solutions could be correlated with a previous
observation of Hurd and Hiskey (11) that varying the acid
concentration of such solutions resulted in a bleaching effect
on the molybdenum thiocyanate complex, and

(3) whether an equilibrium study of the reaction between molybdenum
(V) and the thiocyanate anion would give some information about
the nature of the complexes.

Some of the results of their study have been confirmed by later

workers.

A. Valence state of molybdenum in the complex.

Much work has been done on this subject over a long period

of time. In the conventional analytical method for the estimation

of molybdenum the molybdate solution in 2N HCl is reduced by use of

such reducing agents as stannous chloride (12), the silver

reductor (l3) and metallic mercury (1A). With stannous chloride
as the reducing agent, Bergh and Haight (15) showed that Mo(V) is
the product in 3N HCl and that an equimolar mixture of Mo(V) and
M0(III) is the product in 9N HCl (16). In 3N H01 the Mo(V) dimer

is known to be inert and resistant to further reduction by the

stannous ion (17, 18). Mo(V) in the monomeric thiocyanate complex

is rapidly reduced at room temperature and results in the loss of
absorbance at the characteristic wavelength where the red-colored
species absorbs. This explains why as little as 50 percent of the
theoretical absorbance is obtained when molybdate is determined as
the thiocyanate complex after reduction by excess stannous chloride
(19) since Mo(III) does not form any red-colored complex (2). Thus
in the red-colored complex the valence state of molybdenum is +5

(3 and 16).

The interaction between molybdenum (V) and the thiocyanate anion.
Earlier work on this subject was carried out in aqueous solutions
at various acid concentrations. Interference due to the formation of
a pink complex by thiocyanate ion, and the tendency of the color of
the molybdenum complex to fade once formed made determination of the
complexes formed uncertain. A characteristic phenomenon of acidified
thiocyanate solutions is the formation of a pink—colored substance.
The conditions for this formation are similar to those which prevail
in the study of molybdenum thiocyanate complexes. To circumvent this
difficulty, low acid concentrations were used by Hiskey and Melonche
(2). Perrin (1) conducted his studies in a 60% acetone-water solution
which forms a single phase system in which the complex is sufficiently
stable to allow quantitative measurements to be made. Even in such a
solution the possibility of forming the pink thiocyanate complex
still existed. The present study has been carried out in dry methanol
and in the absence of any mineral acid. No red coloration was observed

when thiocyanate was dissolved in methanol.

The second difficulty encountered by previous investigators
of this problem was the fading of the color which often occured after
the maximum absorbance was attained. Hiskey (2) thought that this
was due to the presence of an acid in the solution. Thus he noted
that this fading was more rapid at high acid concentrations and slower
at low acid concentrations. Wilson and McFarland (A) thought that the
rapid fading was due to the reduction of Mo(V) to Mo(III). No satis-

factory demonstration of the origin of this behavior has yet been made.

Ultraviolet and visible spectra of diammomum oxypentachloromolybdate
(V) and molybdenum-thiocyanate complexes.

In most of the previous studies the molybdenum thiocyanate
complex had been prepared by reducing molybdate in the presence of an
alkali thiocyanate. Since we intended to study the kinetics of
formation of this red complex without reduction it was necessary to
begin with a molybdenum (V) compound. The complex found to be most
suitable was the oxypentachloromolybdate (V) as the diammonium salt

(59). Its spectrum has been studied extensively.

Until 1957 when C. K. J¢rgensen (20) examined the visible
and ultraviolet spectra of the M00015= anion, no band assignments
had been made. Five absorption bands were found for this ion in

8M HCl as shown in Table I.

Table I

Absorption Bands of [MoOClS=]

In Aqueous HCl (20) in dry Methanol (this
wavelength 6 work)

712 mu (broad peak) 12 very broad diffuse(
and weak

AA5 mu smooth peak 16 broad ~ extends to U.V.

355 mu sharp peak 500 not observed

310.5 mu very sharp — sharp

250 mu very sharp — sharp

In methanol as in aqueous solutions the complex ion [M00015=]
shows two weak bands in the visible region“ PThese (I).

are ligand field bands assigned on the basis that the anion is
essentially octahedral but with a strong asymmetry along the
Z—axis resulting from the metal-oxygen bond (21). Ballhausen

and Gray (22) explained this as a domination of the energy diagram
through n-bonding between the metal and-oxygen so that there is

a considerable further splitting of both the E: and 2T2g states.

Two bands are observed in the ultraviolet region at 309 and 257

mu. Figure I.A11en (21) assigned these to transitions involving

transfer of an electron from a bonding n-orbital (en) which is

largely associated with the oxygen atom, to either a nonfbonding or '
an antibonding orbital which is essentially d in character.

To account for the entire spectrum, both visible and ultra-
violet, Gray and Hare (23, 2A) devised a molecular orbital scheme
based on that suggested (22) for the vanadyl ion, [V0]2+.

Comparison of the spectra in concentrated hydrochloric acid
and in methanol solution shows that in methanol the band at
712 mu is very weak, broad and diffuse while the band at 355 mu
is absent. The latter is a relatively weak electron transfer band~
and it appears strongly when molybdenum (III) is partially oxidized.
In 6M hydrochloric acid molybdenum (V) gives a brown solution

which absorbs at 720 and AA5 mu while the shoulder at 355 mu is

blurred out. This is also observed in 6M LiCl in methanol. Evidencet'

has been presented (35) which indicates that this brown hydrolyses
product contains at least two molybdenum atoms per complex. Although
no shoulder appears at 355 mu in highly concentrated solutions of
molybdenum (V) in methanol, a brown solvolysis product was observed.
It is possible that the paramagnetism of [MoOC15=] is changed to
diamagnetism (25) as has been observed in dilute HCl solutions at
'high concentrations of molybdenum. This may also be due to the

formation of oxygen

Figure I.

IA.

IB.

IC.

Ultraviolet spectra of 0.003M (NHu)2M00015.
Spectra run on Unicam S.P. 800 at room

temperature

(NHu)2MOOCl in methanol

5

(NHu)2MOOCl in 6M HCl

5

(NHu)2MoOCl in 6M LiCl

5

 

lO.

 

 

 

 

6m HCl

 

 

 

25'0 360 35'0

Fig. IC

 

6m LiCl

 

 

ll

bridges (26, 27,,28, 29) of,theityD$ encountered in

u;.. -
[C1 -Ru-0—Ru015] and [Cl ReORe015]u . The latter complex is

5 5
in equilibrium (27) with the monomer [Re015(0H)]2‘.

Salts of the series MEMOOClu] have been reported. These
have the same spectra as the MZEMOOXSJ salts indicating that
the anion [MoOXMJ-easily takes up X- ion in solution.

The spectrum of [MOOCIAJ-l has two bands in the visible region
but the position of the band at 712 mu differs from that found
for [M00015]-2 anion. The other ligand field band at A50 mu
occurs at the same position both in the solid and in solution but
differs very little from that found for [Mo0015]"2 (23).

In the presence of a moderately concentrated solution of
thiocyanate there are some pronounced changes in the spectrum.
In the visible region the peak at 712 mu remains broad and diffuse
but a sharp peak having an absorption maximum near A60 mu appears.
This indicates that some new complexation has occurred in solut-
ion. At very high thiocyanate concentration a new peak starts
to form at 510 mu soon after mixing. This peak continues to
build up and reaches a maximum eight minutes after mixing. The
peak position then gradually shifts towards shorter wavelength
reaching a maximum some 80 minutes later at A60'mu:““gth 0'
A possible explanation of this peak shift is disscussed later.

In the ultraviolet region the shoulder at 257 mu becomes a
well-defined peak showing a very high absorption while the peak at

309 mu remains broad and shows no change in absorbance at very

 

12

high thiocyanate concentrations. Since the peak at A60 mu
is well resolved and intense, the kinetic studies of this in-
vestigation were carried out by following the absorbance at this

wavelength.

Dimerization of [Mo0015]= in Solution.

 

Molybdenum (V) in aqueous hydrochloric acid solutions has
been studied spectrophotometrically. The conditions used by
various workers have often been different and it is difficult to
compare their results, particularly in connection with the
phenomenon of polymerization.

It is generally agreed that in HCl solutions greater than
10M the major species is [M00015]=. It is also possible that the
species [MoOCluj- is present. Decreasing the acid strength
produces reduced paramagnetism and enhanced absorbance without
a change in the positions of maximum absorbance (25). These

2...

observations suggest the formation of a dimer, [Mo C18] or

203

[Mo Clu]2+ (32). The nature of the diamagnetic species is

203
not certain but has been formulated with two oxygen bridges (31)
and as a tetramer (32). Spectrophotometric and conductometric
studies indicated that the species [M00C13], [MoOClu]- and
[M00C15]= are formed in formic acid (33). It is thus to be
expected that in methanol both [MoOC15]= and [MoOClu]- can form

as well as polynuclear species.

13

Electron paramagnetic resonance (esr) studies of the
thiocyanates of molybdenum (V) (59, 3A) show the presence of
four nitrogen atoms per complex.

The enhanced absorbance observed with increase in chloride
ion concentration may be due to the formation of M00015= by the
addition of chloride to M0001“- rather than to the breaking up ofs-.
the dimer to monomeric species. This explanation seems plausible
since a plot of the log of the absorbance versus the log of the
concentration of M00015= shown in Figure (8) gives a s10pe of
unity indicating one molybdenum atom per complex. However it is

possible that dimers are present which do not break up upon

dilution.

Hydrolysis of diammonium oxypentachloromolybdate V.

James and Wardlaw (10) proposed a number of hydrolysis
products for the salts of this series. More recently Jezowska-
Trzebiatoska and Mikolaj (35) conducted a study with a magnetic
susceptibility method on the hydrolysis products and possible spec-
ies in equilibrium in solution. From their spectroscopic data
they found that several hydrolytic products are formed (36) and
that these have different spectroscopic pr0perties. While the
absorption spectra changed over the HCl concentration range of

3-5 to 5.5M, the stoichiometric coefficients of hydrogen and

1A

chloride ions remained invariahtoand indicated a change in the
intramolecular arrangement of the primary product. 0n the basis
of these data, and taking into consideration the analytical
results of Jezowska-Trzebiatowska which confirmed that two HCl
molecules and one NH401 molecule are detached from one molecule
of ammonium oxypentachloromolybdate (V), they suggested the

following hydrolytic mechanism

2[MoOCl =] + 2H 0 .4. 2[Mo001 (H 0)]‘ + 2 01' (l)
5 2 ‘r— A 2

- = + ..
2[MOOClu(H2O)] ‘::= [MO2(OH2)O2C16] + 2H + 2 Cl (2)

Mo2(0H)202CI6]‘ + 2H20 2::I[M02(0H2)02Clu(0H)2] + 201’ (3)

[Mo2(0H)202Clu(0H)2]‘ ;::I[M020uClu(H20)2]- (A)

MO204Clu(H2O)2] + 2H

20 -45I(MO2OuCl2(OH2)(H2O)2]

+ 2H+ + 2 Cl" (5)
M020u012(0H)2(H20)2] + 2H20'—J> [Mo2Ou(OH)2(H20)u]

+ 2 Cl' (6)

15

Thus two binuclear species are assumed to be in acid-base

equilibrium and“have the structures

 

 

 

 

’— OH 0 —1=
Cl - OH Cl

\D'q/ \U/

0 0
d
01/” \OH/' \Cl an
. 0 OH ‘-
I

~' H20 0 —"‘"
Cl I 0 I Cl

\M0/ \M0/
01/" \0/ \01
g 0 H20 _.

 

 

II

16

Magnetic susceptibility studies, absorption measurements
and molecular orbital calculations have given evidence for the
existence of these bimolecular complexes (37) in aqueous solutions.
However it should be noted that most of these studies were carried
out at very high molybdemum concentrations and it seems likely

that at low concentrations (<0.008M) dimerizationsmay not occur.

Evaluation of molybdenum to thiocyanate ratios in the complexes.

 

Earlier investigators used different methods to determine
the ratios of molybdenum to thiocyanate in these complexes.
Hiskey and Meloche (2) obtained the ratio 1:3 by considering the
effect which a variation of the thiocyanate concentration has on
the equilibrium constant for the reaction. From their results
they concluded that only one complex exists in solution but they
admitted that this ratio was not established with stoichiometric
exactness from their data. Perrin (l) recognized, using 60%
acetone-water mixtures, that the continuous variation method
gave a rather diffuse absorbance maximum with a thiocyanate to
molybdenum ratio between 2 and 3. He noted that at greater mole
fractions of thiocyanate ion the absorption shifted to longer
wavelengths but its position was not sufficiently sharp for
accurate location. Thus in his determination of the association
constants of the complex he assumed the ratios 2:1 and 3:1 in

the development of his equations. Constant values for the

17

association constants formed his basis for confirming the
existence of these complexes.

Babko (3) had earlier obtained a ratio of 5:1 by U§ec0f
different procedure based on the variation of absorbance of
solutions with reactant concentrations. Thus there has not been
agreement on this question. In the present investigation no
prior assumption was made about the ratio of thiocyanate to

molybdenum in these complexes.

III Experimental

Preparation and purification of materials.

Diammonium oxypentachloromolybdate (V) was prepared by
electrolysis. Chilesotti (51) first showed that by electrolytic
reduction of molybdenum trioxide dissolved in hydrochloric acid
of appropriate concentration, a solution containing molybdenum (V)
could be obtained if a platinized platinum cathode was used. This
principle was utilized in the preparation of diammonium
oxypentachloromolybdate (V).

Molybdenum trioxide (15g) was heated with 75 ml of concen-
trated hydrochloric acid until dissolved. The solution was
evaporated to 37 m1, filtered and made up to 75 ml with the acid.
It thus contained 20 percent molybdenum trioxide in 9N-HCl.

The solution was diluted with an equal volume of water and
placed in the cathode compartment of a diaphragm cell. The anode
compartment contained hydrochloric acid (5N). A polished platinum
electrode served as the anode, while the cathode was of platinized
platinum. A current of 2.5 amperes was employed and on reduction
to the quinquevalent state, a reddish-brown cathodic solution
was obtained. This solution was concentrated to 50 ml under
diminished pressure, a 30 ml sample of ammonium chloride (9g, 5M)
solution was admitted and the mixture heated for one minute. 0n
saturating with hydrogen chloride gas and cooling, emerald green

crystals separated. These were recrystallised by dissolving in a

"minimum volume of water at 80° and saturating with hydrogen chloride

ggas until it had cooled to 25°. The crystals were washed with hydro-

l9

mm 1‘ .
| - .

chloric acid and dried in a vacuum over solid potassium hydroxide.
The methanol (ACS certified) was dried over Drierite and distill-
ed.prior to use.

Since a solution of (NHLI)2M00C15 in methanol shows a de-
crease in absorbance on standing for long periods, fresh solutions
of this reagent were prepared five minutes before use.

Appropriate amounts of (NHu)2M00015 were accurately weighed and
dissolved in calculated volumes of methanol.

The thiocyanate solutions were also prepared just before
use. Ammonium thiocyanate was dried in an oven at 100°C then
allowed to cool in a desiccator. The calculated amount was
weighed out and made up to volume in a volumetric flask.

In cases which required constant ionic strengths calculated
amounts of lithium perchlorate were added to the solutions.
Reagent grade lithium perchlorate was dried in an oven and allow-

ed to cool in a desiccator. This salt was chosen because the

perchlorate ion has little tendency towards complex ioniformation.

The stopped-flow apparatus.

Hartridge and Roughton (A3) introduced a method for studying
reactions with half-lives as short as a few milliseconds. The
method involves forcing two reactants into a special mixing
chamber and determining the composition of the mixture after

suddenly stopping the fluid. More detailed descriptions of this

20

type of apparatus can be found in the references (AA, A5, A6, A7,
A8, A9, 50). Since it was found that air had no effect on the
reaction, it was not necessary to use a closed system.

The apparatus consisted of two glass syringes from which the
reactants can be forced into a mixing chamber. These syringes
were also connected*t0wreactantwreservoirswwithaaItwo—way stopcock.
All of these components were mounted on a framework of properly
cut aluminum plates bolted together and attached to a metal table.
The other parts of the apparatus are: a lever mechanism for
pushing the syringes; a light source (tungsten lamp); a mono-
chromator and appropriate lenses; and a photomultiplier. Rapid
mixing of the reactants was accomplished by pushing the pluggene
manually. The reactants were forced up into the mixing chamber and
then into the observation tube and through a Teflon solenoid
valve which stopped the flow when a microswitch was tripped. This
switch also triggered an oscillosc0pe which monitored the photo-
multiplier current.

The mixing chamber was constructed of Pyreerour jets of
internal diameter 1.0 mm were arranged tangentially to the bore
of the observation tube which was 1.9 mm in internal diameter.

The reactions were followed by measuring the changes in light
absorption as a function of time. Light from the tungsten lamp
passed through the monochromator and was focussed on the center of
the observation tube. After passing through the reacting mixture

the light was focussed onto a photomultiplier tube. The response

21

was amplified and displayed on the screen of the oscillosc0pe.
The trace so obtained was photographed and analysed. Figures
18Ia, b show typical traces obtained in this way.

The rapid-scanning monochromater used in several experiments
has been described elsewhere (58). These experiments were per-
formed with the help of Mr. Earl Hansen. Esr measurements were

made on a Varian X-Band Spectrometer by Mr. Vincent Nicely.

IV Results and Discussion

Association constants for the complexes.

A number of workers have attempted to determine the associat-
ion constants for the various species existing in solution.
Perrin did the pioneer work on this and obtained various values
for these constants on the basis of the following mechanism

(using Perrin's notations):

Mo(V) + CNS‘ —=-M0(V)(CNS) log Kl = 1.7 t 0.22
v—

Mo(V)(CNS) + CNS- :::M0(V)(CNS)2 log K2 = 3.5A - 3.08

Mo(V)(CNS)2 + CNS- :::~MO(V)(CNS)3 log K3 = 3.5A - 2.68

His method involved separating the absorbance into its contribut-
ions from the different species by assuming no absorption from
the 1:1 complex.

In the present investigation the method due to Newman and
Hume (A0) was employed. To determine the association constant

for the highest complex, Equation 18 of the Appendix is plotted

as log AQ—i—A against log Xtm.

Values were assumed for m such that the slope was unity.
A

From the plot, Fig. (2A), Kn was found to be 2.72 t 0.21 x 10 .

The value of m which gives the proper s10pe is two. This value
represents the number of ligand groups added to the lower complex

to give the higher complex. Thus this mechanism can be pictured as

. J: + 2 Cl (1)

[MOO(SCN)n-2Cln—3] + 2(SCN ) i::jMoO(SCN?n015_n

For equation 18 to hold the concentration of the ligand has to

be much greater than that of molybdanum.

23

However in order to determine the association constantcof
the lowest complex the experiment was carried out with the concen-
tration of the ligand comparable to that of the molybdenum. A
new set of equations was developed for this case and a plot of

equation 23) (see Appendix) as

 

 

 

Mt Mt
_ v _ _ I __
A A0 MO A A0 Mo
log ~ versus log [X - 2 '
H Mt t H i 1

A0 T — A A52 — A0
0 X0 MO

gave a straight line of slope 2 and an intercept log K = 2.38.
The slope obtained agrees with that expected and establishes the

mechanism as

: . > =. .v = -2 , N‘W 2
[Mo0(SCN)2Cl3a..—- {Mooclsh (I__(sc3 ,) ( )
or [M0001 (SCN) MeOHlfic—é. —‘['M00CJ."‘MGOHJI_‘.+ 9450A?) (3)
2 2 (“L-l 1“ .- 3
The values of n in equation 1 above remains to be evaluated.

The ratios of the thiocyanate-tofmolybdenumwinnthetdifferent.hm
_species'were.determined.by continuous variation method...”

In this determination two distinct colors were observed.
The first is yellow and forms at low molybdenum to thiocyanate
ratios soon after mixing. The other, which is amber, forms at
higher molybdenum to thiocyanate ratios and takes a long time to
attain maximum absorbance. In View of this; the continuous
variation method was carried out in two stages, namely; one minute
after mixing, andléfisminutes later. Various ratios ranging from

1:1 to 1:8 were chosen.

0.A

10g A0 ; A

 

 

 

 

Fig. 2

Determination of the highest Complex with 2 species, one absorbing.

25

Table 2

Data for the Determination of the Association constant

WWW

log X

.83
.066
.3186
.0686
.A3
.782
.180A
.69A

for the Highest Complex

log

+O.

Ao—A
A

 

57576

.5965
.17386
.4559
.0386
.5597
.1818

.5965

26

 

 

 

 

 

In'5
" ' Mt
log A + A0 M6
" Mt
A0 If; - A
I4
.3 ‘i ' Mt __ .1]
log E-2 A-AO M3 J
_".__'
Ia - 5:
X0 Mo
Fig. 3

Determination of the association constant for
the lowest complex with 2 species both absorbing.

Table 3

Data for the Determination of the association constant

for the lowest complex

 

.9A1
.92

.053
.075

28

Fig. 3 shows that one minute after mixing the highest
absorbance was recorded for the mixture whose molybdenum to
thiocyanate ratio is 1:2. This suggests that the species probably
formed during this time interval is the 1:2 complex. This confirms
the results of the determination of the association constant for
the lowest species which indicated that this species contains two
ligand groups per atom of molybdenum.

After a long time interval, as shown in Fig. A , the highest
absorbance occurs for the l:A ratio. It follows from this that
the complex which has the highest absorbance is the l:A complex,
whose formation constant is 2.72 x 0,21 ‘ 104, Since the highest
complex contains four thiocyanate groups per molecule the value of
n in equation 1 above is four. Thus the two important complexes
existing in solution and which are in equilibrium after a consider-
able time are the 1:2 and l:A complexes.

From the structure of the complex, it was expected that five
ligand groups would be present in the highest complex. In fact,
Babko (3) claimed that such a complex exists in methanol and
absorbs at 500 mu and reaches its maximumtabsorbance ingeight minutes.
The spectra shown in Figure 5 were obtained by use of-avconcentration
of molybdenum of 0.0025M while that of thiocyanate was 0.3M. It
is seen that soon after mixing the build-up starts around 505 mu
and reaches a maximum in eight minutes. Once this maximum had
been attained, the peak position gradually shifted towards short-
er wavelength. The maximum absorbance was reached after 83

minutes at a wavelength of A60 mu.

29

 

 

0.

 

0.1

 

 

L

 

W 05.5. 0.6 of

Mole Fraction of $6783 I
Fig. A

Continuous variation plot at time 0ne Minute after Mixing.

30

If the initial build up was due to the formation of the
1:5 complex, it follows that this species changes with time to

some other complex absorbing in the region of the l:A complex.

Evidence against Dimer Formation

It was found that for a given molybdenum concentration, the
absorbance increased as the concentration of added thiocyanate
increased. However the absorbance upon dilution obeys Beer's law
and yields a straight line as long as the concentration of thio—
cyanate is not much larger than that of molybdenum. When the
concentration of the thiocyanate is much larger than that of
molybdenum the absorbance reaches a maximum indicating that the
highest complex species has been formed. Figure 6A shows the
Beer's law plot for thiocyanate concentrations in the region of
lO-BM while Figure 6B is the plot for higher thiocyanate concen-
trations. Beer's law plots for various solutions containing l:A
ratios of molybdenum to thiocyanate can be seen in Figure 7.

Soon after mixing, the law does not hold but after 1 1/2 hours a
straight line results.

To determine whether there was dimerization of molybdenum (V)
as many workers have suggested for aqueous systems, a log-log plot
of absorbance versus total concentration was made. Figure 8 shows
the straight line that results. The slope of one indicates that
there is but one molybdenum atom per molecule of the complex.

This result which seems to indicate the absence of

31

 

1.5

“V

\‘
0

f. "

 

AWE
.

(O

' ’l

G

 

 

 

0.1 0.3 0.5 0.7 0.9

.Moka Fraction of Thiocyanate

Fig. 5

Continuous Variation plot at time = 195 min. after mixing.

32

 

 

 

 

B
Absorbauce
A
C
L l¥ I A
AlOI~I‘ 500 5“0 580
Fig. 6
Shift in wavelength with time at high thiocyanate concentrat—
ion.
Curve A Time = 1 Min.
Curve B Time = 8 Min (Max. absorbance reached at A = 505mu
Curve C Time = 83 Min. Peak shifted to A = A60mu

33

dimers, may not be conclusive since it is possible that non-
dissociating dimers might have formed.

Electron paramagnetic resonance measurements throw more light
on this problem. As in earlier work on this complex (59)
nitrogen hyperfine interaction was observed on the central line
arising from the interaction of molybdenum isot0pes for which
I=0,with the paramagnetic electron. These hyperfine structures,
shown in Figure (9) consist of nine equally spaced lines having
an intensity distribution which shows interaction of the para-
magnetic electron with four equivalent nitrogen nuclei. This
also gives strong evidence that the thiocyanate bonds through

the nitrogen rather than through sulfur.

1.

3A

 

 

 

AssongAuce

  

 

 

 

2 3 A 5 6
I l l a 1
x10-‘M
Fig. 7

Beer's law plot for the complex at”)

.4

moderate concentrationS‘ ~

35

 

 

A
0 U “‘
1.5
1.0
III
U
2
‘(
I)
1.
0
VS
a
<
0.5
O

 

 

 

 

0.01 0.02 0.03 0.0A 0.05 0.06 0.07M
Fig. 8

Beer's law 91013 for-the complex. at high thiocyanate}concentrations

36

 

 

 

 

 

Q
.9
43%
l
A
V’
m A
u
2 .c
i’
l
d
0
8 . A
Q
25
’0
A
7,? A
0.005 0.01 0.015 0.02
l 4 l L
Concentration of the complex
Fig. 9

Beers law plot for l:A complex
at infinite time and time Zero

37

 

 

 

—log [MoO(SCN)u[Me0H]-
3 2
I
I
I
I
i log A
I -5
I
I
I
I
I
I:
I
:I 1.0
I
I
I
1.5

 

 

 

 

Fig. 8

Log plot of Beer's law for the,complex at moderate«con-
centrations of thiocyanate.

38

 

 

 

 

 

 

 

Fig. 11

Frozen glass epr Spectrum of complex
at high ligand concentration.

 

39

If dimers were present initially or formed by reaction in
the concentration range studied it would be expected that there
would be a change in the esr Spectrum with time. No significant
change with time was observed and it is reasonable to conclude‘
that in the concentration range studied there was either no
dimerization or that the degree of association of these dimers
did not change with time. In moSt investigations on the dimer-
ization of molybdenum(V)the lowest concentration used was about
0.01 molar. Among the latest works are those of I.N. Marov and
co—workers (60) who used ésr methods to establish the presence of
dimers in c0ncentrations greater than 0.01 Molar. R. Colton and
G. G. Rose (61), also working at high concentrations, concluded

that the diamagnetic species [M020 0183-2'containing an 0x0

3
Ibridge, was present.
The presence of only four nitrogen nuclei in the esr
measurements is noteworthy. Five nitrogen atoms were expected
as indicated earlier. Neither the continuous variation method
nor the es? investigation has shown any evidence in Support of
the 1:5 complex. However, it is misleading to state that‘splitting
due to'four nitrogen nuclei eStablish©SLthe presence of a l:A complex.
It is probable that the fifth nitrogen, axial to the oxygen atom,
is not esr sensitive since its bonding to molybdenum is different.
The esr Spectrum of a mixture containing only a 1:2 ratio of

molybdenum to thiocyanate is shown in Figure (10) 'This indicates

no nitrogen interaction.' However, it cannot be concluded from

A0

this that the 1:2 complex does not form but rather that only a
minor amount of the l:A complex forms in this case. This is
confirmed by the Optical spectrum which show little change of a
1:2 mixture even upon long standing.

That ligand substitutional equilibria exist in a solution
containing molybdenum (V) and thiocyanate was established by
D. I. Ryabchikov and his co-workers (62, 63). It was decided to
analyse the system on the basis of a simple kinetic law. Pre-
liminary investigation with the stopped-flow apparatus showed, at
very low ratios of molybdenum to thiocyanate, a very rapid form-
ation of an absorbing species. As the thiocyanate to molybdenum
ratio was increased, it appeared that this initially-formed
species was "swamped-out" by the slower build-up of a more highly
absorbing species. It is speculated that the first species form-
ed is the l:l complex and this is regarded as the starting species
for the subsequent absorbance build up at higher thiocyanate
concentrations caused by the appearance of a 1:2 complex.

Table (A) shows the values of Kpseudo for the different
thiocyanate concentrations when the concentration of molybdenum
is 0.00A113M and ionic strength 0.26M. Figure (13) is the plot
of Kpseudo against the concentration of thiocyanate. It is

evident that as the thiocyanate concentration increases initially

the value of K decreases and reaches a minimum when the

pseudo
ratio of molybdenum to thiocyanate is between 1:2 and 1:3. In

this region it is assumed that there is a rapid formation of some

Al

species, presumably the 1:1 complex. This build up, as shown in
picture IA, page (56), is complete in less than 0.025 second. It
is speculated that at higher thiocyanate concentrations this
complex becomes the starting species for the subsequent build-up
of the 1:2 complex.

After this minimum, K increases with the concentration

pseudo

of the thiocyanate. The values of K in this region is

pseudo
taken as that appropriate to the formation of the 1:2 complex.

 

It is speculated that with further increase in the concentration I
of the ligand the 1:2 species is slowly replaced by a l:A complex. I
This was shown by following the formatinn of the l:A species using
a Unicam Sp. 800 with a constant wavelength scanner. It was noted
that the initial absorbances (i.e. after one minute) for all
mixtures with ratios greater than 1:2, were equal. However the
subsequent build-up depended upon the thiocyanate concentration.
It is reasonable to conclude that the equal absorbances at short
times is due to the presence of the 1:2 complex.
In order to investigate this system further the rapid scann-
ing monochromator was used. It was observed that an absorbing
species formed rapidly. This formation soon reached a maximum and
the absorbance suddenly began to decrease. The decrease also
reached a minimum after which the absorbance built up to and above
the former value. It was thought that such an irregular behavior

was due to a leak in the solenoid. The same behavior was noted

A2

 

 

 

Fig. 12

Frozen glass epr spectrum of 1:2 complex

 

A3

 

1.5

Kpseudo
Ilo"3

 

005

 

 

l l , h

 

 

 

0.01 0.02 0.03 0.0A

Concentration of [SCN] in Moles
Fig. 13

Plot of K versus ligand concentration

pseudo

. ~ A {We—533)“. Hm”

Wi-I

AA

Table A
(NHu)2MoOC15 = 0.00A113M u = 0.26
Valves of Kpseudo for Different (SON‘)
[SON-J Kpseudo
0.0025967 1670 sec-l
0.00A7606 83A
0.005798 A15
0.00669 2A5
0.008028 260
0.0103869 593
0.0151A75 55A
I 0.0202A1 623
0.0251017 1083
0.0302951 1091
0.0350558 118A
0.0A02A92 llA8
0.0A50099 1233

0.050203A 1313

T. .

 

A5

even when the stop-cock close to this solenoid was closed after
mixing. Such a behavior could not be effectively accounted for
but the analysis of the initial rise in absorbance gave results
which agreed with those of the fixed-wavelength stopped-flow
apparatus at high thiocyanate concentrations. However, at low
concentrations the values of Kpseudo were much smaller. This
could probably have resulted from subtraction of the background
absorbance which could have eliminated the very fast part of the
build-up. Table (III) shows the values of Kpseudo obtained from
the Rapid Scanning Monochromater.

It was observed that during the initial build up in absorb-
ance there was a pronounced change in the peak shape as shown in
Figures (1Aa,b,c,d). The change in peak shape is more pronounced
at low thiocyanate concentrations. A well-shaped peak is formed
one minute after mixing. From this change in the peak shape one
concludes that at least one new species was being formed in the
solution. This is supported by the fact that as the peak forms it
moves towards a longer wavelength. Increasing the ratio of
molybdenum to thiocyanate, as seen in Figure 1A (c & d) gives a
smooth peak much earlier than at lower ratios.

This observation supports the speculation that at very low
ratios of molybdenum to thiocyanate, the 1:1 complex forms rapidly

and becomes the starting species for the formation of the 1:2

Species.

A6

The dependence of K on the ionic strength was investi-

pseudo
gated in order to determine the charges on the reactants. A

graph of log K against the square root of the ionic

pseudo
strength, shown in Figure 15, was made. From this plot a slope

of A.l was obtained.‘ The calculated value of the slope, hy‘use Of
the Debye-Huckel limiting law and by assuminguadchargefiof -2 on the
principal reactant, is 16. The value actually obtained indicates
that two species each having a unit negative charge are involved

in the rate determining step. Such a unit charge on the principal
reactant would be possible if one of the chloride ions were
displaced by a molecule of methanol prior to the reaction. A
second possibility is the formation of ion-pairs in solution.
Methanol has a low enough dielectric constant that over the con—
centration range used,ion pairs such as

M+. MoOX5=

are probably important.

This behavior suggests the following as the initial mechanism:

 

 

MoOC15= , e MoOCluMeOH- + 01'
+ +
SON“ SON”
fast ‘fast

MoOClBSCN MeOH
or_
01

1:1 complex.

 

A7

 

 

 

 

 

 

 

 

Fig. 1A

Rapid-scanning monochromater traces
Fig. A ’for 1:1 mixture

Fig. B for'l:2 mixture

 

 

 

A8

 

 

 

 

 

 

 

 

 

 

 

 

 

Rapid-scanning monochromater traces
Fig. 0 for l:A Mixture:

Fig. D for 1:10 Mixture

A9

 

 

 

 

 

5—
L; I I l
01 /u 0.2 03
Fig. 15.

‘Variation of ln Kpseudo with the square root of the ionic

strength. Slope obtained is A.l:expected value is 16.

50

 

 

 

 

 

0.01 0.02 0.03 0.04 0.05 0.06 0.07

Concentration of Ligand

Fig. 16

Variation of Kpseudo with ligand gqncentrationat

Theolineois expected to pass throughighe.origin.

51

The rate constant for the formation of the 1:1 complex was not
determined but is estimated to be about 10“ M"1 sec-1.

After this rapid formation of the 1:1 complex the build-up
of the 1:2 complex begins. At very high thiocyanate concen—

trations the plot of K against the ligand concentration,

pseudo
Figure 16, gives a straight line which does not pass through the

origin. This suggests the following scheme:

MoOC13(SCN)MeOH- + SCN' :——+ MoOCl3(SCN): + MeOH

_ k
MoOCl3(SCN)2_ ——l—+ MoOCl2(SCN)2 MeOH + Cl-

k1
or ————+ MoOCl2(SCN)2Cl

The second step will be represented by

k1

A———-—->B

The first step involves the rapid displacement of a solvent
molecule to form the 1:2 complex with an equilibrium constant K.
In the second step an internal rearrangement is postulated in
which the axial thiocyanate ion displaces one of the equatorial
chloride ions. The rate constant for this process is represent-
ed by k1 and can be obtained from the pseudo-first order rate
constant.

It is speculated that at higher thiocyanate concentrations
the 1:2 complex is rapidly formed and thus becomes the starting
species for the build up of the 1:“ and ultimately the 1:5

complex.

52'

For convenience the pseudo-first order rate constant was
determined by the method applicable to the kinetics of formation
of ferric thiocyanate (51, 52). Although this treatment is
only valid for an excess of oomplexing agent, the same treatment
was formally used at low thiocyanate concentrations. Plots of
log (absorbance) versus time were slightly curved but an average
slope was used.

Assuming the mechanistic scheme described above the follow—

ing is derived:

= klA

[Mooc13(SCN)2=l A
K = MoOCl3SCN MeOH‘ SCN‘ = [MoOCl3SCN MeOH-IESCN']

SCN'MeOH-J + B

(LID:
d 03

 

[MOJT = [MoOCl3(SCN):] + [Mooc1

 

3
[MojT - B = A + A
KtscN‘I
_ 1
’ A {1 + K SCN‘ }
[Mo] - B
A = T
1 + 1
KESCN=T

dB Kl {(Mo)T - B}

d_t-

 

1
1 + KISCN']

= le(SCN ){Mo)T — B}

 

1 + K(SCN')

53

dB _
a? - Kpseudo {(MO)T " B}

le(SCN )

 

Hence K =
pseUdO 1 + K(SCN-)

Kpseudo le ‘
= ‘ - K

(SCN') 1 + KESCN-J Obs

At high thiocyanate concentration a plot of

[MoO(SCN)2C13_]w — [MoO(SCN)2Cl3-]

versus time

 

log

[MoO(SCN)2Cl3=]m - [MoO(SCN)2Cl3=]O

do

observed second order rate constant KObs is calculated as follows.

gives a straight line from which KpS' is obtained. The

 

 

K = Kpsuedo
KlK
Since Kobs = —————— _
l + Kl(SCN )
l = l + (SCN )
Kobs KlK Kl
A plot of % against the thiocyanate concentration gives a
obs
straight line whose slope is %— and intercept %—K
1 l '

Figure 17 shows such a plot while Table 5 gives the various

values of K1 and K. Both K1 and K are independent of ionic

strength.

5N

 

X10 ‘

 

 

 

 

l i
2 u 6
x 10"2

Concentration of (SCN_)

Fig. 1?
Plot of—%— vs Ligand Concentration
obs
K = 33.6

Kl = 1.3 x 103 Mole-l Sec-l

[M00012]

0.00A113
0.0030
0.00226

0.0029

55

Table 5

Table of values of kl and K

k
1 ml
in mole-lsec

2.0 . 0.15 x 103
1.3 . 0.10 x 103
1.6 . 0.12 x 103

1.8 i 0.11 X 103

31.88
33.6
29.4

30.2

0+

0.25

56
SUMMARY

The present investigation has established the presence
of two molybdenyl thiocyanate complexes in methanol. These
are the 1:2 and 1:4 Species.

A reaction scheme consistent with the observed reaction

is postulated as follows:

F:
C Cl Cl
very fast

 

 

01' :01 ' NCS
MeOH k-‘bloumoie‘lsec’1 MeOH
1:1 complex, Amax = 309mu
0 o
c H 01 _ K 01 l' 01
01 : NCS Ke30 : 0.8 01 . NCS
MeOH ~ Nos
i1 - K1 10!
01 01
rearrangement 1 :Cmm (or cis)
01-- , NCS 1.9 : 0.7.x 103 ‘CS
NCS Mole-lSec-l MeOH (or 01-)
1:2 complex, Amax = U60mu
0 0
saw N 01 _ H
A‘Illl" + 2 sow -—___A
slow
01 : NCS NCS
MeOH MeOH
1:“ complex, A '= H60mu

max

57

 
   
 

hail

 

 

‘er
‘I

LAJA

 

 

LAAA JAAA
"" 1"Y

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

i‘AJ All AAALJL‘I L‘Al LAJA AAA—AJ LAAA 41..
4'; T r VYVV 'rVY 'VVV VVYY YWVV vvvv YYYT 71*
0 ‘
.-l
. ’ t
-_ L; L 4’ L_ A
m '31.... .. of 1. 17
s 1 I .
' A ..L. _._ A 4‘ .—
. ' ‘ 'Y' vv'w
1. W-
T7
1
I .
O o-
\
1
.
3
1
?
0
AA "__a‘.“..:’..'-

 

 

Picture I
Oscilloscope traces from the stopped flow apparatus.
IA for Mo: SCN- ratio of 1:1

IB for Mo: SCN- ratio of 1:10

58

V Appendix

A Theoretical

Association constants for the complexes. Absorbance measure-
ments have been used to determine the formation constants of . A
complexes (A0, 41, A2). In this study the method due to Hume was

used to calculate these constants and a summary of the theory

 

 

follows.

For a series of complexes the absorbance is a function of the

IF .mu‘jnnx _.

extinction coefficient E, stepwise formation constants K, ligand
concentration X, and the central atom concentration M.
Thus:
_ 2 _ '
A - E(M) + E1K1(M)(X) + E2K1K2(M)(X) t.... (1)
The two highest stepwise formation constants are defined as

(MXn)

K“ - (Mx >(x>‘n
n-m

(2)

 

M ' '
Kn_m = < Xn~m) p (3)
(MXn_m_p)(X)
If all the three species MXn’ MX _ and MXn—m—p absorb then:
A = En(M§a) + En-m(MXn—m) + En-m-p(MXn-m-p) (A)

where A is the absorbance for the mixture of all three species.

With X in large excess of M such that only the highest complex

is formed

59

A0 = EnMo where M0 is the concentration of the central atom

nearly all of which is present in the form of the highest complex

MX .
n

Mt = (MXn) + (MXn_m) + (MXn_m_p) (5)

Manipulation of equations (1), (3) and (A) in (5) gives the

 

 

following:
. _ Ao. _ m
En - M3, (MXn) - Kn<Mxn_m>(x> (6)
From (3)
(MX ) = A — En(MXn)-En_m(MXn_m)
n-m-p E
n-m-p
A — AO-K (MX )(X)m-E (MX ) (7)
= M; n n-m n-m n—m
E
n-m-p

putting these in (5) we obtain

 

 

A m
A - 0 K (MX )(X) -E (MX )
Mt = K (MX )(x>m + (MX ) + ”O n n-m n'm n-m
n n-m n-m E
n-m—p (8)
_ m
Mt(En_m_p) - (En_m_p)(Mxn_m){(Knx +1)} +
AO m
A - M3 Kn(MXn_m)(X) -En_m(MXn_m) (9)
A
_ m 0 m
A-MtEn_m_p (MXn_mNKKnX +l)En_m_p + H; Kn(X) + En_m} (10)
A - Mt En_m_
(mxn_m) = A *p (11)
O m m
En_m + M— Kn(X) — En_m_p(Kn(X) + 1)

6O

 

Similarly A
M (E + —9 K <X)m) - A{l + K (X)m}
(MX ) = t n—m Mo n n (12)
n-m-p
Ao m m
En-m + M; Kn(X) —En-m—p(l + Kn(X) mm

Substitution of (12) and (13) into (3) gives

A _ Mt En-m—
Kn-m - A J3 (13)
o m m p
[Mt(En—m + M; Kn(X) ]—A(l + Kn(X) )X

Rearranging (1A) gives for the case of 3 Species absorbing

 

when X>>1VIt

 

M
_ _£ _ m _ p
A - Kn_m{(Kn(AoMO A)(Xt) A + En-th)}Xt + (En-m-p)Mt (1A)
Y

in which Xt is the total concentration of ligand.

A plot of A versus Y gives a straight line whose slope is
Kn—m and whoses intercept gives En—m-p'

If both (MXn) and (MXn_m) absorb at a particular wave-
length, and En-m-p = O, we obtain for 3 species present with
2 absorbing

Mt m l A p
A = Kn(AoM— - A) Xt - K——- { /Xt } + En-th (15)
o n-m

If only (MXn) absorbs at a particular wavelength, then En-m=0’

En-m-p = O and we obtain for 3 species with only one absorbing:
Mt m
[K (Ao—— - A)X - A]
log’lr = log n MD t (16)
xp A
t
and for only 2 species present with both absorbing:
t m M
A = Kn(AoM— - A)Xt + En_m t (17)

O

.51

To use equation 17 values are assumed for m such that a straight
line results. When A is plotted versus (A Mt/ - A) X m. If a
——————— 0 MO t
wavelength is selected such that only the higher of the two species
is absorbing, En-m becomes zero and Eq. 18 upon rearrangement
gives for ' 2 species, only 1 absorbing, MXn-m-p = O,
= O, E = O

E
n-m-p n-m

AOME - A
1 ( Mo )
og ———_TK_-

= -m log X - log Kn (18)
a plot of the left—hand side versus log X gives a slope of n and
an intercept equal to -log Kn'

In order to obtain the first formation constant it is

necessary that the concentrations of both the molybdenum and

thiocyanate be comparable.

As before A = E(M) + Eq(MXq) (19)
Mt = [M] + (MXq) (20)
x = (X) + q(qu) . (21)

t

 

62.

Solving for Kq as before we obtain

 

 

 

' A - E(Mt)
Kq = A(E«-E) - E (A-E(Mt)) A-E(Mt) q (22)
———q ‘1 Xt—q(————)
E E -E
. 0
By using A0' = EMo, A0" = Eh X0 and substituting
1
59 for E and £9" for EA we obtain for 2 s ecies both
" Mo X0 q p
absorbing and qu+r = O, Eq+r = 0
V I
A-Ao %% A- Ao % ‘
log -w————- = q {logEXt - q —w———-— ] + log Kq (22)
A Mt :
0 KS " A L0 - &.
Xo Mo
' Mt
A-Ao ——
Ao —— - Ag - Ao
XO M3

~using suitablewvalues-for qlgivesva straight line ofislope q
’The intercept of this plot is 10g.K«

7 plot is '

.L '. .' "

l.

\OCDNONU'I

10.
11.
12.
13.
l“.
15.
l6.
17.
18.

19.

20.

63

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