Coordination chemistry of chromium (III) ion containing bidentate, tridentate and quadridentate organic ligands and kinetics of ligand substitution reactions of some haloamine complexes

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Indian Journal of Chemistry Vol. 34A, January 1995, pp. 31-37

Coordination chemistry of chromium (ill) ion containing bidentate, trident- ate and quadridentate organic ligands and kinetics of ligand substitution

reactions of some haloamine complexes

S C Pal&M T H Tarafder"

Department of Chemistry, University of Rajshahi, Rajshahi 6205, Bangladesh Received 5 April 1994;.revised 27 June 1994; accepted 18 July 1994

A number of new dichloro, trichloro, thiocyanato and azido complexes of chromium(III) using diethylenetriamine (det), triethylenetetramine (tet),N,N, bis 2-aminoethyl 1,3-propanediamine (2,3,2 tet) and o-phenylenediamine have been synthesised and characterized by elemental analyses, magnetic and conductivity measurements, IR and electronic spectral studies.The complexes have the com- positions, [Cr(det)X^3] and [Cr(L)X²]X; [X=Cl, NCS or N^3; L=tet, 2,3,2-tet or o-phenylenediamine].

The molar conductance data reveal that most of the dichloro chromium complexes and the thiocyanato complex with o-phenylenediamine are 1:1 electrolytes in DMSO while those of det, tet and 2,3,2-tet are nonelectrolytes but the azide complexes are found to be insoluble in almost all common organic solvents. Magnetic and spectral data support octahedral stereochemistry for the complexes.

Kinetics of aquation of chloro complexes of Cr(III) with 1,3-diaminopropane, diethylenetriamine (det) and N,N-bis 2-aminoethyl-1,3-propanediamine (2,3,2-tet) have been investigated. This reveals that chelate effect of ligands as well as metal-ligand ring size in the complexes governs the rates of aquation. The first and second aquation rate constant values decrease with increase in derived chelate effect of the ligands. The chelate effect of the ligands used increase in the order 1,3-diaminopropane <det<2,3,2 tet.

There have been a great deal of interest in the synthetic!" aspect of Cr(III) complexes of macrocycles. A number of reports of complexes of different metals involving these ligands have appeared recently?'!". Kinetics and mechanism of aquation of dihalo complexes of different metals involving macrocylic ligands have also been the subject of interest in the recent years!':". This type of kinetic studies is of importance in under- standing the model synthesis similar to biological process, such as, dephosphylation of ATP19.21.

Studies indicate that the unique structural char- acter of macrocycles can be related to physical and chemical properties of their complexes.". Li- gand field strength of secondary amine donors are largely affected by the change in the ring size of macrocycles+". A strain energy model has been developed by some workers for the complexes involving macrocyclic Iigands+-".

There have been no reports on complexation of dichloro-chloro, isothiocyanato and azido substi- tuted Cr(III) with diethylenetriamine (det), triethylenetetramine (tet), o-phenylenediamine and N,N-

~is-2-aminoethyl 1,3-propanediamine (2,3,2-tet) hgands. Kinetic data showing the dependence of

chelate effect on the rates of acid catalysed aquation of such type of dichloro complexes have not been reported. We thus report herein the synthetic aspect of the aforesaid complexes as well as kinetics and mechanism of acid-catalysed aquation of some of the above dichloro complexes in- cluding the dichloro complexes containing 1,3-diaminopropane, synthesis of which has been reported in one of our earlier papers35. The present study also includes the relation between mechanism of aquation and chelate effect and metal-ligand ring size.

Materials and Methods

IR spectra (as KBr discs) were recorded with a Pye-Unicam SP3-300 IR spectrophotometer. Con- ductivities of 10 -³ M solutions of the complexes in dimethyl sulphoxide (DMSO) were measured at 30°C using a WPA CM 35 conductivity meter and a dip type cell with platinized electrodes.

Electronic spectra were recorded with shimadzu

UV-160 UV-visible spectrophotometer. Magnetic susceptibility measurements were made on a Johnson-Matthey magnetic susceptibility balance.

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32 INDIAN J CHEM. SEe. A, JANUARY 1995

All chemicals were reagent grade and were used as supplied by E. Merck except for ethanol which was purified by distilling the crude 99%

ethanol with magnesium turnings and-iodine.

Carbon, hydrogen and nitrogen analyses were carried out by the Microanalytical services at the University of St. Andrews, Scotland and Sophisti- cated Instrumentation Centre, Central Drug Re- search Institute, Lucknow.

Preparation of the complexes

Preparation of [O(det)Cl^3] (1 )-CrCI3 '6H²0 (0.01 mol) was heated in dimethylformamide (DMF) (50 cm ') at 70°C for about an hour with continuous stirring. To this hot dark green solution a solution of diethylenetriamine (det) (0.01 mol) in ethanol (15 em") was added. The resultant bluish purple product was filtered, washed with cold ethanol and finally dried in vacuo over fused CaCI^2•

General method of preparation of [CrLClz]Cl [L= NHz - CHz - CHz - NH- CHz - CHz - NH- CHz - CHz - NHz (tet), NHz - CHz - CHz - NH- CHz CHz - CHz - NH - CEz - CHz - NHz (2,3,2 tet) (2 and 3)-To a hot solution of CrClz'6HzO (0.005 mol) in ethanol (30 ern"), a solution of the tetramine ligand (0.005 mol) in the same solvent (15 cm ') was added with stirring for about 10 mi- nutes. The resultant purple product was separated, washed successively with cold ethanol and stored as above.

Preparation of [O(C⁶H⁴(NHz)z)zClz]Cl'2HzO (4)-An ethanolic solution of CrCI³'6HzO (O.005mol) (25 ern") and that of o-phenylenediamine (0.01 mol, 75 ern") were refluxed for about 2 hours. The volume of the mixture was then re- duced to 50% by evaporation and left in a refrigerator for overnight. It was- then titurated with ether when brownish black product appeared.

The product was separated, washed successively with ether and dried in vacuoover ^P401O'

Preparation of [O(detXNCS)3] (5)-A solution of Cr(N03k9HzO (0.005 mol) in ethanol (30 crrr') was mixed to a solution of KSCN (0.015 mol) in the same solvent (40 cm-) when a white precipitate separated out. This was filtered off. To the pink coloured filtrate an ethanolic solution of diethylenetriamine (det) (0.005 mol, 10 ern") was added when a bluish product obtained. The product was then separated, washed with ethanol and .stored as above.

General method of preparation of [CrI1..NCSb]

NCS-xHzqL= NHz - CHz - NH- CHz - CHz- NH- CHz - CHz - NHz (tet), NH2 - CH2 - CH2- NH- CH2 - CHz - CHz - CHz - NH- CHz - CHz

- NHz (2,3,2 tet)](6 and 7)-To an ethanolic solution of Cr(N03k9HzO (0.005 mol) (30 em") a solution of KSCN (0.015 mol) in the same solvent (40 cm '] was added when white precipitate of potassium nitrate was formed. It was filtered off. To the pink coloured filtrate an ethanolic solution of the ligand (0.005 mol) (60 cm ') was added. The resultant bluish pink product was filtered, washed with ethanol and dried in vacuo over P⁴0^IO•

Preparation of ^[Cr(C⁶H⁴(NH2lzlz(NCSlz]NC5 (8)-To the ethanolic solution of Cr(N0³k9HzO (0.005 mol) (30 ern") a solution of KSCN (0.015 mol) in the same solvent (40 cm ') was added when white precipitate of potassium nitrate separated out. The precipitate was filtered off. An ethanolic solution of o-phenylenediamine (0.01 mol, 60 cm ') was added to the filtrate and left over night in the refrigerator. Cold water was then added in large excess when greyish black product appeared. The product was then filtered, washed with cold ethanol and dried in vacuoover ^P401O'

Preparation of [Cr(det)(N.')3](9)- The complex 1 (0.2 g) was suspended in water (50 crn ') and was heated on a water bath for an hour when a bluish purple turbid solution was obtained. To this solution an aqueous solution of sodium azide (0.5 g) (10 cm ') was added and the mixture was heated for an hour at - 7 5°C with constant stirring. A blue product gradually developed. This was left overnight at room temperature. It was then filtered, washed with cold water and dried in vac- uo and stored as above. The m.p. of the complex was> 235°C and yield 68%.

Preparation of[CrI1..N³lz]N³[L= NHz- CHz- CH2 - NH- CHz - CHz - NH- CHz - CHz - NHz (tet), NHz - CH2 - CHz - NH- CHz - CHz - CHz - NH

- CHz - CH2 - NH] (2,3,2 tet)](10 and 11)- The complex 2 or 3 (0.2 g) was suspended in water (50 crrr') and was heated on a water bath for half an hour when a pink solution was obtained. To this solution an aqueous solution of sodium azide (0.6 g, 10 cm '] was added and heated for 45 mi- nutes at - 90°C with constant stirring. The blue product that gradually developed during heating was left at room temperature for overnight and dried in vacuo and stored as above. The m.p. of the complex was> 235°C and yield around 50%_

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PAL et al.:STUDY OF Cr(III)-liALOAMINE COMPLEXES 33

Table I-Analytical data and colour of the complexes

Carbon(%) Hydrogen (%) Nitrogen (%)

Compound Colour Calc. Found Calc. Found Calc. Found

[Cr(det)C131(1) Bluish purple lS.4 lS.l 5.0 4.9 16.1 15.9

[Cr(tet)C12]Cl (2) Purple 23.6 23.5 5.9 5.7 lS.4 18.3

[Cr(2,3,2-tet)C121Cl (3) Purple 26.4 26.3 6.3 6.2 17.6 17.6

[Cr(C6H4(NH2hhCI21Cl'2HP(4) Dark brown 35.1 34.9 4.9 4.7 13.6 13.4

[Cr(det)(NCShl (5) Pale blue 25.3 25.1 3.9 3.8 25.5 25.3

[Cr(tet)(NCShJNCS'4H20 (6) Bluish pink 19.7 19.6 5.4 5.2 17.2 17.1

[Cr(2,3,2-tet)(NCShlNCS (7) Bluish pink 31.1 31.0 5.2 5.2 25.4 25.4

[Cr(C6H4(NH2hh(NCShlNCS (8) Dark grey 40.7 40.6 3.6 3.5 22.2 22.2

[Cr(det)(N3hl (9) Dark blue 17.1 17.0 4.6 4.6 59.8 59.7

[Cr(tetXN3hJN3 (10) Dark blue 22.2 22.1 5.6 5.5 56.2 56.0

[Cr(2,3,2-tet)(N3hlN3 (11) Dark blue 24.9 24.7 5.9 5.9 53.9 53.8

Table 2-Physical properties of the complexes

Compound Molar Conductance "M Icorrl X 106 P.rr Electronic spectral (Q-I crn-mol : ') (C.G.S.) (B.M.) bandmaxima(nm)

1 10.0 5908.4 3.77 536,341,300,243

2 25.2 5748.4 3.72 605,340,300,246

3 27.0 6947.7 4.09 536,341,300,243

4 2S.5 5522.5 3.64

5 5.0 5891.6 3.76 550,400,300,246

6 8.0 6077.8 3.82 550,411,311,247

7 7.5 6620.9 3.99 550,340,300,243

8 30.0 5416.6 3.61

9 6063.9 3.82

10 6548.5 3.96 580,341,297,246

11 6875.5 4.07

Kinetic studies

All the aquation kinetic measurements on the above complexes were made spectrophotometri- cally using a LKB Biochrom Ultrospec 4053 at appropriate wave lengths in aqueous HCI medium over a concentration range 0.177-0.886 mol dm^"?

at 30°C and constant ionic strength, 1=1.0 mol dm -^3. The ionic strength was maintained with the help of NaCI. In all the kinetic runs 1 m mol of each of the complexes were used. For this, weighed quantities of the complexes were added quickly to the acid solution and was stirred vigor- ously for .dissolution. The clear portion of the solution was transferred to a 1 em. quartz cell where the rate of aquation was monitored spec- trophotometrically with time. Chloride titration was done by Mohr method using K²Cr0⁴ as an indicator.

RemJlts andDiscussion

Dichloro- and trichloro-complexes of Cr(III)

have been prepared from CrCI³·6H²0 containing diethylenetriamine (det), triethylenetetramine (tet), N,N, bis diaminoethyl 1,3-propanediamine (2,3,2 tet) and o-benylenediamine as colligands. Using the above complexes some azido complexes have been isolated by metathesis reaction", Some thiocyanato complexes have also been synthesized and characterized.

The structures of the complexes have been for- mulated on the basis of elemental analyses and conductance measurements (Tables 1 and 2).

These have been supported by the magnetic IR and electronic spectral data (Table 2). All of the dichloro-chloro complexes except o-phenylenediamine are sparingly soluble in dimethyl sulphoxide (DMSO) and show molar conductance values of 25-36 ohm -\ em- at 30°C consistent with 1:1 electrolyte". Trichloro complex containing diethylenetriamine (det) is apparently nonelectrolyte giv- ing conductance value of 10 ohm^-I em". Con- ductance measurements on thiocyanato complexes

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34 INDI~N J CHEM. SEe. A, JANUARY 1995

Table 3-Values of k, and k2 for the aquation ot· Cr complexes in HCI3 medium at 30°C and at constant ionic

strength, J= 1.0 mol dm?

Wavelength [HCI) 102k,(s -') 102k2(s -') (nm) (mol dm-3)

[Cr( 1,3-diaminopropanehCI2)CI

410 0.886 0.94 2.81

0.708 0.75 2.33

0.531 0.42 1.05

0.354 0.34 0.75

0.177 0.22 0.39

341

[Cr(det)C13]

0.886 0.708 0.531 0.354 0.177

1.50 1.13 0.71 0.53 0.33 0.31

0.25 0.20 0.11 0.048

341

[Cr(2,3,2 tet)C12]CI

0.886 0.25

0.708 0.19

0.531 0.11

0.354 0.10

0.177 0.08

~56 0.42 0.28 0.18 0.12

in DMSO, in which they are sparingly soluble, exhibit conductance value of 1:1 electrolytes only for the complex with o-phenylenediamine whereas those containing det, tet and 2,3,2 tet are none- -, lectrolytes. Conductance measurements on azido complexes could not be made because they are insoluble in almost all organic solvents.

Infrared spectra

Infrared spectral data of the complexes are shown in the Table 3. Complexes 1-3, 5-7 and 9- 11 show ^v(NH²⁾ and ^v(NH) stretching modes^4•9•29 at 3080-3240 cm-Î and 2780-2920 cm-Î respectively lower than the free triamine (3280- 3385 cm^"! and 2860-2940 cm-Î) and tetramine (3200-3385 cm^"! and 2820-2940'cm-^l) ligand values. These shifts clearly support the amino nitrogen coordination to Crill in the complexesv/-".

Complexes 4 and 8 exhibit v(NH2) stretching modes^4•9,29 at 2900-3150 em^-I lower than the free o-phenylenediamine ligand values (3040- 3400 em -I).This shift towards the lower frequencies clearly suggests the amino nitrogen coordination. This is further evident by the appearance of

v(Cr-N) modes in the region 420-570 cm-^I in the far IR spectra of all the complexes ^1ll.29.30.

-6-0L.-__ -L_--~----L-....J

o 2 "

Timc(min)

6

Fig. 1- Plot of In(A, - A.,) versus time for aquation of [Cr(I,3-diaminopropanehCI2]CI in aqueous HCI medium

(C=0.531 mol dm+') at 30°C and J= 1.0 mol dm-3 (NaCl).

A.=41O nm.

-2·0

~ -3-0 -e

I

<t

'£ -4·0

-50

- 60 '--- '--, --'-I --'- ..l

o ² ⁴ ⁶ ⁷

Time (rnin)

Fig. 2 - Plot of In(A, - A., ) versus time for aquation of [Cr(det)CI3] in aqueous HCI medium (C~0.886 mol dm-3) at

30°C and 1=1.0 mol dm"?(NaCl). A.=341 nm.

The thiocyanato complexes, 5-8 display v(CN) stretching mode at 2070-2080 cm -^I characteristic of N-bonded thiocyanato moiety, thereby sup- porting that the metal ion behaves as hard acid centre^30.31.32. The Cr- NCS bonding is further supported by the appearance of v(CS) modes at

higher frequencies, 1038-1125 cm-^I in the complexes.

The azido complexes 9-11 show absorption at 2080-2090 em -Î and at 1315-1340 em -Î arising from the vâsys (NNN) and v^sys (NNN) modes, characteristic of coordinated and ionic azides? respectively.

Compounds 4 and 6 display bands at - 3400 em -^Icharacteristic of lattice water molecules.

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PALet al.:STUDY OF Cr(llI)-HAWAMINE COMPLEXES 35

3.0 _,,,-,

,

, '\,,/diaquo .peei ••

I I

1.0

-

- -

-

~2.0

• e

a_o

...

•

C

Trichloro .peei ••

-.•.

----

0~20~0~----~3~0~0---4~0~0~--- )..(nm)

Fig. 3-A portion of the UV-visible spectra of [Cr(det)CI3]

before and after aquation. Solid line ( ) and broken line(...)indicate trichloro and diaquo species respectively.

Magnetic measurements and the electronic spectra Magnetic measurements and the electronic spectra of the complexes were also obtained in order to have information on their probable geometries. Magnetic measurements at 25°C indicate that all the complexes, 1-11 are paramag- netic (,ueff =3.61-4.09 B.M.) corresponding to three unpaired electrons. Magnetic measurements support O, symmetry of the molecules".

Structural information is further supported from d-d spectral bands in the complexes (Table 2). Complexes 1-3, 5-7 and 10 exhibit three spin allowed transitions 4A2g _4 Tzg (550-605 nm), 4A2g_4~JF) (340-400 nm) and 4A2g_4~g(P) (297-300 nm). These are all consistent with O, symmetry of chromium(IIl) complexes<". The fourth band around 240-247 nm is probably caused by charge transfer.

K inetic studies

The kinetics of aquation reaction of the complexes [Cr( 1,3-diaminopropane hCl1]CI, [Cr(det)CI3]

and [Cr(2,3,2 tet)Cl2]Cl have been studied at 410 nm, 341 nm and 341 nm, respectively. The synthetic and structural aspects of the complex

[Cr(1,3 diamino propanel.Cl.K'l has been report-

ed". In general, the overall aquation reaction of the complexes can be represented by Eqs (1 )-(3).

[Cr(det)CI,] ~HO [Cr(det)CI(H~OhF + + 2Cl- ... (2)

"

I I

, ,

,

3.0 I I

,

\'\,/ dioquo .p.el ••

, V

d/chloro .peel ••

I

• 2.0

uc

...

a

li

• ...

C 1.0

\,

o~2~00~---~3~0~0---~4~0~0---

A (nm)

Fig. 4-A portion of the UV-visible spectra of [Cr(2,3,2 tet)ClzJ+before and after aquation. Solid line ( )and broken line (...)indicate dichloro and diaquo species respec-

tively.

[Cr(2,3,2 tet)Clzr H2

0.

[Cr(2,3,2 tet)(H2

0hF+

+2CI- ... (3)

The aquation of these complexes proceed in two steps, each involving a displacement of one chloride by a neutral water molecule. These steps are two consecutive irreversible first order reactions" represented by

... (4) where X,Y,Z are original dichloro, monoaquo and diaquo complexes respectively.

First order rate equation (5) was used to deter- mine the rate constants k, and k2 for the first and second steps solvolytic displacement of coordinated chlorides, viz.

In(AI-Aoo)=ln(A()-A",)-kl ... (5) Figures 1 and 2 show representative plots of In(AI - Aoo) versus t for acid hydrolysis of the complexes. The first order rate constants k^I and k2 for the initial and final steps of aquation were obtained from the corresponding linear portions of the curves. The rate constants k, and kl are summarised in Table 3.

With the increase in donor capacity of the ligands, the entropy of the system increases thus enhancing the stability of the resultant complexes.

This chelate effect stabilizing the chloro moiety affects its substitution by water molecules. The chelation increases in the order 1,3-diaminopropane

<

detx 2,3,2-tet, thus explaining the order of

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36 INDIAN J CHEM. SEe. A,JANUARY 1995

CI

For N~N

l,3diaminopropan. "'-.I/' -CI- and 2.3.2It I /' , •.••..•..-

N~N CI Originalcompound

N~N C.

N~N

/i~

, H20

First aquotlon product

l'~:1

^N_Transition _stcte^N

H20

N....--N1 <,N

'" / -CI-

For d.t Cr •

(1/\

^"'CI

CI

Original c emplex

N....---N----N

Hp '-....1/

•(1./Cr1'-....CI

H20

First oquation product

[ _N",I/N....--N<,+

1 /""

c.

(I (I

Transition stat.

Scheme 1

rate constants (k^l and k2) for the aquation of these complexes. The difference in the rate constant values for the first and second aquation steps in these complexes may presumably be related to the difference in the activation energies involved in those two steps.

The lability of the haloamine species is believed to be related in part, to the flexibility of the metal-amine rings. Complexes with 1,3-diaminopropane contains two six membered amine rings, whereas complex with det contains two condensed five membered amine rings and that with 2,3,2 tet contains three condensed rings of which one is a six and other two are five membered ones. Excepting det, all other complexes contain two axial chloride ligands in the trans positions. In the case of det all the three chloride ligands occupy the cis positions. The flexibility of the complexes probably follows the previous order of chelations. Such flexibility may be responsible for the ease of formation of the transition state by a mechanism which is primarily dissociative in na- turc" .

Since mechanism of aquation of haloamine Cr(Ill) complexes is essentially Sr'!I, we can pre- dict that the electronic effects play an important role in changing the rates of aquation as the ring size changes. The rates should be greater for the complexes having smallest ring size and the least for the complex with largest rings.". Our results

are consistent with the mechanism. Again the electronic (cis) effect in the complex containing det may play a secondary role in the rates of aquation of the complexes.

In our studies the rates of aquation reactions of the complexes can be correlated with the strain energies of the chelated ligands in the starting complexes and of the monoaquo complexes. The largest source of strain energies arises from the bond angle stretching and torsional deformation":

The strain energies are probably relieved largely in the transition state (activated complex) and we believe that its magnitude maintains the order as was found for rate constants.

The transition state for the substitution reaction at chromium (III) in the first aquation step is presumably a five coordinate species. The exact con- formation of the complex is not retained and the remaining chloride/chlorides exerts its steric force on one side of the planar rings. This is shown in the.following Scheme 1.

The stepwise removal of chlorine from the complexes by water molecules is further supported by the existence of two different slopes in the plots of In(A,- Aao) versus time for the complexes in HN0³ medium. Chloride titration also supports the replacement of chloride from the complexes by the water molecules. Figures 3 and 4 which show comparative UV-visible spectra of chloro and diaquo complexes under investigation further support the above statement.

Acknowledgement

We are grateful to Prof. R.W. Hay, Department of Chemistry, University of St. Andrews, Scotland for providing us with C, Hand N analytical data.

Financial assistance provided by the Bangladesh University Grants Commission, Dhaka and that from the Department of Chemistry, Rajshahi Uni- versity is solemnly acknowledged.

References

I House D A, Hay R W & Ali M A, Inorg chirn Acta, 72 (1983)239.

2 Martin L Y, Hettayes L J, Zompa L J & Busch D H, J A m chem Soc.96(1974) 4046.

3 Erikssen J & Monsted O. Acta chem Scand, Ser, A 37 (1983) 579.

4 Sperati C R, Ph.D. Thesis, Ohio State University, 1971;

Diss Abstr, 32B (1972) 6282. Chern Abstr, 77 (1972) 107189 K; House D A & Hay R W, Inorg chirn Acta, 54 (1981)Ll45.

5 Swisher R G, Brown G A, Smierciak R C & Blina E L, Inorg Chern.20 (1982) 394.

o Samuels G I & Espenson J H. lnorg Chern, 19 (1980) 233.

7 Poon C K&Pun K C,Inorg Chern, 19 (1980) 568.

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PALet al.:STUDY OF Cr(I1I)-HALOAMINE COMPLEXES 37

8 FergusonJ &Tobe N L,Inorg chim Acta, 4 (1970) 109.

9 Hay R W & Tarafder M T H, J chem Soc, Dalton Trans, (1991) 823.

10 Westland AD &Tarafder M T H, Inorg Chern, 21 (1982) 3228.

11 Massoud S S&Milburn R M,Polyhedron, 8 (1989) 275.

12 Martha E S & Tobe M L, J chem Soc Dalton Trans, (1986)427.

13 David T R, Kafi Adzamoh I, Peter Leupin & Geoffery Sykes, Inorg Chem, 23 (1984) 3065.

14 Yann Hunn & Daryle H Busch, J Am chem Sac, 99 (1977) 20.

15 Poon CK &Tobe M L,Coord chem Rev,1 (1968) 81.

16 Poon C K,Coord chem Rev, 10 (1973) 1.

17 Hay R W & Lawrence G A, J chem Soc Dalton Trans, (1975) 1556.

18 Poon C K&Tobe M L,J chem Soc A,(1968) 1549.

19 Tafesse F & Milburn R M, Inorg chim Acta, 135 (1987) 119.

20 Tafasse F, Massoud S S'&Milburn R M, Inorg Chern, 24 (1985)2591.

21 Rawji G, Hediger M & Milburn R M, Inorg chim Acta, 79 (1983) 247.

22 Hung Y, Martin L Y, Jackesis S C, Tait A M &Busch D H,JAm chem Soc,99, 4029 (1977).

23 Martin L Y, De Hayes L J, Zompa L J & Busch D H, J Am chem Soc,96 (1974) 4046.

24 Martin L Y, Sperati C R & Busch H, J Am chem Soc, 99 (1977) 2968.

25 Walkins D D, Riley D P, Stone J ~ & Busch D H, Inorg Chern,15 (1976) 387.

26 Warner L G & Busch D H, Coord chem Proc, John C Bailer,JrSymp,1 (1969).

27 Poon C K&Busch D H,Inorg Chem, 12 (1973) 2010.

28 Geary W J, Coord chem Rev,7 (1971) 81.

29 Tarafder M T H &Islam A A M A, Polyhedron, 8 (1989) 109.

30 Nakamoto K, Infrared spectra of inorganic and coordina- tion compounds, 2nd Edn, (John Wiley, New York), (1970) p 155.

31 Basolo F, Baddley W H &Burmeister J L, Inorg Chem, 3 (1964) 1202.

32 Lauer J L, Perkin M E, Burmeister J L, Johnson K A &

LimJ C,Inorg Chern,11 (1972) 907.

33 Lever A B P, Inorganic electronic spectroscopy, Chapter 6 (Elsevier, Amsterdam), 1964.

34 Figgis B N, Introduction to ligand fields, (John Wiley, New York) 1966.

35 Tarafder M T H&Pal S C,J Indian chem Soc,(in press).

36 Moore J W & Pearson R G, Kinetics and mechanisms, 3rd Edn (John Wiley, New York) 1981, p. 290.

37 Jonasson J R, Murray R S, Stranks D ~ & Yandell J K, Proc 12th Int Conf Coord Chern, (Sydney, Australia), (1966), p. 32.