Indian Journal of ChemistryVol. 16A, April 1978, pp. 333-334

Solvolysis & Substitution Reactions of Some Tin (IV) Complexes InTriethylphosphate

R. C. PAUL, O. D. SHARMA & J. C. BHATIADepartment of Chemistry, Panjab University, Chandigarh 160014

Received 5 August 1977; accepted 6 October 1977

Solvolysis and substitution reactions of (2-CHspy).SnCl., (3-CHsPy)zSnCl., (4-CHspy)zSnCl••(quinoline)zSnCI. and (isoquinoline).SnCl. have been studied in triethylphosphate (TEP). Thekinetic parameters for (I) the replacement of Cl- by triethylphosphate (TEP) and (ii) substitutionof amine molecule by CI- from CHaCOCI in the complexes (3-CH3PY)2SnCI•• (4-CH3Py).SnCla and(iso-C.H7N).SnCI, in TEP have been determined. (2-CHapy).SnCI. and (C,H7N).SnCI. are labilewith respect to solvolysis in TEP. The inductive and steric effects of the amine moleculesappear to influence these reactions which have been proposed to follow a dissociative mecha-nism.

WE have already described! the kinetic behaviourof (pY)2SnCI4 (I) in TEP. We report herea kinetic study of the solvolysis and substi-

tution reactions of (2-CHaPy) 2SnCl4(II) , (3-CHaPY)2SnCl4 (III), (4-CHsPy)zSnCI4 (IV), (C9H7N)2SnCI4(V) and (iso-CgH7NlzSnCl4 (VI) in TEP, where Py= pyridine and C9H7N = quinoline.

Materials and MethodsThe compounds used were of reagent grade

quality and were purified by known methods. Thecomplexes were prepared and analysed as describedearlier". The methods of rate measurements andcalculations of rate and activation parameters werethe same as reported earlier". The infrared spectrawere re~rded on a Perkin-Elmer 621 spectrophoto-meter.

Results and DiscussionThe plots of the conductances of the present

complexes in TEP at infinite time (24 hr],Am •• , versus square-root of the concentrations (inthe range 0·20-0·80 mMl-l) are linear with slopesless than the values calculated theoretically on thebasis of the limiting law. This suggests that theion-pair formation is negligible in the solvent.

Whereas the dilute solutions of the complexes(II) and (V) immediately attain Am values corres-ponding to 1:1 electrolytes, which do not changewith time; conductances of dilute solutions of thecomplexes (III), (IV) and (VI) increase exponen-tially with time. Therefore, complexes (II) and(V) are labile while others are inert with respectto solvolysis in TEP, although the base strengthsof 2-CHsPy and quinoline are of the same order asthose of Py, 3-CH3Py, 4-CH3Py and isoquinoline.The lability of the Sn-Cl bond in complexes (II)and (V) may be due to steric hindrance as a con-sequence of the presence of a C-C linkage at «-positionto nitrogen of the pyridine molecule. The stericfactors also restrain the resulting cations (2-CH3PyhSnCli and (C9H7N)2SnCI; from taking up bulky TEP

molecule into coordination sphere of tin. The farIR spectra of different solutions of complexes (II)and (V) in TEP support the formation of thecations. There is no evidence of the formation oftin-oxygen bond and the P = 0 frequency of TEPremains unchanged.

The AI11a> values for solutions of the complexes(III), (IV) and (VI), determined for two separateruns for each complex, correspond to the value fora uni-univalent electrolyte in TEp3. This suggestssolvolysis of the complex as shown below (Eq. 1).

ks(amine)2SnCI4+TEP -+ [(aminelzSn(TEP)Cla]+ -l-Cl"

... (1)

The presence of the cation in solution has beeninferred from a comparison of the spectrum of thesolution with the spectra of the pure solvent andthe substrate. No significant shift was observedin the positions of vP = 0 and vSn-CI bands in thespectrum of fresh solution. However, in the spectrarecorded 48 hr after mixing of the reactants, theP = 0 stretching vibration of the solvent shiftedto 1183 crrr+. There is a small correspondingincrease in stretching vibrations of P-(O-C) and(P-O)-C groups. Amine ring vibrations shift to620 em"! from 600 crrr+, Two new bands appearat 465 and 530 em"! which are due to Sn-O stretch-ing vibrations''.

The rate of reaction depends on [complex] accord-ing to the rate equation; rate = k, [complex].Apparently the reaction is independent of [TEP] ,yet participation of TEP, which makes solvolysis apseudo-first order process, cannot be disregarded.The first order kinetics may be due to either apurely dissociative mechanism or a bimoleculardisplacement mechanism. Differentiation betweenthese two mechanistic paths is not possible from theobserved order of the reaction because [solvent]remains almost constant during the course ofreaction. The evidence in favour of SNI path isindirect. It has been indicated that in the case of

333

INDIAN ]. CHEM., VOL. 16A, APRIL 1978

:omplexes (II) and (V), the product of the reactionIS (a.min~)2SnCl~ cation which suggests that thereaction IS controlled only by the process involvingdissociation of Sn-Cl bond.

The rates of solvolysis of the complexes are inthe following decreasing order: IV>I1I>I andVI> 1. Whereas the PKa values of amines differby a factor of twenty, the increase in the rate ofsolvolysis for their complexes is only three-fold.!he results are consistent with the deduction thatIncreased base strength of the amine moleculeenhances electron density on tin as a result of whichSn-Cl bond becomes more polar. If considered onthe basis of +1 effect alone, the observed order ofreacti'1t.i~s is in ~ccordance with the expected ordero~ basicities of differently substituted methyl pyri-dines,

The. observed rates are temperature-dependentaccordmg to the Arrhenius equation. The valuesof aH~ (Table 1) are in the following decreasingorder: I> IV> III and I> VI. aHt values decreasewit~ increase in the rates. This also supports theassignment of SNl mechanism and the deductionstha~ bo~h +1 and steric effects of the orientingarmnes Influence the dynamics of reaction. The a9values also support the assigned mechanism.

Substitution of the amine molecules of the com-plexes by Cl" in TEP medium occurs in the presenceof C~3COCI. The examination of IR spectra ofsolutions of the complexes in presence of CH3COCIshows that .vp = 0 of the solvent does not changeand there IS no absorption characteristic of Sn-Ob?nd. However, vSn-CI of the complexes, which is~hsplaced to lower region by about 25 crrr+, appearsm the range expected for pentachloro or hexachloro-stannate ions", Therefore, the probable reaction maybe the replacement of amine molecule in the com-plex by chloride ions resulting in the formation of(amine) SnCl;; or SnCl2jj ions with TEP acting as asolvent only.

CHsCOCI ~ CH3CO+ +Cl- ... (2)

"(amine2SnCI4+Cl- -+ (amine) SnClfi+ amine

CHaCOCl+amine ~ CH3CO+-amine+Cl-

... (3)

... (4)

The exponential increase in conductance withtime for the reactions (2-4), has been followed fordifferent mole ratios of the complex and chlorideion and the rate constant k has been evaluated.While AmID value for complex (I) corresponds tothat for a 1: 2 electrolyte, it is in the range for 1 : 1electrolytes for all other complexes. Thus, the ions(3-CH3Py)SnClii, (4-CHaPy)SnCI; and (iso-C9H7N)-SnCl;; are stable in TEP. The reaction follows arate expression, rate = As+kobs [complex], and islargely insensitive to [chloride ion]. As has beenobtained as the intercept in the plot of kobSversus[Ct-]. However, As values are not of that muchsignificance as the k values for the reaction. Thus,the reaction appears to follow a mechanism in which

334

TAB~E 1 - AVERAGE VALUES OF RATES AND b.Ht AND65. FOR THE SOLVOLYSIS (EQ. 1) AND SUBSTITUTION

(EQ. 3) REACTIONS OF Sn(IV} COMPLEXES IN TEP

Complex ks or k X 105 (see-I) at ARt 65+

SOLVOLYSIS REACTION

(Py).SnCI,(3-MePy}.SnCI,(4-MePy}.SnCI.[isoquinolinej,

seer,

25·6727·9930·9230·23

59·6960·7770·9249·07

163·7190·5124·885·07

34·931·728·624·6

40·830·220·331·6

SUBSTITUTION REACTION

(Py).SnCI,(3-MePy}.S;,Cl,(4-MePy).SnCI.(isoquinolino),

seer,

41·0849·1443·3345·01

91·0282·5976·7689·21

175·09149·5140·0134·4

26·723·924·717·7

17·512·46·3

12·8

Temperature was maintained within ± 0.10•

the rate is co ntrolled by the step involving cleavageof Sn-N bond.

Amine molecules of the substrate behave both asrep!ace~ as well as orienting ligands. Therefore,est I!flahon of the contribution to rate by inductive,conjugate and steric effects is not as simple as inthe solvolysis reaction. The inductive effect isunlikely to' favour the displacement of amine mole-cule. Sin:ilarly, the conjugate effect (well pro-nou?ccd I1l 4-CH3-Py molecule) is likely to goagainst the proposed mechanism. Therefore, thesteric factors appear to control the rate of the re-action. The observed decreasing order of the rates:III> IV> I and VI> I, is consistent with theobservation that the rates are higher for a bulkymolecule. Although the correlation of a9 valueswith the mechanism of a reaction is not simple,yet the observed positive values of ~st may bedue to dissociation of Sn-N bond being the ratecontrolling step. The variation of as: with theamine molecule does not contradict the postulatethat for unimolecular reactions ast is expected tobe independent of the standard state. In fact,.thi~ change in ~St for different complexes indirectlyindicates that electrophilic attack by amine onCH3COCI produces Cl in the system. Under theconditions of the experiment, evaluation of abso-lute entropy of activation for displacement of aminemolecule from the complex is not possible.

References1. PAUL, R. C., KAPILA, V. P. & BHATIA, ]. C., J. inorg:

nucl, Chem., 35 (1973), 833.2. PAUL, R. C., SINGH, M., SHARMA, O. D. & BHATIA, J. C.,

Indian J. Chem., 14A (1976), 379.3. FERRARRO, J. R., Low frequency vibrations of inorganic

coordination compounds (Plenum Press, New York),1971, 122.