The cis-trans product ratio for the base-catalyzed solvolysis of the 15NH3 labelled
~~~~S -[ C O ( N H , ) , ( ~~N H ~) C I ] ~+ion has been determined as a function of the solvent. The solvents used were H20-CH30H and H20-dmso (dimethyl sulphoxide) mixtures, as well as anhydrous CH30H and CH3NH2. The trans-[C0(NH,),(~5NH~)(drnso)]3+ ion was included also in the study. The base hydrolysis results do not show any solvent or leaving-group dependence, within experimental error (2%). All trans systems give (44 f 1)% rearrangement. Small, significant differences are shown by the penta-amminechloro-complex in CH3NH2 (50% rearrangement) and by the dmso complex (48% rearrangement in H20 and 46% in CH30H). The results are interpreted as indicating the adequacy of a mechanistic model involving a five-co-ordinate intermediate.Stereochemical change in base hydrolysis is still to a large extent an unsolved problem in the conjugate base (c.b.) mechanism. This mechanism generally consists of a fast proton transfer pre-equilibrium, followed by the rate-determining dissociative elimination of the leaving g ~o u p . ~J Early stereochemical theories assumed the five-co-ordinate intermediate, inherent in the dissociative activation, to have a fixed geometry. This idea was combined with that of a stereospecific entry of the incoming ligand, directed by the amido-group.4Vs Recent results on the ' red ' s and p (secondary and primary) isomers of the [CoL(NH3)XI3 + I 2 + ion [L = 2,2'-diamino-2"methylaminotriethylamine, X = C1, Br, N3, or NH3], bearing on the lifetime of the intermediate,'jn7 have been interpreted as indicating that the five-co-ordinate species do not relax towards a common trigonal-bipyramidal structure, although this process would require only a small change in bond angle. The resulting more dynamic picture is the immediate capture of a solvent molecule, before bond angles can equilibrate.If indeed the process of capture of the entering group by the five-co-ordinate species competes in rate with angle equilibration and solvent structure rea~rangement,~.~ relative rates and consequently product stereochemistry should be affected by the nature of the solvent. This conclusion does not seem consistent with the fact that experiments in our laboratory revealed a close agreement in product stereochemistry for the base-catalyzed reactions of [ c ~( e n ) ~X Y ] ~+ ' + (en = ethylenediamine; X = C1, Br, or dmso; Y = Cl, NCS, NO2, N3, or NH3) complexes in liquid ammonia and in aqueous solut i ~n . ~* ~ In this report we investigate the solvent dependence in the base-catalyzed solvolysis reaction of tran~-[Co(NH~)~-(15NH3)X]3+/2+ (X = dmso or C1) ions, a reaction for which a fixed five-co-ordinate intermediate seems to explain aqueous stereochemical results well.