KINETICS AND STEREOCHEMISTRY OF AMMONIATION OF DICHLOROBIS (ETHYLENEDIAMINE) COBALT(III) PERCHLORATE

The rate and stereochemical course of the ammoniation of cis- and trans-[Co(en)2Cl2]·(ClO4) down to [Co(en)2(NH3)2]·(ClO4)3 have been determined photometrically in the temperature range -40 to -55°C. Trans-[Co(en)2Cl2]·(ClO4) and cis-[Co(en)2(NH3)Cl]·(ClO4)2 react under retention of configuration, whereas cis-[Co(en)2Cl2]·(ClO4) gives 20% and trans-[Co(en)2(NH3)Cl]·(ClO4)2 gives 30% retention. These figures closely agree with the ones obtained for the analogous base hydrolysis reactions. The consecutive solvolysis steps can be described by a conjugate-base mechanism. Acid dissociation constants have been determined pH-metrically for trans-[Co(en)2Cl2]·(NO3) (pKa = 7.84 at -65°C) and cis-[Co(en)2(NH3)Cl]·(NO3)2 (pKa = 7.71 at -65°C). Activation parameters of the overall reaction and the rate determining step are discussed and compared to those of hydrolysis. In contrast to the stereochemical course, the reactivity pattern of ammoniation differs from the one found for hydrolysis. These differences seem to be related to structural changes in the second coordination sphere. The higher rate in liquid ammonia is for a large part due to the more favourable heat of solvation of the proton in liquid ammonia compared with water. In agreement with the ammoniation of the pentaammines the exceptionally large reactivity of the N-conjugate base of the en-complexes seems to be mainly due to an entropy effect located in the second coordination sphere (ΔS≠ has a value around 100 J K-1 mol-1). © 1979.