Kinetics of basic hydrolysis shows that the cis(N^*) mer-isomers (m1 or m2) are about 100-fold more reactive than the trans(N^*) forms (m3 or m4) due to steric strain caused by the pyridyl residue in the amp. No facial isomers have been found in the experiments. Detailed stereochemical studies using progressive NMR spectroscopy reveal that all mer reactants give a common product distribution in the basic hydrolysis reaction. Four substituted mer-isomers have been observed. Rearrangements of the isomers in DMSO at 100℃ show that the m1-Cl complex is the most reactive one, the cis(N^*) isomers (m1 and m2) yield m3-Cl complex first. A subsequent equilibrium between chloro species of m3 and m4 then takes place. Experiments of deuteration of the active protons in each isomer indicate that reactivity of an isomer is not proportional to the H-D exchange ratio which is observed before hydrolysis and rearrangement of the isomer. Ab initio computational results (RHF/LANL2DZ optimised structure) are well consistent with the crystal structural parameters of four geometric mer-isomers of [Co(2, 3-tri)(amp)Cl]ZnCl~4 with an maximum error of ±3%. The cis-mer isomers (m1 and m2) are calculated to be about 4 kJ/mol less stable than the trans-mer isomers (m3 and m4). However, the facial isomers are calculated to be at least 17 kJ/mol less stable than the mer isomers. Each isomer are calculated to be at least 5 kJ/mol more stable when the effect of solvent (water) is included using the Onsager model within the solvent continuum reaction field method. The distortions for the different isomers from the idealised octahedron are correlated with reactivity. The more severely distorted species is predicted to be more reactive.

Key words: COBALT COMPLEX, ISOMER, REARRANGEMENT REACTION, SUBSTITUTION REACTION, AB INITIO CALCULATION, P, STRUCTURE ACTIVITY RELATIONSHIP