The kinetics and stereochemistry of base hydrolysis of the seven isomers of [Co(dien)(ampy)Cl]²⁺ and [Co(dien)(ibn)Cl]²⁺

The kinetics and stereochemistry for the base catalysed substitution reactions of all seven isomers (4 mer and 3 fac) of both [Co(dien)(ibn)Cl]²⁺ and [Co(dien)(ampy)Cl]²⁺ have been studied in detail, for water and azide ion as entering groups. The stereochemistry for the azide ion anation of some of the [Co(dien)(diamine)OH]²⁺ species have also been investigated. The mer isomers are of comparable reactivity and amongst the fastest reacting pentaaminechlorocobalt(III) complexes known. They are also much faster to hydrolyse than the fac species. In both the ibn and ampy systems, a common product stereochemistry is observed for the four reactant mer isomers (the product is a mixture of all four mer configurations), for both azide ion and water as nucleophiles, but not for the three fac reactants (H₂O as nucleophile). The kinetic and equilibrium distributions are quite different. For the mer isomer reactions, a common trigonal bipyramidal five-coordinate intermediate deprotonated at the sec-NH of the dien is overwhelmingly implicated. The substitution mechanisms are argued in detail. Other data reported include isomerisation rates and equilibrium distributions for some mer-hydroxo and a mer-aqua complex of exceptional reactivity, equilibrium distributions for the mer-phosphato complexes in the ampy system under different pH conditions, the crystal structure for the isolated m1-[Co(dien)(ampy)OP(OH)₃]Cl₃ · 2H₂O species, and a rationale for its predominance at neutral pH based on internal H-bonding.