Rates of isotopic ligand exchange of Ni(II) complexes of α-amineoximes

Abstract

The kinetics of the ligand exchange of [Ni(AO) 2-H] +, [Ni(MeAO) 2H] + and [Ni(EnAO)H] + have been investigated using C 14 labeled ligands. It was found that the exchange for the series follows the general law: R = k 1[complex] T + k 2[complex] T [ligand] F . The overall rates are in the order AO > MeAO ⩾ EnAO. With [Ni(AO) 2H] + the exchange is rapid and carried predominately by the ligand dependent, k 2 term. With [Ni(MeAO) 2H] + at constant [MeAO] T, R shows a minimum in the region of pH = 8. In the basic region the faster rate is due to increased free ligand concentration while the acidic catalysis is interpreted on the basis of a half-bonded intermediate. For [Ni(EnAO)H] + the exchange is carried by the k 1 term with k 2 being small. The order of the ligand dependent step, k 2, is AO > MeAO > EnAO, and is due to steric hindrance in the formation of a five co-ordinate activated state. This is supported by the catalysis of exchange by foreign chelating ions or molecules. These results are compared with the kinetics of reaction with EDTA and a mechanistic picture involving simple dissociation and a CN = 5 activated state is postulated and discussed.