Publication details

Hydrolysis of chlorido complexes of d8 metals: Old models, new facts



Year of publication 2019
Type Article in Periodical
Magazine / Source Inorganica Chimica Acta
MU Faculty or unit

Faculty of Science

Keywords Square-planar substitution; Hydrolysis; Aquation kinetics; Charge-separation effect; Cisplatin; d8 complexes
Description The discovery of the antitumor activity of cisplatin in 1969 has accelerated research in platinum chemistry. In particular, cisplatin hydrolysis, the rate-determining process activating the drug for reaction with DNA, was extensively studied and discussions of the mechanism of square-planar substitutions were rekindled. One intriguing aspect of cisplatin hydrolysis is the fact that the rates of the two aquation steps are almost identical, in spite of the charge-separation effect which would be expected to slow down the second step. This result is analyzed here in the context of the more general observation made by Martin et al. that aquation of chloridoammine complexes of Pt(II) proceeds at similar rates, independently of complex charge. Ancient hypotheses put forward to explain this observation are confronted with recent data. The discussion is extended to aquation of chlorido-am(m)ine complexes of Pd(II) and Au(III), and to chloride substitutions by thiourea and pyridine. It is shown that the charge independence is not limited to chloride substitution by water. This new correlation of published data invalidates our recent hypothesis according to which non-conventional hydrogen bonding from the attacking water molecule to the Pt(II) center as acceptor counterbalances the charge separation effect. Recent data also show that the frequently used kinetic model for square-planar substitutions featuring a metastable five-coordinate intermediate does not apply to complexes involving only simple o-donating ligands such as amines, water, or chloride. By inference, the Martin-Basolo model, explaining the charge-independence of aquation rate constants with the mutual compensation between charge separation and charge neutralization in the transition state, remains the only model compatible with available data.

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