MOLECULAR MECHANICAL AND QUANTUM-CHEMICAL STUDY ON THE SPECIES INVOLVED IN THE HYDROLYSIS OF CIS-DIAMMINEDICHLOROPLATINUM(II) AND SUBSTITUTED BIS(ETHYLENEDIAMINE)DICHLOROPLATINUM(II) COMPLEXES .1. REACTANTS AND PRODUCTS

Cisplatin and its substituted ethylenediamine derivatives, cis-PtCl2(R 2en) (en = ethylenediamine, R = H, Ph (phenyl), 2-, 3- and 4-PhOH) hav e been studied with respect to the first step of their hydrolysis reac tion. The geometry of the reactants and products was determined by mol ecular mechanics (MM). The MM optimized structures were used to calcul ate by the extended Huckel method the charge distribution and relative electronic energies. The MM and EH calculations were carried out with different ligand conformations. Due to increased non-bonded repulsion , with increasing ligand bulkiness, the square planar arrangement is t he preferred geometry also by the MM results. This additional (to the electronic) stabilization of the square planar arrangement around Pt(I I) is unfavorable for the aquation process. The thermodynamic stabilit ies correlate with the rate of hydrolysis of meso-, (+)- and hydroxyph enyl)ethylenediamine]dichloroplatinum(II) (3-PtCl2). The slower rate o f hydrolysis of the meso diastereoisomer as compared with that of the d, l species of 3-PtCl2 is explained by the presence of a 5th Pt-O con tact in the meso diastereoisomer which hinders the entrance of the wat er molecule and makes the hydrolysis slower.