Molecular mechanical and quantum chemical study of the species involved in the hydrolysis of cis-diamminedichloroplatinum(II) and substituted bis(ethylenediamine)-dichloroplatinum(II) complexes. Part 2. Simulated transition states

Abstract

Cis-diamminedichloroplatinum(II) (cisplatin) and its substituted ethylenediamine derivs. cis-PtCl2(R2en) (en = ethylenediamine, R = H, Ph, 2-, 3-, and 4-PhOH) were investigated with respect to the possible structures of the hypothetical transition state complexes (TSC) of the hydrolytic SN2 reaction in which 1 Cl is replaced by H2O. TSCs with trigonal bipyramid (TBP) and square pyramid (SP) geometry (coordination no. 5), were studied by mol. mechanics (MM) and extended Huckel (EH) methods. The EH and MM energies as well as the no. of occurrence (entropy factor) for the cisplatinum compd. point to a preferred TBP TSC geometry with NH3 and Cl in axial positions. However, for en and substituted en compds., TSCs with SP geometries (CI in apical position) are preferred. The calcd. EH and MM energies of the TBP and SP structures do not differ significantly and TBP .tautm. SP interconversions may play an essential role in TSC formation. To improve the discrimination, the MM-optimized geometries were treated in terms of displacement coordinates for D3h (TBP) and C4v (SP) by calcg. the total distortion vectors (DV). DV identified once again the TBP with NH3 and Cl in axial position as the least-distorted conformer, but it also revealed the combinations of displacement coordinates which shape the TSC geometry.