MECHANISTIC ASPECTS OF THE CHEMICAL-REACTIVITY OF PT(II) DITHIOCARBONATO COMPLEXES TOWARD VARIOUS NUCLEOPHILES - A THEORETICAL-STUDY

EHMO-SCCC calculations have been used in the analysis of the electronic structure, chemical reactivity, and related properties of eight model platinum(II) complexes constituting the set of compounds formed in the reactions of planar d8-bis(dithiocarbonato) and/or bis(dithiocarbamato)platinum(II) complexes with various nucleophiles. On the basis of the molecular orbital description of the complexes, the frontier molecular orbital approach of chemical reactivity, and the calculated potential energy surfaces, plausible mechanisms of their formation reactions and fluxional behavior have been deduced. Accordingly, a mechanism involving an early square-pyramidal transition state followed by a trigonal bipyramidal intermediate and a late distorted square-pyramidal transition state is proposed for the nucleophilic substitution of the one sulfur atom by the phosphine nucleophile in the Pt(S2COR)2 complex. Moreover, [Pt(S2COR)3]- is formed via a direct attack of the Pt(S2COR)2 by the ROCS2- nucleophile through a charge-controlled nonrigid distorted square-pyramidal transition state. The inability of Pt(S2CNHR)2 to undergo analogous reactions is also discussed. An intramolecular mechanism is proposed for the conversion of [Pt(S2COR)3]- into [Pt(S2C=O)(S2COR)]-, which, upon reaction with an excess of phosphine gives Pt(S2C=O)(PH3)2, through a distorted trigonal bipyramidal transition state. The different reaction behavior of amines and/or phosphines with the xanthato complexes comparing to that of the ROCS2- nucleophiles is also discussed. Finally, several suggestions concerning future experimental efforts are also proposed.