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 electroni c structure, chemical reactivity, and related properties of eight mode l platinum(II) complexes constituting the set of compounds formed in t he reactions of planar d8-bis(dithiocarbonato) and/or bis(dithiocarbam ato)platinum(II) complexes with various nucleophiles. On the basis of the molecular orbital description of the complexes, the frontier molec ular orbital approach of chemical reactivity, and the calculated poten tial energy surfaces, plausible mechanisms of their formation reaction s and fluxional behavior have been deduced. Accordingly, a mechanism i nvolving an early square-pyramidal transition state followed by a trig onal bipyramidal intermediate and a late distorted square-pyramidal tr ansition state is proposed for the nucleophilic substitution of the on e 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 disto rted square-pyramidal transition state. The inability of Pt(S2CNHR)2 t o undergo analogous reactions is also discussed. An intramolecular mec hanism is proposed for the conversion of [Pt(S2COR)3]- into [Pt(S2C=O) (S2COR)]-, which, upon reaction with an excess of phosphine gives Pt(S 2C=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 al so discussed. Finally, several suggestions concerning future experimen tal efforts are also proposed.