The nature of the platinum-phosphine bond. An ab initio Hartree-Fock and density functional study

The electronic structure of model complexes [Pt(PX3)(2)] (X = H or F) has been investigated by ab initio calculations, carried out with several theoretical approaches, of different sophistication, based on the Hartree-Fock method and density functional theory. Density functional calculations including correlation and relativistic effects gave highly reliable theoretical values for the equilibrium geometries and stabilities of the complexes, more accurate than those previously reported. The best estimates of the Pt-P bond distances are 2.27 and 2.25 Angstrom, for [Pt(PH3)(2)] and [Pt(PF3)(2)], respectively. The corresponding dissociation energies are 5.3 eV and 5.1 eV. The differences in chemical behaviour exhibited by PH3 and PF3 can be summarised as follows: the sigma-donated charge from PX3 to the metal is found to be 0.12 and 0.39 electron, for X = H and F; the pi-back-donated charge is 0.48 and 0.59, respectively. Therefore the PF3 ligand should be classified as a stronger sigma donor and pi acceptor than PH3, but its platinum complex is not more stable than [Pt(PH3)(2)]. The role of the polarisation functions centred on the ligand atoms should be interpreted more generally than that of the conventional sigma-pi mechanism, due to the fact that sigma- and pi-orbital occupancies are related not only to metal-ligand charge exchange but also to a charge rearrangement internal to the ligands, which cannot be fully interpreted in terms of a simplified orbital picture. Indeed, the whole electronic structure of the platinum phosphine complexes is largely dominated by electron correlation and relativistic effects.