Effects of relativity on the NiCO, PdCO, and PtCO bonding mechanism: a constrained space orbital variation analysis of density functional results

Using a density functional based constrained space orbital variation technique, we have analyzed the importance of relativistic effects on the various mechanisms which affect the metal-CO bond in Ni, Pd, and Pt monocarbonyls. The bonding is dominated by the π back-donation mechanism which, for Pd and Pt, is considerably reinforced by relativistic effects. The weaker PdCO bonding is rationalized by reduced back-donation. The combined effect of relativistic bond length contraction and of the atomic d orbital energies determines the trend in the metal-CO bond strengths. This trend is different from the one found for the coordinatively saturated M(CO)4 tetracarbonyls.