BLOCK-LOCALIZED WAVEFUNCTION ENERGY DECOMPOSITION (BLW-ED) ANALYSIS OF σ/π INTERACTIONS IN METAL-CARBONYL BONDING

The bonding features in metal-carbonyls including neutral MCO (M = Ni, Pd, Pt) and MCO+ (M+ = Cu+, Ag+, Au+) complexes have been elucidated at the DFT level with relativistic compact effective potentials for transition metals and 6-311 vertical bar G(d) basis sets for C and O by the block-localized wavefunction (BLW) method. The BLW method can decompose the intermolecular interactions in terms of Heitler-London, polarization, and charge transfer energy contributions. Since the metal-CO bonding involves two synergic interactions, namely the sigma-dative bond from the carbon lone electron pair to an empty d(sigma) orbital on the metal, and the pi back-donation from filled d(pi) orbitals to the empty 2 pi* orbital on CO, the present BLW-ED analyses quantitatively demonstrated that in neutral MCO complexes the pi-bonding dominates over the sigma-bonding, whereas in cationic MCO+ complexes, the sigma-bonding plays a major role. But in both neutral and cationic species, the CO polarization induced by the metals enhances the C-O bond and increases the C-O vibrational frequencies, while the pi back-donation tends to weaken the C-O bond and decrease the C-O vibrational frequencies. For neutral complexes, the latter is more prominent than the former, and consequently, there is a red-shifting of the C-O vibrational frequencies. In contrast, the pi back-donation is insignificant in MCO+ cations, and the C-O eventually vibrates at higher frequencies than the free CO frequency.