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

The bonding features in metal-carbonyls including neutral M CO (M = Ni , Pd , Pt ) and M CO + (M + = Cu + , Ag + , Au + ) complexes have been elucidated at the DFT level with relativistic compact effective potentials for transition metals and 6-311+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 σ-dative bond from the carbon lone electron pair to an empty d σ orbital on the metal, and the π back-donation from filled d π orbitals to the empty 2π* orbital on CO , the present BLW-ED analyses quantitatively demonstrated that in neutral M CO complexes the π-bonding dominates over the σ-bonding, whereas in cationic M CO + complexes, the σ-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 π 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 π back-donation is insignificant in M CO + cations, and the C–O eventually vibrates at higher frequencies than the free CO frequency.