Quantum chemical study of CO and NO bonding to Pd-2, Cu-2, and PdCu

The bonding of CO and NO to the dimers Pd-2, Cu-2, and PdCu was studied with the quantum chemical linear combination of Gaussian-type orbitals-model core potential-density functional (LCGTO-MCP-DF) method. Bonding occurs preferably at the bridge site on Pd and at the linear end-on site on Cu-2. On PdCu, CO binds in a bridge or a-top geometry on Pd, while NO binds preferably in The a-top fashion. The binding energy of CO and NO to the Pd (Cu) atom of PdCu is slightly smaller (larger) than that for the homonuclear dimer Pd-2 (Cu-2), These trends are in line with the propensity to back-donation of the various metal dimers. The geometry of the complexes is determined by the back-donation and by the ability of the metal to accommodate the incoming ligand charge (smallest for Cu-2, largest for Pd-2). In PdCuL (L = CO, NO) the sigma-sigma repulsion is reduced by an increased participation of the 4p sigma orbital in accepting the lone pair. In contrast, the lack of Pd hybridization in CuPdL precludes an efficient donation to Pd. We discuss trends in bond lengths, angles, and vibrational frequencies in relation to the magnitude of the sigma-sigma repulsion and attractive pi-pi interaction.