Binding of Small Gas Molecules by Metal-Bipyridyl Monocationic Complexes (Metal = Cu, Ag, Au) and Possible Bond Activations Therein.

The viability of a series of small gas molecules (H, N, CO, CO, HO, HS, CH, CH, CHCl, CH, and CH) bound [M-(bipy)] (bipy = bipyridyl; M = Cu, Ag, Au) complexes is investigated at the PBE0/cc-pVTZ/cc-pVTZ-PP level with a special emphasis on the possible bond activation within the bound ligands. While the bond dissociation energy, enthalpy change, and free energy change are computed to show the stability of the complexes with respect to the dissociation into [M-(bipy)] and free gas molecule (L), natural bond orbital, electron density, and energy decomposition analyses in conjunction with natural orbitals for chemical valence are carried out to characterize the nature of L-M bonds. For a given L, the L binding ability is the highest for Au followed by Cu and Ag complexes, except for quite loosely bound CO. For all ligand cases, the dissociation processes from the respective bound complexes are endergonic in nature at room temperature, except for the H-, CH-, and CH-bound Ag complexes and CO-bound Ag and Au complexes. The interaction between L and M centers is supported by orbital and ionic interactions with latter being more dominant over the former. The delocalization index and local energy density values support the covalent character in L-M bonds in most of the cases. These M centers can act as a mild bond activation agent for L, Au being the best candidate in this series for this purpose. Particularly, the H-H bond in H, C═C bond in CH, C≡C bond in CH, and C-H bonds in CH and CH (the last two are for Au) are elongated along with a significant red-shift in the corresponding stretching frequency, compared to those in free molecules. These can be explained by the significant π-back-donation populating the lowest unoccupied antibonding molecular orbital of L in these complexes.