Theoretical Studies of Organometallic Compounds. 6. Structures and Bond Energies of M(CO)_n⁺, MCN, and M(CN)₂⁻ (M = Ag, Au; n = 1–3)

Quantum mechanical ab initio calculations using relativistic effective core potentials for the metals are employed to predict theoretically the geometries, vibrational spectra, and metal–ligand bond energies of the title compounds. The comparison with available experimental data shows very good agreement. The dicarbonyls Ag(CO)₂⁺ and Au(CO)₂⁺ are predicted to be more stable than the mono- and tricarbonyls, respectively. The Au(I) complexes are stronger bound than the Ag(I) complexes, and the cyanides have much stronger metal–ligand bonds than the carbonyls. The topological analysis of the electronic structures and the natural bond orbital analysis show that the origin of the metal–ligand bonds in the carbonyls and cyanides is dominantly electrostatic, but covalent contributions are analyzed for Ag(CO)₂⁺, AuCO⁺, Au(CO)₂⁺, and the nitriles M(CN)₂. The covalent contribution to the metal–ligand bond increases from MCO⁺ to M(CO)₂⁺, but it decreases strongly from M(CO)₂⁺ to M(CO)₃⁺. The covalent character of the MC bonds arises from σ donation of the ligand with negligible π back-donation, which agrees with the calculated and observed frequency shift toward higher wavenumbers for the CO and CN stretching modes.