Theoretical studies of organometallic compounds, Part XXXV: Trends in molecular geometries and bond strengths of the homoleptic d¹⁰ metal carbonyl cations [M(CO)_n]^x+-(M^x+ = Cu⁺, Ag⁺, Au⁺, Zn²⁺, Cd²⁺, Hg²⁺; n = 1-6): A theoretical study

Quantum chemical investigations at the MP2 and CCSD(T) level with relativistic effective core potentials for the metals are reported for homoleptic carbonyl complexes of the Group 11 and Group 12 d¹⁰ metal cations with up to six carbonyl ligands. Additional calculations for some compounds were carried out using density functional theory (DFT) methods (BP86 and B3LYP). There is good agreement between theoretical CCSD(T) and experimental bond dissociation energies (BDEs), which are known for eight of the 36 complexes studied. The bond energies predicted by DFT are too high. The complexes [Cu(CO)_n]⁺ and [Au(CO)_n]⁺ are predicted to be bound species for n = 1-5 only, whereas [Ag(CO)_n]⁺ and the Group 12 carbonyls [M(CO)_n]²⁺ are bound species for n = 1-6. The metal-CO bonding has been analyzed with the help of the natural bond orbital (NBO) method and the charge decomposition analysis (CDA) partitioning scheme. The Group 11 species exhibit more covalent metal - CO bonds than those of Group 12, but coulombic interactions are dominant even for the Group 11 species. The dicarbonyls of Cu⁺, Ag⁺, and Au⁺ have shorter M-CO bonds than the monocarbonyls, and the bond dissociation energies are higher for [M(CO)₂]⁺ than for [M(CO)]⁺. This is explained by the polarization (s - d_σ hybridization) of the metal valence electrons in [M(CO)]⁺. The metal-CO bond energies of the tricarbonyls are significantly lower than those of the dicarbonyls, because the favorable charge polarization at the metal is not effective. The drop in the bond energy is particularly great for [Au(CO)₃]⁺, because the Au⁺-CO bonds in [Au(CO)]⁺ and [Au(CO)₂]⁺ are enhanced by covalent contributions. [Au(CO)]⁺ and [Au(CO)₂]⁺ have stronger metal-CO bonds than the copper and silver analogues, but the tri-and tetracarbonyls of Au⁺ have weaker bonds than those of Cu⁺ and Ag⁺. The M²⁺-CO bond energies of the Group 12 carbonyls are significantly higher than those of the respective Group 11 carbonyls. Since many of the complexes studied in this paper, particularly those of the Group 12 dications, have not been synthesized yet, the results should prove useful to experimentalists.