Trends in molecular geometries and bond strengths of the homoleptic d(10) metal carbonyl cations [M(CO)(n)](x+) (Mx+ = Cu+, Ag+, Au+, Zn2+, Cd2+, Hg2+; n=1-6): A theoretical study

Quantum chemical investigations at the MP2 and CCSD(T) level with relativis tic effective core potentials for the metals are reported for homoleptic ca rbonyl complexes of the Group 11 and Group 12 d(10) metal cations with up t o six carbonyl ligands. Additional calculations for some compounds were car ried out using density functional theory (DFT) methods (BP86 and B3LYP). Th ere is good agreement between theoretical CCSD(T) and experimental bond dis sociation energies (BDEs), which are known for eight of the 36 complexes st udied. The bond energies predicted by DFT are too high, The complexes [Cu(C O)(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 coval ent metal-CO bonds than those of Group 12, but coulombic interactions are d ominant 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 explaine d by the polarization (s-d(sigma) hybridization) of the metal valence elect rons in [M(CO)](+). The metal-CO bond energies of the tricarbonyls are sign ificantly 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 t hose of Cu+ and Ag+. The M2+ - CO bond energies of the Group 12 carbonyls a re significantly higher than those of the respective Group 11 carbonyls, Si nce 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.