Resonance Character of Copper/Silver/Gold Bonding in Small Molecule⋅⋅⋅M-X (X=F, Cl, Br, CH3, CF3) Complexes.

The resonance character of Cu/Ag/Au bonding is investigated in B⋅⋅⋅M-X (M=Cu, Ag, Au; X=F, Cl, Br, CH3, CF3; B=CO, H2O, H2S, C2H2, C2H4) complexes. The natural bond orbital/natural resonance theory results strongly support the general resonance-type three-center/four-electron (3c/4e) picture of Cu/Ag/Au bonding, B:M-X↔B(+) -M:X(-) , which mainly arises from hyperconjugation interactions. On the basis of such resonance-type bonding mechanisms, the ligand effects in the more strongly bound OC⋅⋅⋅M-X series are analyzed, and distinct competition between CO and the axial ligand X is observed. This competitive bonding picture directly explains why CO in OC⋅⋅⋅Au-CF3 can be readily replaced by a number of other ligands. Additionally, conservation of the bond order indicates that the idealized relationship bB⋅⋅⋅M +bMX =1 should be suitably generalized for intermolecular bonding, especially if there is additional partial multiple bonding at one end of the 3c/4e hyperbonded triad.