Abstract
The importance of relativity and dispersion in metallophilicity has been discussed in numerous studies. The existence of hybridization in the bonding between closed shell d(10)-d(10) metal atoms has also been speculated, but the presence of attractive MO interaction in the metal-metal bond is still a matter of an ongoing debate. In this comparative study, a quantitative molecular orbital analysis and energy decomposition is carried out on the metallophilic interaction in atomic dimers (M(+)···M(+)) and molecular perpendicular [H(3)P-M-X](2) (where M = Cu, Ag, and Au; X = F, Cl, Br, and I). Our computational studies prove that besides the commonly accepted dispersive interactions, orbital interactions and Pauli repulsion also play a crucial role in the strength and length of the metal-metal bond. Although for M(+)···M(+) the orbital interaction is larger than the Pauli repulsion, leading to a net attractive MO interaction, the bonding mechanism in perpendicular [H(3)P-M-X] dimers is different due to the larger separation between the donor and acceptor orbitals. Thus, Pauli repulsion is much larger, and two-orbital, four-electron repulsion is dominant.