Organometallics versus P4 Complexes of Group 11 Cations: Periodic Trends and Relativistic Effects in the Involvement of (n−1)d, ns, and np Orbitals in Metal−Ligand Interactions

The nature of the bonding in ethylene and η 2 -P 4 complexes, M(C 2 H 4 ) 2 + and M(η 2 -P 4 ) 2 +, of group 11 metal cations (M = Cu, Ag, Au) has been explored by density functional calculations. On the basis of the evaluation of symmetry orbitals, the contributions from the interactions of ligand orbitals with metal n s, n p, and ( n −1)d orbitals have been investigated. The analysis shows that the metal−ligand bonds in the organometallics and phosphorus complexes fit to a unified scheme, whereas traditional concepts such as the isolobality principle would hardly predict such a bonding analogy between C 2 H 4 and P 4 complexes. Bond energies increasing in the order Ag < Cu < Au have been predicted. The stronger metal−ligand bonds in the gold(I) compounds compared to those in the silver(I) compounds can be elucidated by the relativistic stabilization of the orbital interactions, particularly of those involving 6s and 5d orbitals. The stronger metal−ligand bonds in Cu(η 2 -P 4 ) 2 + compared to those in the experimentally known Ag(η 2 -P 4 ) 2 + can be attributed partly to the strong back-donation from metal 3d orbitals to vacant ligand orbitals. This result stands in sharp contrast to the common belief that first-row transition metals form weaker bonds to ligands than do their second-row analogues because of a comparably small overlap between ligand orbitals and metal 3d orbitals.