The nature of the bonding in ethylene and h2-P4 complexes, M(C2H4)2+ and M(h2-P4)2+, of group 11 metal cations (M = Cu, Ag, Au) has been explored by d. functional calcns. On the basis of the evaluation of symmetry orbitals, the contributions from the interactions of ligand orbitals with metal ns, np, and (n-1)d orbitals have been investigated. The anal. 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 C2H4 and P4 complexes. Bond energies increasing in the order Ag < Cu < Au have been predicted. The stronger metal-ligand bonds in the gold(I) compds. compared to those in the silver(I) compds. 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(h2-P4)2+ compared to those in the exptl. known Ag(h2-P4)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 analogs because of a comparably small overlap between ligand orbitals and metal 3d orbitals. [on SciFinder (R)]