Orbital symmetry as a tool for understanding the bonding in Krossing's cation

The geometric and electronic structure of Krossing's cation, Ag(eta(2)-P(4))(2)(+), which shows an unexpected planar coordination environment at the metal center and D(2)(h) symmetry both in solution and in the solid state, have been investigated using density functional theory and orbital-symmetry-based energy decomposition. The analysis reveals that the contribution from electrostatic interactions to the bond energy is greater than that of orbital interactions. Partitioning of the latter term into the irreducible representations shows that, in addition to the 5s orbital, 5p orbitals of silver act as acceptor orbitals for electron donation from sigma(P-P) orbitals (a(1)(g), b(1)(u)) and n(P) orbitals (b(3)(u)). Back-donation from the 4d(10) closed shell of Ag into sigma orbitals of the pnictogen cages (b(2)(g)) is also important. However, this contribution is shown not to determine the D(2)(h) structure, contradicting conclusions from the pioneering study of the title cation (J. Am. Chem.Soc. 2001, 123, 4603). The contributions from the irreducible representations to the stabilizing orbital interactions in the D(2)(h) structure and in its D(2)(d)-symmetric conformer are analogous, indicating that the planar coordination environment at the metal center in Ag(eta(2)-P(4))(2)(+) is induced by intermolecular rather than by intramolecular interactions. Because ethylene coordination to a metal ion is an elementary reaction step in industrial processes, the bonding in Ag(C(2)H(4))(2)(+) has been analyzed as well and compared to that in Krossing's cation. Surprisingly, similar contributions to the bond energies and an involvement of metal 4d and 5p orbitals have been found, whereas a recent atoms in molecules analysis suggested that the metal-ligand interactions in silver(I) olefin complexes fundamentally differ from those in tetrahedro P(4) complexes. The only qualitative difference between the bonding patterns in Ag(eta(2)-P(4))(2)(+) and Ag(C(2)H(4))(2)(+) is the negligible energy contribution from the b(3)(u) irreducible representation in the ethylene complex because a respective symmetry-adapted linear combination of ligand orbitals is not available.