The role of cyclo-triphosphorus (P-3) in-plane sigma orbitals in coordinating transition metal fragments

New consideration of the chemical bonding in [L3M(mu-P-3) M'L-3](n+) complexes (triple-deckers with bifacial coordination of the unit P-3) focuses on the active role played by the P-3, sigma bonding network. The argument, developed from an extended Huckel molecular orbital (EHMO) analysis, is based largely on the rigorous symmetry properties of the MO interactions. In particular, for 30 electron species, only the in-plane sigma bonding orbitals of P-3, can match the in-phase metal d(pi), combinations (e' symmetry for an ideal D-3h model). Additionally, an in-phase combination of metal sigma-hybrids interacts with the total-symmetric sigma bonding MO of P-3, (a(1)'). Only through the latter interactions, are the low lying and empty metal levels partially saturated. On increasing the total electron count, the donor-acceptor interactions of e' symmetry are reduced and, for 34 electron systems, the attractions transform into electron repulsions without destroying the primary triple-decker assembly. Finally, the role of the P-3, sigma orbitals is re-examined for some monomeric complexes of the type LnM(eta(3)-P-3) and for the white phosphorus molecule (P-4) itself. Thus, the variations of the P-P bond lengths can be interpreted in terms of a different involvement of the P-3 a orbitals for both the mononuclear and binuclear adducts.