THE ROLE OF CYCLO-TRIPHOSPHORUS (P-3) INPLANE SIGMA-ORBITALS IN COORDINATING TRANSITION-METAL FRAGMENTS

New consideration of the chemical bonding in [L3M(mu-P-3) M'L-3](n+) c omplexes (triple-deckers with bifacial coordination of the unit P-3) f ocuses on the active role played by the P-3, sigma bonding network. Th e 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, a n in-phase combination of metal sigma-hybrids interacts with the total -symmetric sigma bonding MO of P-3, (a(1)'). Only through the latter i nteractions, are the low lying and empty metal levels partially satura ted. On increasing the total electron count, the donor-acceptor intera ctions of e' symmetry are reduced and, for 34 electron systems, the at tractions transform into electron repulsions without destroying the pr imary triple-decker assembly. Finally, the role of the P-3, sigma orbi tals 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 v ariations of the P-P bond lengths can be interpreted in terms of a dif ferent involvement of the P-3 a orbitals for both the mononuclear and binuclear adducts. (C) 1998 Elsevier Science S.A. All rights reserved.