VALENCE-BOND STUDIES OF CLI5 AND CLI6

Ab initio valence bond calculations with an STO-6G basis set are used to study aspects of the electronic structure of trigonal bipyramidal C Li5. With lithium 2p orbitals initially omitted, a valence bond demons tration is provided for the molecular orbital description of the origi n of the hypercoordination, namely Li(eq)-Li(ax) as well as C-Li(ax) b onding. The axial Li(ax)-C-Li(ax) bonding involves primarily a three-e lectron three-centre bonding unit, for which there are eight canonical Lewis-type VB structures in a minimal basis set treatment. Resonance between these structures is equivalent to resonance between the ''incr eased-valence'' structures (A) and (B), [GRAPHICS] in which the three axial electrons occupy C-Li(ax) bonding molecular orbitals. The Li(ax) -Li(eq) bonding arises when Li(ax)- --> sigma(CLi(eq)) electron deloc alizations occur from relevant Lewis structures, to generate four-elec tron three-centre bonding units, and ''increased-valence'' structures of the type (C) [GRAPHICS] for the three atoms that are involved in th e bonding unit. Structure (C) is equivalent to resonance between the L ewis structures (D) and (E), with Li(eq)-Li(ax). bonding occurring in (E). Inclusion of lithium 2p orbitals in exploratory calculations lead s to a substantial shortening of the axial bonds. Some consideration i s given to three-electron three-centre bonding for CLi6, and to an ele mentary increased-valence bonding index for this type of bonding unit.