AB-INITIO CALCULATIONS ON H(3)X(2)SILI AND H(3)X(2)SI(-) (X=N, P)

While the local minima obtained for H3SiN2Li are planar or almost plan ar molecules, those for H3SiP2Li deviate significantly from planarity. In both cases the isomers lowest in energy correspond to cyclic non-c lassical structures with the lithium atom coordinated by either two ni trogen or two phosphorus atoms. The relative energy of the two most st able H3SiN2Li isomers is maintained when the lithium atom is complexed by an oxygen-contaning solvent modelled by water. The first solvation energy of about -19 kcal mol(-1) for both species was found to be qui te similar to those obtained by other authors for several first-row li thium compounds. The energetic difference between the two most stable H3SiN2Li isomers might, however, be reduced or even change its sign wh en a second solvent molecule is added. Upper bounds to the change of e nergy associated with the dimerization of the two most stable nitrogen compounds were calculated, and at -44 to -54 kcal mol(-1) they are le ss negative than the value obtained for the dimerization of lithiated aminonitrile. Among the nitrogen compounds the isomers containing diva lently bonded silicon are lower in energy than those to which they are formally related by a 1,2 hydrogen shift. This result is in keeping w ith the well-known fact that aminosilylene is lower in energy than sil animine. Stabilization of a silylene by interaction of the silicon ato m with a lone pair of an adjacent atom is less effective for phosphoru s than for nitrogen. As in the case of silanephosphimine and phosphino silylene the silylenes are now higher in energy than those isomers to which they are formally related by a 1,2 hydrogen migration. The inter action of the molecular subunits in some H(3)SiX(2)Li and H(3)SiX(2)(- ) isomers (X = N, P) was evaluated by means of bond separation reactio ns and the results were compared with the corresponding values for H(3 )CX(2)Li and H(3)CX(2)(-).