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 planar molecules, those for H3SiP2Li deviate significantly from planarity. In both cases the isomers lowest in energy correspond to cyclic non-classical structures with the lithium atom coordinated by either two nitrogen or two phosphorus atoms. The relative energy of the two most stable 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 quite similar to those obtained by other authors for several first-row lithium compounds. The energetic difference between the two most stable H3SiN2Li isomers might, however, be reduced or even change its sign when a second solvent molecule is added. Upper bounds to the change of energy associated with the dimerization of the two most stable nitrogen compounds were calculated, and at -44 to -54 kcal mol(-1) they are less negative than the value obtained for the dimerization of lithiated aminonitrile. Among the nitrogen compounds the isomers containing divalently 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 with the well-known fact that aminosilylene is lower in energy than silanimine. Stabilization of a silylene by interaction of the silicon atom with a lone pair of an adjacent atom is less effective for phosphorus than for nitrogen. As in the case of silanephosphimine and phosphinosilylene the silylenes are now higher in energy than those isomers to which they are formally related by a 1,2 hydrogen migration. The interaction 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 reactions and the results were compared with the corresponding values for H(3)CX(2)Li and H(3)CX(2)(-).