CONTRASTING STABILITIES OF CLASSICAL AND BRIDGED PYRAMIDAL SI(3)H(3)XMOLECULES (X=BH-,CH,N,NH+,NO,SIH,P,PH+, AND PO)

Trigonal-pyramidal Si(3)H(3)X systems have been studied at HF/6-31G, MP2(FC)/6-31G, and Becke3LYP/ 6-31G* levels. The classical trigonal-p yramidal structure (5) is a higher order stationary point for X = BH-, CH, NO, SiH, P, PH+, and PO, whereas it is a minima for X = N and NH, at the MP2(FC)/6-31G level. An alternative pyramidal structure (6, C-3 nu,) with three SiHSi 3c-2e bonds is minima, lower in energy than 5 by 47.7 (X = BH-), 39.1 (X = CH), 31.7 (X = N), 25.0 (X = NH+), 20.6 (X = SiH), 20.7 (X = P), 16.1 (X = PH+), and 18.2 (X PO) kcal/mol. Is olobal analogy connects 6 with various triply hydrogen bridged pyramid al structures in organometallics.