Contrasting stabilities of classical and bridged pyramidal Si₃H₃X molecules (X = BH^-, CH, N, NH⁺, NO, SiH, P, PH⁺, and PO)

Abstract

Trigonal-pyramidal Si₃H₃X systems have been studied at HF/6-31G^∗, MP2(FC)/6-31G^∗, and Becke3LYP/6-31G^∗ levels. The classical trigonal-pyramidal 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_3v) 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. Isolobal analogy connects 6 with various triply hydrogen bridged pyramidal structures in organometallics.