Theoretical examination of substituent effects on the stabilization of a Sn-Y (Y = Sb and Bi) multiple bond

The potential energy surface for the unimolecular rearrangement XSn equivalent to Y --> TS --> Sn =YX (Y = Sb, Bi) was investigated using the B3LYP and QCISD methods. To explore electronic effects on the relative stability of XSn equivalent to Y and Sn = YX, the first-row substituents (X = H, Li, BeH, BH2, CH3, NH2, OH, F) have been used. Our theoretical findings suggest that the doubly bonded Sn = YX species are always both kinetically and thermodynamically more stable than their corresponding triply bonded isomers, XSn = Y, regardless of the electronegativity of the substituent X. Nevertheless, our model calculations indicate that an aryl group can, if sufficiently bulky, stabilize triply bonded XSn equivalent to Y molecules with respect to both isomerization and polymerization. That is to say, it is not electronic but steric effects that play a dominant role in stabilizing both Sn equivalent to Sb and Sn equivalent to Bi triple bonds. (C) 2004 Wiley Periodicals, Inc. Int J Quantum Chem 102: 72-79, 2005.