PI-COMPLEXATION AND SIGMA-COMPLEXATION IN C3X (X = BE, BH, SI) CLUSTERS - A STUDY OF BOND STRETCH ISOMERISM

Rhomboidal (C2-upsilon) C3X (X = Si, BH, and Be) structures have been studied with ab initio molecular orbital theory using Pople's G1 method. Each system contains a set of bond stretch isomers that are related via a level crossing mechanism. In all cases the species containing a short cross-ring C-C distance (a) is more stable than the one (b) with a long C-C separation, but the energy differences are relatively small. Analysis of the MP2 one-electron density, employing a triply split basis set with diffuse and polarization functions, indicates that isomers a have more concentration of charge within the structural frame than isomers b. The topological properties of these systems suggest that isomers a, with their short cross-ring C-C bonds, may bc viewed as a interactions between X and "cyclic" C3, which is covalent in C3BH and of a pi-nature in C3Be. Isomers b, which have short cross-ring C-X distances, are better represented as interactions between X and a bent C3 unit, which gives a covalent T-structure for C3BH and a pi-complex for C3Be.