π and σ Complexation in C₃X (X = Be, BH, Si) clusters. A study of bond stretch isomerism

Rhomboidal (C_2v) 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 be viewed as a interactions between X and "cyclic" C₃, which is covalent in C₃BH and of a π 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 π complex for C₃Be.