Structures, Stabilities, and Bonding in CBe₂, C₂Be, and C₂Be₂

The structures and energies of the binary beryllium compounds CBe₂, C₂Be, and C₂Be₂ in their energetically lowest singlet and triplet states have been investigated by means of ab initio quantum chemical methods, employing second order Moller-Plesset (MP2) perturbation theory and, for the triatomic molecules, CASSCF techniques. Nonlinear geometries are predicted to be the global minima for CBe₂ (¹A₁) and C₂Be (¹A₁) by both methods, MP2/6-31G//6-31G and CASSCF using a DZ+P basis set and an active space of eight electrons in nine orbitals. For C₂Be₂, the MP2/6-31G//6-31G results favor a linear triplet isomer (³Σ_u⁺) by more than 80 kcal/mol relative to the next low-lying structure (a cyclic form, ¹A₁). The results of one electron density analysis indicate that charge transfer from Be to C is responsible for the stabilities of the nonlinear isomers and that there is little covalent CBe bonding in the singlet nonlinear minima of CBe₂ and of C₂Be.