The Chemical Bond in C₂

Quantum chemical calculations using the complete active space of the valence orbitals have been carried out for H_nCCH_n (n=0-3) and N₂. The quadratic force constants and the stretching potentials of H_nCCH_n have been calculated at the CASSCF/cc-pVTZ level. The bond dissociation energies of the C-C bonds of C₂ and HC≡CH were computed using explicitly correlated CASPT2-F12/cc-pVTZ-F12 wave functions. The bond dissociation energies and the force constants suggest that C₂ has a weaker C-C bond than acetylene. The analysis of the CASSCF wavefunctions in conjunction with the effective bond orders of the multiple bonds shows that there are four bonding components in C₂, while there are only three in acetylene and in N₂. The bonding components in C₂ consist of two weakly bonding σ bonds and two electron-sharing π bonds. The bonding situation in C₂ can be described with the σ bonds in Be₂ that are enforced by two π bonds. There is no single Lewis structure that adequately depicts the bonding situation in C₂. The assignment of quadruple bonding in C₂ is misleading, because the bond is weaker than the triple bond in HC≡CH. What bonding? Quantum chemical calculations at the CASSSCF and CASPT2-F12/cc-pVTZ-F12 level suggest that C₂ has a weaker C-C bond than acetylene. There are two normal π bonds and two weak σ bonds in C₂ (see scheme).