The Chemical Bond in C 2

Quantum chemical calculations using the complete active space of the valence orbitals have been carried out for H n CCH n ( n =0–3) and N 2 . The quadratic force constants and the stretching potentials of H n CCH n have been calculated at the CASSCF/cc‐pVTZ level. The bond dissociation energies of the C−C bonds of C 2 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 C 2 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 C 2 , while there are only three in acetylene and in N 2 . The bonding components in C 2 consist of two weakly bonding σ bonds and two electron‐sharing π bonds. The bonding situation in C 2 can be described with the σ bonds in Be 2 that are enforced by two π bonds. There is no single Lewis structure that adequately depicts the bonding situation in C 2 . The assignment of quadruple bonding in C 2 is misleading, because the bond is weaker than the triple bond in HC≡CH.