In a recent study, based on a correlated valence bondapproach,8Shaiket al.proposed that a fourth bond is necessaryto explain the C2spectrum at low energy. Within a correlated res-onating valence bond approach, they have found that the 2σand 3σmolecular orbitals, afters–phybridization, change their nature ascompared to standard molecular orbital theory and show a corre-sponding bonding character. By taking into account the remainingtwoπorbitals, obviously bonding, they argued that an unexpectedquadruple bond should be a more appropriate description of theC2molecule. This result was rather surprising, especially consideringthat quadruple bonds should very rarely occur.

In main group elements, however, the maximum number ofbonds between two atoms has remained at three,6–8although amaximum bond order of four in C2and the isoelectronicspecies CN+, BN and CBhas been proposed.9,10However, thisproposal has been largely debated and has stimulated heateddiscussion.11–13Shaiket al.9reported that the fourth bondin C2has a non-negligible bond dissociation energy ofB12–17 kcal mol1based on high level valence bond calculations.However, the experimental bond dissociation energy of C2wasfound to be lower than that of HCRCH.14This has igniteddoubt in assigning true quadruple bonding in C2.11–13Theexperimental14C–C distance in C2(1.255 Å) is longer than thatin HCRCH (1.203 Å). The calculated force constant values arealso smaller for C2than HCRCH.9,15The effective bond order(EBO) introduced by Rooset al.16based on the complete-active-space-self-consistent-field (CASSCF) theory provided a value of2.16 for C2and 2.86 for HCRCH.12All these have created adilemma in ascertaining a true quadruple bond in C2, one of the