On the maximum bond multiplicity of carbon: unusual C≡U quadruple bonding in molecular CUO

Carbon is known to form single, double and triple bonds. An educated search led us to a novel view on the bonding of carbon in the triatomic uranium carbide oxide molecule CUO. When comparing various empirical properties and theoretical indices of a whole set of related species, some of those molecules are best described with carbon being quadruply bonded, with the ubiquitous one sigma- and two pi-bonds, plus a non-negligible, albeit weak, rearward sigma-bond. Observable indicators of bond strength suggest a rather high C-U bond order, significantly above three for a series of CUE molecules with different electrophilic ligands E. Several orbital-based indices count only a little more than three C-U bonds, although with additional ionic bonding. The bonding in the CU unit of CUE differs from the recently claimed quadruple bonding in C-2 owing to the rich U-pfds valence shell. CUE molecules are comparatively stable units with a high nucleophilicity at the carbon end, and they may become chemically interesting intermediates, for instance in water decomposition reactions.