We have used photo-ionisation efficiency spectroscopy to determine the ionisation potentials (IPs)
of the niobium–carbide clusters, Nb5Cy (y = 0–6). Of these clusters Nb5C2 and Nb5C3 exhibit the
lowest IPs. Complementary density functional theory calculations have been performed to locate
the lowest energy isomers for each cluster. By comparing the experimental IPs with those
calculated for candidate isomers, the structures of the Nb5Cy clusters observed in the experiment
are inferred. For all these structures, the underlying Nb5 cluster has either a ‘prolate’ or ‘oblate’ trigonal bipyramid geometry. Both Nb5C5 and Nb5C6 are shown to contain
carbon–carbon bonding in the form of one and two molecular C2 units, respectively
