The gas-phase reactions of Nb-n(+) (n = 2-6) with C2H4 are studied by Fourier transform mass spectrometry. Collision-induced dissociation (CID) and ion-molecule reactions are used to probe the product ion structures. The sequential reactions of Nb-2(+) are believed to involve ligand coupling, while for the larger clusters this mechanism is less important. The bonding between the Nb atoms and the C atoms of the ligand in the product ions becomes stranger with increasing cluster bite. One of the major product ions from the reaction of Nb-4(+) with ethylene, Nb4C4+, is especially stable and believed to be the distorted 2 x 2 x 2 cubic "nanocrystal" structure observed in supersonic expansions. The thermochemical limits, D-o(Nb-n(+) - C-2) greater than or equal to 186.3 kcal/mol (n = 3-6), were derived on the basis of the observation of the complete dehydrogenation of ethylene.