A compact supersonic source is employed with a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer to generate and investigate the metallocarbohedrene, Nb8C12+. Nb8C12+ reacts with water and alcohols via sequential attachments and dehydrogenations leading to the initial truncation products Nb8C12-(OR)(4)(+) (R = H, CH3, C2H5, C4H9) Also, Nb8C12+ reacts with NH3 via sequential attachments to produce the initial truncation product Nb8C12(NH3)(4)(+). Nb(8)C(12)L(5-8)(+) peaks for ammonia and alcohols are also observed, but they grow in slowly. A measurement of the relative rate constants for the sequential addition reactions of NH3 confirms that addition of the fifth NH3 is at least an order of magnitude slower than the first four attachments. In reactions with acetonitrile and benzene, a maximum of four attachments of these ligands is observed. Sequential halide abstractions with CH(3)X yield Nb(8)C(12)X(n)(+) truncating at n = 4 for X = Cl and n = 5 for X = Br and I. The "titration" results and the relative rate constants for the sequential addition reactions of NH3 provide supporting evidence that the geometric structure of Nb8C12+ is the theoretically more stable T-d or D-2d symmetry having two sets of four equivalent metal sites, as opposed to the T-h symmetry with all eight equivalent metals. Finally, the results for Nb8C12+ are compared to those previously obtained for V8C12+ and Ti8C12+.