Gas-phase metal carbonyl cations of the vanadium-group metals (V+, Nb+, Ta+) were produced in a molecular beam by laser vaporization and then mass-analyzed and size-selected in a time-of-flight spectrometer and studied with IR Laser photodissociation spectroscopy in the carbonyl-stretching region. The abundances in the mass spectra, the fragmentation patterns, and the IR spectra provided a combined approach that revealed the coordination numbers in these systems. Although seven-coordinate structures would have 18 electrons in each case, V(CO)(6)(+) was found to be formed rather than V(CO)(7)(+) center dot Nb+ formed both six- and seven-coordinate species, while Ta+ formed only the Ta(CO)(7)(+) complex. Density functional theory computations were used to predict the IR spectra for these systems, which are dramatically different for the six- and seven-coordinate structures and in excellent agreement with the measurements. V(CO)(6)(+) and Nb(CO)(6)(+) have structures slightly distorted from octahedral, white Nb(CO)(7)(+) and Ta(CO)(7)(+) have C-3v capped octahedral structures.