Scandium and yttrium carbonyl cations produced in the gas phase via laser vaporization are mass selected and studied with infrared laser spectroscopy in the C-O stretching region. Mass spectra, ion fragmentation behavior, and infrared spectra, complemented by computational chemistry, establish the coordination numbers and structures of these complexes. Sc+ does not form the eight-coordinate 18-electron complex but instead produces a 16-electron seven-coordinate species. However, Y+ forms the anticipated eight-coordinate structure. Density functional theory computations provide structures and corresponding vibrational spectra for these complexes. Sc(CO)(7)(+) has a C-3v, capped octahedral structure, while Y(CO)(8)(+) forms a D-4d square antiprism. The C-O stretches at 2086 and 2087 cm(-1) for Sc(CO)(7)(+) and Y(CO)(8)(+), respectively, are among the most red-shifted frequencies measured for any transition metal carbonyl cation.