We describe recent experiments in which mass spectrometry and laser spectroscopy are combined to characterize Li+-H2, Na +-H2, B+-H2 and Al +-H2 complexes in the gas-phase. The infrared spectra, which feature full resolution of rotational sub-structures, are recorded by monitoring M+ photofragments as the infrared wavelength is scanned. The spectra deliver detailed information on the way, in which a hydrogen molecule is attached to a metal cation including the intermolecular separation, the force constant for the intermolecular bond and the H-H stretching frequency. The complexes all possess T-shaped equilibrium geometries and display a clear correlation between the length and force constant of the intermolecular bond and the dissociation energy. In contrast, the data do not support any straightforward correlation between the frequency shift for the H-H stretch mode and the dissociation energy.