In this perspective article we describe recent infrared spectroscopic investigations of mass-selected M +-H 2 and M +-D 2 complexes in the gas-phase, with targets that include Li +-H 2, B +-H 2, Na +-H 2, Mg +-H 2, Al +-H 2, Cr +-D 2, Mn +-H 2, Zn +-D 2 and Ag +-H 2. Interactions between molecular hydrogen and metal cations play a key role in several contexts, including in the storage of molecular hydrogen in zeolites, metal-organic frameworks, and doped carbon nanostructures. Arguably, the clearest view of the interaction between dihydrogen and a metal cation can be obtained by probing M +-H 2 complexes in the gas phase, free from the complicating influences of solvents or substrates. Infrared spectra of the complexes in the H-H and D-D stretch regions are obtained by monitoring M + photofragments as the excitation wavelength is scanned. The spectra, which feature full rotational resolution, confirm that the M +-H 2 complexes share a common T-shaped equilibrium structure, consisting essentially of a perturbed H 2 molecule attached to the metal cation, but that the structural and vibrational parameters vary over a considerable range, depending on the size and electronic structure of the metal cation. Correlations are established between intermolecular bond lengths, dissociation energies, and frequency shifts of the H-H stretch vibrational mode. Ultimately, the M +-H 2 and M +-D 2 infrared spectra provide a comprehensive set of benchmarks for modelling and understanding the M +⋯H 2 interaction.