Upon deposition of Al, Ga, or In atoms (M) together with phosphine in a solid argon matrix, metal atom complexes M.PH3 are formed. Photolysis of the matrices at lambda = 436 nm results in the tautomerization of the adduct species to the insertion products HMPH2 and H2MPH. In addition, PH is formed from the reactions with Ga and In, with HMPH2 being its most likely precursor. Further photolysis into the absorption maximum of HMPH2 near 550 nm results in decomposition of HMPH2 with partial re-formation of the adduct M.PH3 and further buildup of PH. H2MPH is photostable under these conditions but suffers decomposition under the action of UV light (200 < or = lambda < or = 400 nm). All the molecules have been identified by their IR spectra, the assignments being attested by the effects of deuteration and also by comparison either with the vibrational properties anticipated by density functional theory (DFT) calculations or with those of known, related molecules. The resulting analysis is elaborated for the light it sheds on the structures and properties of the new molecules and on the mechanisms of the reactions affording, or disposing of, them.