Isolation of aluminum, gallium, and indium atoms together with ammonia in a solid argon matrix is shown to result in a thermal reaction yielding the metal atom complex A1 . NH3, Ga . NH3, or in NH3, which has been characterized by its IR and UV-visible spectra. Upon photoactivation at lambda = 436 nm, the metal atom inserts into an N-H bond of ammonia to form the divalent compound HMNH2 (M Al, Ga, or In). This product is photolabile, decomposing to the corresponding monovalent compound MNH2 on exposure to broad-band UV-visible light (200 less than or equal to lambda less than or equal to 800 nm). All the molecules have been identified by means of their IR spectra, the assignments being underwritten by the effects of D and N-15 substitution, and also by comparison either with the vibrational properties of known, related molecules or with those predicted by quantum chemical (DFT) calculations. The resulting analysis is elaborated for the light it sheds on the structures and electronic properties of the molecules.