Following studies of the thermal and photoactivated reactions of the Group 13 metal atoms Al, Ga or In (M) with NH₃ or PH₃ in solid argon matrices, the properties of the observed or potential reaction products are discussed. These are molecules with the general formula H_mMEH_n, where M = Al, Ga or In, E = N or P, and m, n = 0–3. All the molecules have been characterised structurally, energetically and vibrationally by Density Functional Theory (DFT) calculations. The following products have been identified experimentally by their IR spectra: the adducts M·EH₃, the metal(II) insertion products HMEH₂, the metal(III) derivatives H₂MNH₂ and H₂MPH, and the metal(I) amides MNH₂. The observation of most of the IR-active fundamentals for different isotopic forms of the molecules MNH₂ and H₂MNH₂ allowed normal coordinate analysis calculations to be performed, thereby endorsing the vibrational assignments and affording reliable force constants. Observed and hypothetical H_mMEH_n molecules have been compared with particular reference to structural and energetic differences according to whether E = N or P. Questions of bonding have been addressed through calculations of barriers to rotation of planar molecules and of barriers to inversion for molecules containing pyramidal MPH₂ moieties. π-Type interactions play only a minor part in the bonding of planar molecules like H₂MNH₂. The much larger barrier to inversion of PH₃ compared with NH₃ results in pyramidal MPH₂ but planar MNH₂ fragments. With the help of an appropriate MO scheme it is possible to rationalise how the inversion barrier changes when one hydrogen of PH₃ is replaced by an MH_m group.