A series of group 13-15 compounds of the general formula [MYR2](n) (M = B, Al, Ga; Y = N, P, As; n = 1, 2; R = H, CH3) have been theoretically studied at the B3LYP/TZVP level of theory. The stability of different isomer structures is discussed to reveal the competitiveness of group 13-13, group 13-15, and group 15-15 bonding. Preferential bonding patterns and trends in the stability with respect to M and Y are also discussed. For the dimeric compounds, C-2v symmetric [HMYH](2) rings are the lowest in energy, with the single exception of Ga2N2H4, for which a somewhat unexpectedly C-2v symmetric [GaNH2](2) ring is found to be the energy minimum, followed by the planar H2NGaGaNH2 chain. The higher stability of the GaNH2 bonding pattern in oligomer compounds may be rationalized in terms of the increasing stability of the oxidation state I as compared to that for the boron and aluminum analogues. Methylation significantly reduces the energetic differences between monomeric MYMe2 MeMYMe, and Me2MY, isomers, especially for the AlP, AlAs, and GaAs systems, thus allowing a variety of structural types to be competitive in energy.