A series of group 13-15 compounds of the general formula [MYR ₂]_n (M = B, Al, Ga; Y = N, P, As; n = 1, 2; R = H, CH ₃) 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_2u symmetric [HMYH]₂ rings are the lowest in energy, with the single exception of Ga₂N₂H₄, for which a somewhat unexpectedly C_2u 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 A1P, AlAs, and GaAs systems, thus allowing a variety of structural types to be competitive in energy.