H3AXHn (A = B, Al, and Ga; XHn = NH3, PH3, AsH3, H2O, SH2 and SeH2) donor–acceptor complexes are studied from conformational and coordination-mode points of view at B3LYP/6-311+G(3df,2p) and CCSD(T)/6-311+G(3df,2p) levels of theory. The metallic character of gallium atom is responsible of the irregular structural and coordination mode trends in free and complexed gallane (GaH3). The intuitive staggered conformation is not adopted by all compounds because there is, in some cases, competition between H(donor)–H(acceptor) electrostatic interaction and hyperconjugative electronic delocalisation. All H3AXH3 (X = N, P, and As) complexes are staggered. In accordance with Pophristic–Goodman’s study (V. Pophristic, L. Goodman, Nature 411 (2001) 565), hyperconjugation is behind this geometrical preference. For the H3AYH2 series, the H3BOH2 staggered conformation is also favoured by this interaction. Nevertheless, H3AlOH2 and H3GaOH2 are curiously eclipsed and not favoured by the hyperconjugation. This paradox is clarified by both electrostatic and energetic delocalisation interactions analysis. The coordination mode is horizontally and vertically discussed in N and O groups of the periodic table within these complex sets.