An ab initio MP2/aug-cc-pVTZ study has been carried out on XH4+ ... H-2 and [XH4+ (H-2)(4)]... H-2 complexes (where X = N, P, As, Sb, Bi). The investigated complexes are energetically stable and characterized by the existence of X-H...sigma and X...sigma a interactions. The interaction energy decomposition indicates that differences in the nature of interactions for face and vertex isomers are not qualitative. The AIM theory has been also applied to analyze the properties of the bond critical points: their electron densities and their Laplacians. (C) 2004 Published by Elsevier B.V.