The stabilities of different structures for [ReH4(CO)(PH3)(3)](+) are predicted from ab initio calculations at the MP2 level with effective core potential, split-valence basis sets. Two structures have been found to be of almost equal energy. One has a dodecahedral geometry (1) with the four hydride ligands occupying the B sites, and the other has a pentagonal-bipyramidal geometry (2) with one eta(2)-H-2 ligand. These two structures represent the classical and nonclassical tautomers observed experimentally in a solution of [ReH4(CO)(PMe(2)Ph)(3)][BF4]. The hydride-hydrogen site exchange in the nonclassical structure (2) and the interconversion process between 1 and 2 have also been studied. The interconversion was found to be faster (calc barrier 4 kcal/mol) than the site exchange process (calc barrier >8 kcal/mol). From these calculations, we proposed a new interpretation of the NMR experiments. The different chemistry observed for [ReH4(PMe(2)Ph)(4)](+) is explained by the interaction of metal d orbitals with the CO pi* obitals.