The structure and bonding of cis- [Fe(PH3)(4)H(H,)](+), as well as of the transition state for intramolecular hydrogen exchange is investigated by pure density functional calculations (BP86), as well as by hybrid methods (B3LYP). The calculated Fe-P distances as well as the Fe-H separations of the dihydrogen ligand are significantly longer in the B3LYP case than for BP86. Both sets of calculations predict a low activation energy for intramolecular hydrogen exchange, consistent with experimental findings, but the value of DeltaE(phi) is twice as large for B3LYP compared to that of BP86 (19 and 9 kJ/mol, respectively). The chemical bonding is described according to a topology analysis of the electron density, based on the theory of "atoms in molecules". According to the BP86 calculations, the transition state for the intramolecular hydrogen exchange process clearly has to be classified as a trishydride complex, whereas the B3LYP approach not only results in Fe-H bonds but also in H-H bonding interactions, thus suggesting the presence of a trihydrogen moiety.