The intramolecular reaction involving exchange of a hydrogen atom between molecular hydrogen and hydride ligands is studied in the model system cis-[Fe(PH3)4H(H2)]+ by means of the molecular orbital ab initio method. Several possible mechanisms are considered for the reaction and compared according to their energetic barriers. The favored mechanism is the so-called open direct transfer, which consists of a single step transfer of the hydrogen atom between the two ligands. In the transition state of this mechanism the central hydrogen atom is bound to the other two hydrogens. Traditional σ-bond activation processes like oxidative addition and σ-bond metathesis are compared to the most favored mechanism arising from our results. This mechanism carries essential differences from the others, thus constituting a model for a new kind of reaction. © 1992, American Chemical Society. All rights reserved.