The photoelimination of molecular hydrogen from H2Fe(CO)4 has been studied through contracted configuration interaction calculations of the potential energy curves that connect the ground and the lowest excited states of the reactant to the ground and excited states of the primary products. The calculations were carried out under C2-upsilon constraint, with a basis set that is at least of double-zeta quality. The most affected geometrical parameters during the course of the reaction were optimized at the HF level for each point along the reaction path. The multireference CCI calculations, which correlate the 3d electrons and four electrons of the two Fe-H bonds, were based on a unique CASSCF wave function with six electrons in six active orbitals (1-sigma(g)Fe-H2, 1-sigma-*(g)Fe-H2, 1-sigma(u)Fe-H2, 1-sigma-*(u)Fe-H2, 3d(y2-z2), 2-sigma(g)Fe-H2), optimized for the state 5A1, the principal configuration being (sigma-g)2(3d(y2-z2))1(sigma-*g)1(sigma-u)1(sigma-*u)1. It is proposed that excitation of H2Fe(CO)4 at 254 nm will bring the molecule from the ground-state a 1A1 into the nearly degenerate a 1B2 3d --> sigma-*u and b1A1 3d --> sigma-*g excited states. From there the system has the choice between two reactive channels: (i) the dissociation along the b1A1 potential energy curve with formation of the products H2 and Fe(CO)4, the latter in b1A1 excited state; (ii) after intersystem crossing to the lowest a3B2 3d --> sigma-*u state, the dissociation along the a3B2 potential energy curve with a small energy barrier with formation of the products H2 and Fe(CO)4 in their ground state. On the basis of spin-orbit coupling consideration, it is predicted that the reverse oxidative addition as well as the direct thermal process is allowed, with a low energy barrier (less than 10.0 kcal/mol) for the reverse reaction. The recombination of H2 with Fe(CO)4 observed in low-temperature matrix experiments is discussed regarding the nature of the electronic state of the Fe(CO)4 fragment generated after irradiation.
