Density functional theory (DFT) methods, with three different gradient-corrected functionals (BP86, BLYP, and B3LYP), have been used to calculate molecular geometries and energies along the pathway for oxidative addition of H-2 to Fe(CO)(4) leading to H2Fe(CO)(4). The geometry of H2Fe(CO)(4), optimized using each of the different functionals, is in good agreement with experimental results. The enthalpies for reductive elimination of H-2 from H2Fe(CO)(4), referenced to both the first excited singlet and the triplet ground state of Fe(CO)(4), have been calculated using the BP86, BLYP, and B3LYP functionals and are compared to the experimental result of 21 +/- 2 kcal mol(-1). The mechanism for the oxidative addition of H-2 to Fe(CO)(4) is discussed and compared with experimental observations. The geometry of a dihydrogen intermediate, (eta(2)-H-2)Fe(CO)(4), and the transition state between (eta(2)-H-2)Fe(CO)(4) and H2Fe(CO)(4), along the reaction path, has also been optimized.