Modelling spin-forbidden reactions: recombination of carbon monoxide with iron tetracarbonyl

New density functional theory and ab initio computations on the [Fe(CO)(5)] system are reported. Careful exploration of basis set and correlation effects leads to best values for the difference in energy DeltaE(1,3) between ground state (3)[Fe(CO)(4)] and the singlet excited state of ca. 8 kcal mol(-1), and for the bond dissociation energy BDE(3) of [Fe(CO)(5)] with respect to ground state fragments (3)[Fe(CO)(4)] + CO of ca. 40 kcal mol(-1). A modified form of the B3PW91 functional is used to explore the potential energy surface for the spin-forbidden recombination reaction of CO with (3)[Fe(CO)(4)]. A C-s-symmetric minimum energy crossing point ( MECP) between the reactant ( triplet) and product ( singlet) potential energy surfaces is found, lying 0.43 kcal mol(-1) above the reactants. The rate coefficient for recombination is computed using a non-adiabatic form of transition state theory, in which the MECP is treated as the critical point in the reaction. Semi-quantitative agreement with experiment is obtained: the predicted rate coefficient, 8.8 x 10(-1)5 cm(3) molecule(-1) s(-1), is only six times smaller than the experimental rate. This is the first computation from first principles of a rate coefficient for a spin-forbidden reaction of a transition metal compound.