The phenyl cation is known to have two lowenergy minima, corresponding to 1 A 1 and 3 B 1 states, the ®rst of which is more stable by ca. 25 kcal/mol. The minimum energy crossing point between these two surfaces, located at various levels including a hybrid method ®rst described here, lies just above the minimum of the triplet, 0.12 kcal/mol at the CCSD(T)/cc-pVDZ// B3LYP/SV level, and there is signi®cant spin-orbit coupling between the surfaces at this point. On the basis of these results, the lifetime of the triplet is expected to be very short.

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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 Cs-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(-15) cm3 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.

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