Spin forbidden chemical reactions of transition metal compounds. New ideas and new computational challenges

Many reactions of transition metal compounds involve a change in spin. These reactions may proceed faster, slower-or at the same rate as-otherwise equivalent processes in which spin is conserved. For example, ligand substitution in [CpMo(Cl)(2)(PR3)(2)] is faster than expected, whereas addition of dinitrogen to [Cp*Mo(Cl)(PMe3)(2)] is slow. Spin-forbidden oxidative addition of ethylene to [Cp*Ir(PMe3)] occurs competitively with ligand association. To explain these observations, we discuss the shape of the different potential energy surfaces (PESs) involved, and the energy of the minimum -energy crossing points (MECPs) between them. This computational approach is of great help in understanding the mechanisms of spin-forbidden reactions, provided that accurate calculations can be used to predict the relevant PESs. Density functional theory, especially using gradient-corrected and hybrid functionals, performs reasonably well for the difficult problem of predicting the energy splitting between different spin states of transition metal complexes, although careful calibration is needed.