Spin−Orbit Coupling Effects on the Metal−Hydrogen Bond Homolysis of M(H)(CO)3(H-DAB) (M = Mn, Re; H-DAB = 1,4-Diaza-1,3-butadiene)

The spin−orbit coupling (SOC) induced mixing between the low-lying singlet and triplet electronic excited states of M(H)(CO) 3 (H-DAB) (H-DAB = 1,4-diaza-1,3-butadiene) (M = Mn, Re) is investigated through SOC-CI calculations, using a one-electron effective spin−orbit operator at the metal center. On the basis of the spin−orbit interactions calculated between the low-lying singlet and triplet nd → π* DAB (Metal-to-Ligand Charge Transfer) and σ M - R → π* DAB (Sigma-Bond-to-Ligand Charge Transfer) excited states, it is shown how the spin−orbit effect may control the metal−hydrogen bond breaking of this class of complexes. In the manganese complex, the spin−orbit interactions between the low-lying singlet and triplet states range between 0 and 100 cm -1, whereas in the rhenium complex they are calculated between 100 and 560 cm -1 . The spin−orbit splitting of the lowest triplet excited states is negligible in the manganese complex with values of a few tens of wavenumbers, whereas it becomes significant (between 80 and 1200 cm -1 ) in the rhenium complex. The spin−orbit interactions between the 1 MLCT state, populated after visible irradiation, and the 3 SBLCT state, dissociative for the metal hydrogen bond homolysis, are estimated at 75 and 314 cm -1 in the manganese and rhenium complexes, respectively. In order to evaluate the consequence of the spin−orbit interaction increase on the probability of dissociation of the M−H bond through 1 MLCT → 3 SBLCT intersystem crossing, preliminary wave packet propagations have been performed on idealized spin−orbit coupled potentials in the diabatic representation with coupling terms of 500 and 80 cm -1 . In both cases the metal−hydrogen bond breaking probability never exceeds 1% in 1 ps.