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 --> pi(DAB)* (Metal-to-Ligand Charge Transfer) and sigma(M-R) --> pi(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 spinorbit 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 (MLCT)-M-1 state, populated after visible irradiation, and the (SBLCT)-S-3 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 (MLCT)-M-1 --> (SBLCT)-S-3 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.