Ultrafast X-ray spectroscopy as a probe of nonequilibrium dynamics in ruthenium complexes

► We provide an explanation of the spin crossover transition in Ru complexes. ► We explain the stability of the triplet metal-to-ligand charge transfer state. ► The nonequilibrium time-dependence of a transition-metal L-edge X-ray absorption spectrum is calculated. ► The time-dependent X-ray absorption can provide detailed information on nonequilibrium dynamics.