The geometrical structures of molecular species during chemical reactions as well as relevant electronic configurations and spin-states are crucial information on understanding various photochemical and photobiological processes. For this purpose, time-resolved X-ray spectroscopy has been developed as a complimentary tool for the ultrafast optical spectroscopies. Essentially, due to the element-specific valence-to-core level transition of probing X-rays, time-resolved X-ray spectroscopy can be sensitive spectral probes of metalligand interactions of solvated transition metal complexes. We have focused on these elementspecific probing natures of X-ray pulses and elucidated structural dynamics in several

important classes of transition metal complexes. In this seminar, I will describe our recent works on the ultrafast linkage isomerization of Ru-based photochromic complex. More specifically, we have utilized Ru L-edge and S Kedge X-ray spectroscopies to unravel details of excited state dynamics. X-ray spectra reveal the spin and valence charge of the Ru atom and provide experimental evidence that metalcentered

excited states mediate isomerization. Complementary X-ray spectra of the functional ligand S atoms probe the nuclear structural rearrangements, highlighting the formation of two metal-centered states with different metal-ligand bonding. Our results address an essential open question regarding the relative roles of transient charge-transfer and metal-centered states in mediating photoisomerization.