Tracking Co(I) Intermediate in Operando in Photocatalytic Hydrogen Evolution by X‑ray Transient Absorption Spectroscopy and DFT Calculation

X-ray transient absorption spectroscopy (XTA) and optical transient spectroscopy (OTA) were used to probe the Co­(I) intermediate generated in situ from an aqueous photocatalytic hydrogen evolution system, with [RuII(bpy)3]­Cl2·6H2O as the photosensitizer, ascorbic acid/ascorbate as the electron donor, and the Co-polypyridyl complex ([CoII(DPA-Bpy)­Cl]­Cl) as the precatalyst. Upon exposure to light, the XTA measured at Co K-edge visualizes the grow and decay of the Co­(I) intermediate, and reveals its Co–N bond contraction of 0.09 ± 0.03 Å. Density functional theory (DFT) calculations support the bond contraction and illustrate that the metal-to-ligand π back-bonding greatly stabilizes the penta-coordinated Co­(I) intermediate, which provides easy photon access. To the best of our knowledge, this is the first example of capturing the penta-coordinated Co­(I) intermediate in operando with bond contraction by XTA, thereby providing new insights for fundamental understanding of structure–function relationship of cobalt-based molecular catalysts.