Electronic and Molecular Structure of the Transient Radical Photocatalyst Mn(CO)₅ and Its Parent Compound Mn₂(CO)₁₀

We present a time-resolved X-ray spectroscopic study of the structural and electronic rearrangements of the photocatalyst Mn₂(CO)₁₀ upon photocleavage of the metal-metal bond. Our study of the manganese K-edge fine structure reveals details of both the molecular structure and valence charge distribution of the photodissociated radical product. Transient X-ray absorption spectra of the formation of the Mn(CO)₅ radical demonstrate surprisingly small structural modifications between the parent molecule and the resulting two identical manganese monomers. Small modifications of the local valence charge distribution are decisive for the catalytic activity of the radical product. The spectral changes reflect altered hybridization of metal-3d, metal-4p, and ligand-2p orbitals, particularly loss of interligand interaction, accompanied by the necessary spin transition due to radical formation. The spectral changes in the manganese pre- and main-edge region are well-reproduced by time-dependent density functional theory and ab initio multiple scattering calculations.