Transition-metal complexes have long attracted interest for fundamental chemical reactivity studies and possible use in solar energy conversion1,2. Electronic excitation, ligand loss from the metal centre, or a combination of both, creates changes in charge and spin density at the metal site3,4,5,6,7,8,9,10,11 that need to be controlled to optimize complexes for photocatalytic hydrogen production8 and selective carbon–hydrogen bond activation9,10,11. An understanding at the molecular level of how transition-metal complexes catalyse reactions, and in particular of the role of the short-lived and reactive intermediate states involved, will be critical for such optimization. However, suitable methods for detailed characterization of electronic excited states have been lacking. Here we show, with the use of X-ray laser-based femtosecond-resolution spectroscopy and advanced quantum chemical theory to probe the reaction dynamics of the benchmark transition-metal complex Fe(CO)5 in solution, that the photo-induced removal of CO generates the 16-electron Fe(CO)4 species, a homogeneous catalyst12,13 with an electron deficiency at the Fe centre14,15, in a hitherto unreported excited singlet state that either converts to the triplet ground state or combines with a CO or solvent molecule to regenerate a penta-coordinated Fe species on a sub-picosecond timescale. This finding, which resolves the debate about the relative importance of different spin channels in the photochemistry of Fe(CO)5 (refs 4, 16,17,18,19 and 20), was made possible by the ability of femtosecond X-ray spectroscopy to probe frontier-orbital interactions with atom specificity. We expect the method to be broadly applicable in the chemical sciences, and to complement approaches that probe structural dynamics in ultrafast processes.
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This work was supported by the Volkswagen Stiftung (M.B.) the Swedish Research Council (M.O.), the Carl Tryggers Foundation (M.O.), the Magnus Bergvall Foundation (M.O.), the Collaborative Research Centers SFB 755 and SFB 1073 (I.R., S.G., W.Q., M.S. and S.T.) and the Helmholtz Virtual Institute ‘Dynamic Pathways in Multidimensional Landscapes’. W.Z., R.W.H. and K.J.G. acknowledge support through the AMOS program within the Chemical Sciences, Geosciences, and Biosciences Division of the Office of Basic Energy Sciences, Office of Science, US Department of Energy. Portions of this research were performed on the Soft X-ray Materials Science (SXR) Instrument at the Linac Coherent Light Source (LCLS), a division of SLAC National Accelerator Laboratory and an Office of Science user facility operated by Stanford University for the US Department of Energy. The SXR Instrument is funded by a consortium whose membership includes the LCLS, Stanford University through the Stanford Institute for Materials Energy Sciences (SIMES), Lawrence Berkeley National Laboratory (LBNL), the University of Hamburg through the BMBF priority program FSP 301, and the Center for Free Electron Laser Science (CFEL).
The authors declare no competing financial interests.
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Wernet, P., Kunnus, K., Josefsson, I. et al. Orbital-specific mapping of the ligand exchange dynamics of Fe(CO)5 in solution. Nature 520, 78–81 (2015). https://doi.org/10.1038/nature14296
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