Photoinduced bond oscillations in ironpentacarbonylFe(CO)5 give delayed, synchronous bursts of carbonmonoxide(CO) release

The early excited state dynamics in the photodissociation of transition metal carbonyls determines the chemical nature of short-lived catalytically active reaction intermediates. However, time-resolved experiments have not yet revealed the mechanistic details in the sub-picosecond regime. Hence in this study, the photoexcitation of ironpentacarbonyl Fe(CO)5 has been simulated by semi-classical surface-hopping excited state molecular dynamics based on time-dependent density functional theory. We find that the bright metal-to-ligand charge-transfer (MLCT) transition induces Fe-C oscillations in the trigonal bipyramidal complex leading to periodically reoccurring release of predominantly axial CO. Metaphorically the photoactivated Fe(CO)5 acts as a CO geyser, which we explain in terms of dynamics in the potential energy landscape of the axial Fe-C distances and non-adiabatic transitions between manifolds of bound MLCT and dissociative metal-centered(MC) excited states. The predominant release of axial CO ligands and a delayed release of equatorial CO ligands are explained in an unified mechanism based on the (sigma*)(Fe-C) anti-bonding character of the receiving orbital in the dissociative MC states both in the Franck-Condon region and at distorted geometries.