Berry pseudorotation enabled photodissociation of Fe(CO)5 observed by ultrafast electron diffraction

The structural dynamics of metal carbonyls are central to processes ranging from catalysis to organometallic synthesis. Here we investigate the photodissociation of a prototypical transition metal carbonyl, Fe(CO)5, using mega-electron-volt ultrafast electron diffraction. By separately tracking structural evolution along the axial and equatorial directions, we provide an atomistic, angle-resolved view of the nuclear motions preceding CO dissociation and infer key features of the excited-state potential energy surface from the experimental observations. We further show that vibrational coupling before reaching the conical intersection facilitates the loss of a random carbonyl ligand via the Berry pseudorotation mechanism. Iron pentacarbonyl undergoes CO dissociation under UV excitation. While prior works suggested preferential loss of axial ligands, MeV ultrafast electron diffraction experiments show photolysis affects random CO ligands via a Berry pseudorotation.