Chemical bonding alteration upon electronic excitation in transition metal complexes

Chemical bonding alteration accompanying various processes, such as direct fast ligand dissociation, bond formation, isomerization, linkage isomerism or ring closing/opening in a wide range of transition metal complexes coordinated to specific ligands is discussed on the basis of electronic density fluctuations upon UV/vis irradiation. Based on recent experimental studies completed by computational examination, it is shown that simple analysis in terms of bonding and anti-bonding orbitals involved in one-electron excitations is too limited to decipher the complicated mechanisms that underlie light-activated reactive channels leading to unsaturated metal complexes, meta-stable conformers or photo-switches, the properties of which are used in fast-evolving fields of chemistry, biology and advanced materials. Combining time-resolved spectroscopy and various methods of quantum chemistry and dynamics should improve our knowledge of the early events occurring within the first ps. Within this time-scale the original electronic density will evolve rapidly following reactive channels or being trapped in minima, the branching ratio between the different decay pathways being controlled by the active vibrational normal modes of the molecule in its specific environment. As soon as the molecule is trapped into the low-lying potential energy surfaces associated with the triplet states its journey can be chaotic due to shallow minima.