A review on molecular electrochemistry of metallocene dichloride and dimethyl complexes of group 4 metals: Redox properties and relation with opticalligand-to-metal charge transfer transitions

Emphasis is given to redox, photophysical, and photochemical properties of homologous bent metallocenes of group 4 transition metals. Comparative anal ysis of a variety of electron-transfer induced transformations and ligand-t o-metal charge-transfer excited states is performed for bent metallocene co mplexes upon systematic variation of the identity of the metal ion (Ti, Zr or Hf), ancillary pi- and monodentate sigma- (Cl, Me) ligands. For such org anometallic pi -complexes, linear correlations exist between energies of op tical and redox HOMO-to-LUMO electron transitions. It is suggested that com bination of spectroscopic and electrochemical techniques provides important diagnostics to determine "ionisation potential" and "electron affinity" in solution (relative energies of frontier molecular orbitals obtained as red ox potentials) and the energy gap in metallocene complexes. Some of earlier instructive cases of direct relationship between optical transition energi es and differences in redox potentials revealed for inorganic and coordinat ion compounds are discussed.