Electronic structure, spectroscopy, and photochemistry of group 8 metallocenes

Spectral and photochemical studies of ferrocene, ruthenocene, and several of their benzoyl-substituted derivatives have provided valuable information about the electronic structures and excited state reactivities of these group 8 metallocenes. The presence of a benzoyl group on one or both cyclopentadienyl rings of the metallocene strongly influences the electronic transitions observed in the UV–vis spectral region. While the parent complexes display low-intensity, solvent-insensitive ligand field absorption bands, the monobenzoyl and 1,1′-dibenzoyl derivatives exhibit bands that are much more intense and sensitive to the solution environment. This behavior results from the mixing of appreciable metal-to-ligand charge transfer (MLCT) character into the low-energy excited states of the benzoyl-substituted complexes. Resonance Raman data lend strong support to this MLCT assignment. Irradiation of a metallocene dissolved in ethyl 2-cyanoacrylate results in the anionic polymerization of the electrophilic monomer. Studies of the kinetics and mechanism of this photoinitiated polymerization process reveal that it occurs by two competing charge transfer-induced pathways, the relative importance of which depends upon the identity of the metal and the presence or absence of the benzoyl substituent.