The first experimental approach to probing frontier orbitals and HOMO–LUMO gap in bent metallocenes

Combination of optical and electrochemical methods provided first experimental estimate of both HOMO and LUMO relative energies and HOMO–LUMO gap in organometallic π-complexes. Energies of redox and optical HOMO-to-LUMO electron transitions in (π-L)2MX2 (Ti, Zr, Hf) linearly correlate viz. ELMCT∝ΔEredox,Eem(0–0)∝ΔEredox. Replacement of Cl− with Me− σ-ligands increases the HOMO–LUMO gap, predominantly, by destabilization introduced in the LUMO via Me-ligands; that is experimental justification for a hypothesis by Harrigan et al. [J. Organomet. Chem. 81 (1974) 79]. Effect of introduction of larger π-ligands and a bridging group is progressive reduction of frontier orbitals energy gap.