Combination of optical and electrochemical methods provided first experimental estimate of both HOMO and LUMO relative energies and HOMO-LUMO gap in organometallic rc-complexes. Energies of redox and optical HOMO-to-LUMO electron transitions in (pi-L)(2)MX2 (Ti, Zr, Hf) linearly correlate viz. E-LMCT proportional to DeltaE(redox), E-em(0-0) proportional to DeltaE(redox), Replacement of Cl- with Me-sigma-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 pi-ligands and a bridging group is progressive reduction of frontier orbitals energy gap. (C) 2002 Elsevier Science B.V. All rights reserved.