Rydberg transitions in titanium sandwich complexes: Gas-phase and solution-phase photoabsorption study of (η7-C7H7)(η5-C5H5)Ti, (η8-C8H8)(η5-C5H5)Ti, and (η8-C8H8)(η5-C5Me5)Ti

The gas-phase electronic absorption spectra of (Ch)(Cp)Ti, (Cot)(Cp)Ti and (Cot)(Cp*)Ti (Cp = η5-C5H5, Cp* = η5-C5Me5, Ch = η7-C7H7, Cot = η8-C8H8) have been measured for the first time and compared with the spectra of the complexes in n-pentane solution to reveal Rydberg transitions. The gas-phase spectrum of (Ch)(Cp)Ti shows no clearly defined Rydberg bands but an increased absorbance in the 32 000–42 000 cm−1 region testifies to contribution of the e2 → R4p Rydberg excitation. The spectra of gas-phase cyclooctatetraene complexes reveal the features which correspond to the transitions originating at the non-bonding a1 HOMO and terminating at the lowest Rydberg s, p and d levels. These are the first observations of Rydberg excitations from a titanium-localised valence-shell orbital in polyatomic molecules. The a1 → R4px,y band in the (Cot)(Cp)Ti spectrum shows a vibronic structure with components separated by 230 cm−1. The intensity distribution in the vibrational progression is indicative of an unusually large change of the equilibrium metal–ligand distances on the Rydberg excitation.