Spectroscopy in the Ultraviolet and Visible Regions As Studied by Solution-Phase and Vapor-Phase Photoabsorption in the Ultraviolet and Visible Regions

The electronic absorption spectra of (η 7 -C 7 Η 7 )(η 5 -C 5 Η 5 )V and (η 7 -C 7 Η 7 )(η 5 -C 5 H 4 Me)Ta in n -heptane solution and in the vapor phase have been measured. Assignments of absorption features in the solution-phase spectra have been suggested on the basis of evaluated energies of electronic excitations. The spectra of both complexes change dramatically on going from the solution to the vapor phase due to the appearance of intense Rydberg bands which have been assigned unambiguously on the basis of their term values. All of them originate at the metal-localized d(σ+) orbital. The vapor-phase spectrum of (η 7 -C 7 Η 7 )(η 5 -C 5 Η 5 )V reveals the lowest Rydberg s(σ+) and p(σ+) excitations as well as two first members of the R n p(π) series. These Rydberg excitations are observed also in the spectrum of gas-phase (η 7 -C 7 Η 7 )(η 5 -C 5 H 4 Me)Ta, which shows additionally two absorption bands arising from the lowest Rydberg d transitions. The first member of the R n d series in (η 7 -C 7 Η 7 )(η 5 -C 5 Η 5 )V is broadened beyond detection due to an admixture of a valence shell excitation. The IE corresponding to the detachment of an electron promoted onto the vacant valence shell and Rydberg orbitals has been estimated. On this basis, an IE diagram of unoccupied orbitals including both intravalency and Rydberg levels has been constructed for the two compounds. The lowest vacant level in both complexes is composed mainly of the δ (e 2 ‘‘) orbital of the cycloheptatrienyl ring. The second empty level corresponds to the antibonding d(π) orbital in (η 7 -C 7 Η 7 )(η 5 -C 5 Η 5 )V and the lowest Rydberg s(σ + ) level in (η 7 -C 7 Η 7 )(η 5 -C 5 H 4 Me)Ta. The diagram shows that the role of Rydberg orbitals should be taken into consideration when dealing with the vapor-phase physical and chemical processes involving low-lying vacant levels of transition-metal (η7-cycloheptatrienyl)(η5-cyclopentadienyl) derivatives.