ULTRAVIOLET VAPOR-PHASE ABSORPTION-SPECTRA OF D6-METALLOCENES

The electronic absorption spectra of vaporous Cp2M, Cp2*M (Cp = eta-5-C5H5, Cp* = eta-5-C5Me5; M = Fe, Ru, Os) and CpRuCp* have been investigated and compared with those of complexes in pentane solution. Vapour-phase spectra differ strongly from those measured in solution. This is due to the presence of Rydberg excitations in the spectra of vaporous metallocenes. The Cp2M spectra show two sharp absorption bands which disappear on going from the vapour phase to the solution phase. They have been unambiguously assigned to the symmetry allowed nd(a1g) --> R(n + 1)p(e1u) and nd(a1g) --> R(n + 1)p(a2u) Rydberg transitions (n = 3, 4, 5 for M = Fe, Ru, Os, respectively). One peak in the vapour-phase spectra of Cp2*M corresponds to these promotions. Two Rydberg bands have been found in the spectrum of vaporous CpRuCp*. The excitations from the 4d(a1) nonbonding orbital to the R5s and R5p Rydberg orbitals are responsible for these bands. The electronic absorption spectra of d6 metallocenes in the vapour phase, in contrast to those in solution, appear to be sensitive to the changes in molecular symmetry.