Electronic excited states of bis(η6-arene) tungsten: an investigation by ultraviolet photoabsorption spectroscopy in the vapour and solution phases Original Research Article

The electronic absorption spectra of (Bz)2W and (Tol)2W (Bz = η6-C6H6, Tol = η6-C6H5Me) in the gas phase show comparatively narrow bands which are absent from the spectra in heptane solution. These bands form two Rydberg series converging on the first ionization potential. Series 1 is characterized by the quantum defect δ = 3.40 in both compounds. Series 2 shows δ = 3.15 in (Bz)2W and δ = 3.21 in (Tol)2W. Series 1 arises from the a1g → Rnp(e1u) transitions. Its first and second members are split into two components on going from (Bz)2W to (Tol)2W. Series 2 corresponds to the a1g → Rnp(a2u) excitations. In addition to these two series, the (Tol)2W vapour-phase spectrum contains the a1g → R6s and a1g → R6d transitions which were interpreted on the basis of their term values. The first member of the Rnp(a2u) series is broadened beyond detection by admixture of valence-shell transitions. The interaction between electronic excited states results in deviation of the position of the second Rnp(e1u) series member from that predicted by the Rydberg formula. The frequency of the lowest Rnp(e1u) transition deviates also from the theoretical value but this is mainly due to stronger penetrating properties of the corresponding terminating orbital in comparison with the higher Rydberg orbitals. The vapour-phase spectra of (Bz)2W and (Tol)2W show no Rydberg features which might be considered a consequence of spin-orbit coupling caused by the presence of the heavy metal atom.