Photoabsorption study of electronic excited states of (η 5-cyclopentadienyl)(η 6-arene)manganese

Abstract

The electronic absorption spectra of the (Cp)(arene)Mn compounds (Cp = η 5-C 5H 5, arene = η 6-C 6H 6, η 6-C 6CH 3, η 6-1,3-C 6H 4(CH 3) 2) in the vapour phase and in pentane solution have been investigated for the first time. Most of the sharp bands revealed by vapour-phase photoabsorption disappear from the solution spectra. Electronic excited states corresponding to these bands have a Rydberg origin. The vapour-phase spectra show electron transitions from the non-bonding 3d(σ +) orbital to ns, np, and nd Rydberg levels ( n = 4, 5, 6). These excitations have been assigned on the basis of their quantum defects and term values. The Rydberg structure in the spectrum of (η 5-cyclopentadienyl)(η 6-benzene)manganese can be adequately described under the C ∞ν point group. The introduction of methyl substituents into the benzene ring disturbs the axial symmetry. This is indicated by the splitting of the Rydberg degenerate 4p(π) orbital. On methyl substitution, the energy of the lowest R4s(σ( +) state is shifted in a way which is parallel to that of the neutral-molecule ground electronic state. At the same time, R5p, R5d, and R6p levels change in line with the ground electronic state of the [(Cp)(arene)Mn] + cation. These facts have been interpreted in terms of the size and the penetrating properties of the Rydberg orbitals.