PHOTOABSORPTION STUDY OF ELECTRONIC EXCITED-STATES OF (ETA(5)-CYCLOPENTADIENYL)(ETA(6)-ARENE)MANGANESE

The electronic absorption spectra of the (Cp)(arene)Mn compounds (Cp = eta(5)-C5H5, arene = eta(6)-C6H6, eta(6)-C6H6CH3, eta(6)-1,3-C6H4(CH3 )(2)) in the vapour phase and in pentane solution have been investigat ed for the first time. Most of the sharp bands revealed by vapour-phas e photoabsorption disappear from the solution spectra. Electronic exci ted states corresponding to these bands have a Rydberg origin. The vap our-phase spectra show electron transitions from the non-bonding 3d(ac ) orbital to ns, np, and nd Rydberg levels (It = 4, 5, 6). These excit ations have been assigned on the basis of their quantum defects and te rm values. The Rydberg structure in the spectrum of (eta(5)-cyclopenta dienyl)(eta(6)-benzene)manga can be adequately described under the C-i nfinity v point group. The introduction of methyl substituents into th e benzene ring disturbs the axial symmetry. This is indicated by the s plitting of the Rydberg degenerate 4p(pi) orbital. On methyl substitut ion, the energy of the lowest R4s(sigma(+)) state is shifted in a way which is parallel to that of the neutral-molecule ground electronic st ate. At the same time, R5p, R5d, and R6p levels change in line with th e ground electronic state of the [(Cp)(arene)Mn](+) cation. These fact s have been interpreted in terms of the size and the penetrating prope rties of the Rydberg orbitals.