ORIGINS OF STRUCTURE IN THE E(1) IONIZATION BANDS OF MAGNESOCENE AND OSMOCENE - A PHOTOELECTRON SPECTROSCOPIC STUDY OF M(ETA-C5D5)(2), M=MGAND OS

HeI photoelectron spectra of the e(1) bands of M(eta-C5D5)(2), M = Mg and Os, are reported and compared with those of the undeuterated analo gues. In both cases, the observed fine structure is identical to that of the protonated analogue ruling out any C-H vibrational excitation a s its cause. The infrared spectra of both deuterated molecules are als o reported. In the case of magnesocene, a vibrational sequence of 1452 +/- 200 cm(-1) is observed in the e(1g) ionisation band, which corres ponds most closely with that found for the E C-C vibrations for the pa rent molecule. The E(2g) vibration is expected to be the Jahn-Teller a ctive vibration in the molecular ion in a (2)E(1g) excited state. This indicates that Jahn-Teller distortion of [Mg(eta-C5H5)(2)](+) is dyna mic rather than static, The e'(1) ionisation band of Os(eta-C5H5)(2) a nd Os(eta-C5D5)(2) shows a second maximum 0.37 +/- 0.005 eV above the vertical ionisation energy, together with other fine structure. The fi ne structure in this case is assigned to spin-orbit coupling as a resu lt of an Os p contribution to the e'(1) wave function. The departure f rom the anticipated 1-to-1 intensity is ascribed to the Ham effect.