OPTICAL-ABSORPTION STUDY OF HALF-OPEN FERROCENE AND OPEN FERROCENE

The electronic absorption spectra of vaporous (eta(5)-cyclopentadienyI )(eta 5-2,4-dimethylpentadienyl)iron (1) and bis(eta(5)-2,4-dimethylpe ntadienyl)iron (2) have been measured and compared with those of the c omplexes in pentane solution as well as with the vapor-phase spectrum of 1,1'-dimethylferrocene (3). The photoabsorption of vaporous 1 shows two shoulders at 33 000 and 35 400 cm(-1) which disappear on going to the condensed phase and have the term values appropriate for their as signment as the lowest ns and np Rydberg transitions, respectively. Th ese excitations originate at the molecular orbital (MO) corresponding to the first peak in the photoelectron spectrum of 1. The data obtaine d indicate that ''half-open ferrocene'' (1) is between ''open ferrocen e'' (2) and ''closed ferrocene'' (3) with respect to the 3d character of their MO's.