Experimental investigation of the electronic structures of enneacarbonylbis(.mu.3-X-methylidyne)triiron complexes (X = H, F, Cl, Br) by means of He I/He II gas-phase UV photoelectron spectroscopy

The electronic structures of three novel trinuclear clusters of iron {[Fe3(CO)9(CX)2], X = H, Cl, Br} have been investigated by means of He I/He II gas-phase UV photoelectron spectroscopy. A detailed assignment of the photoelectron spectra of the title molecules has been achieved by making use of the first principle theoretical calculations recently published for the isoelectronic and isostructural [Fe3(CO)9(CF)2]. The experimental data confirmed that both the e(g)-like and the t2g-like levels of the Fe3(CO)9 fragment actively participate in the interaction between the Fe3(CO)9 and the (CX)2 moieties. Furthermore, the analysis of the relative intensity variations on passing from the He I to the He II ionizing source allowed us to discriminate among the different extents of bonding between the two fragments along the series, confirming, at the same time, the leading role played by the [Fe3(CO)9]-C(apical) interaction in stabilizing the whole cluster. Interestingly, in the [Fe3(CO)9(CH)2] and [Fe3(CO)9(CBr)2] derivatives at least one component of the two C(apical) lone pairs has been experimentally detected. Additionally, in the He II spectrum of the former compound it has been identified as the band due to the ionization from the molecular orbital responsible for the direct Fe-Fe sigma bond. The presence of a common bonding scheme for the investigated molecules agrees fairly well with electrochemical measurements.