Electronic structure of strained silicon- and sulfur-bridged [1]ferrocenophanes and an analogous dicarbon-bridged [2]ferrocenophane: An investigation by photoelectron spectroscopy and density-functional theory

He I and He II photoelectron (PE) spectra of [Fe(eta-C5H4)(2)SiMe2] (1), [Fe(eta-C5H3Me)(2)SiMe2] (2), [Fe(eta-C5H4)(eta-C5Me4)SiMe2] (3), [Fe(eta-C5Me4)(2)SiMe2] (4), [Fe(eta-C5H4)(2)C2H4] (5), and [Fe(eta-C5H4)(2)S] (6) have been measured and assigned. The d bands of 1-5 show less structure than that of ferrocene, consistent with a loss of degeneracy of the e(2) orbitals on bending. Compound 6, which has the largest inter-ring angle of the series, shows two separate d bands. The trend in the first ionization energy closely parallels the variation in oxidation potential. Density functional calculations on 1, 5, and 6 give geometries in good agreement with the structures found from X-ray diffraction. Ionization energies calculated were also in excellent agreement with experiment. Good agreement was also found between the calculated d-d transitions and the position of the first spin-allowed band in the optical spectra of 1 and 6. Estimates of strain energy in bending ferrocene and octamethylferrocene were obtained, and octamethylferrocene was shown to be significantly more difficult to bend. Compounds 1 and 6 were both shown to have a low-lying empty orbital, partially located on the ipso carbon, which is a possible site for nucleophilic attack in the polymerization process undergone by these compounds.