Synthesis, structure, electrochemistry, and Mössbauer effect studies of (ring)Fe complexes (ring = Cp, Cp∗, and C6H7). Photochemical replacement of benzene in the cyclohexadienyl complex [(η5-C6H7)Fe(η-C6H6)]+

Visible light irradiation of cation [(η5-C6H7)Fe(η-C6H6)]+ (1+) in acetonitrile results in substitution of the benzene ligand giving the labile acetonitrile derivative [(η5-C6H7)Fe(MeCN)3]+ (2a+). The stable isonitrile and phosphite complexes [(η5-C6H7)FeL3]+ [L = tBuNC (2b+), P(OMe)3 (2c+), P(OEt)3 (2d+)] were obtained by reaction of 1 with L in MeCN. The structures of 2cPF6, [CpFe(η-C6H6)]PF6 (3PF6), and Cp∗Fe(η-C6H6)]PF6 (4PF6) were determined by X-ray diffraction. The redox activity of the cyclohexadienyl complexes 1+, 2b+−2d+ has been investigated by electrochemical techniques and compared with that of the related cyclopentadienyl complexes 3+ and 4+. DFT calculations of the redox potentials and the respective geometrical changes were performed. Variable temperature Mössbauer (ME) spectroscopy has elucidated the relationship between structure and formal oxidation state of the iron atom in these complexes. In the case of 3+ an unexpected pair of crystallographic changes has been observed and interpreted in terms of both a second and first order phase transition. The mean-square-amplitude-of-vibration of the metal atom has been compared between the ME and X-ray data. ME measurements in a magnetic field have shown that in 4+ the quadrupole splitting is positive as it is in ferrocene.