“…In particular, the qcyclopentadienyl (Cp) derivatives [CpFeL2R] (L = CO, PR3, R = alkyl, acyl, silyls) have been examined in great detail. Infrared spectroscopy provided evidence for the rotamers and allowed the thermodynamic properties to be determined experimentally (3c, 3e, 4), with the interpretations aided by theoretical studies using Extended Hiickel, a b initio, and molecular modelling calculations (4)(5)(6). There has been some controversy about the precise interpretation (4,5), but it is currently thought that the more stable rotamers are A and B, with C at higher energy.…”