A series of η 6-(biphenyl)-η 5-(cyclopentadienyl)iron(II) hexafluorophosphates have been prepared. Demethylation occured during the synthesis of the 2′-OMe derivatives to yield the correspoding 2′-OH product. The mechanism of this process is discussed. In all cases the complexation involved the unsubstituted phenyl ring. From 13C NMR data, values of Hammett resonance parameters σ R, were calculated which show that the [CpfeC 6H 5] + group behaves as an electron-withdrawing substituent comparable in strength to the cyano group. Approximate values of the biphenyl interplanar angle (θ) were obtained. θ appeared to be significantly lower when electron-releasing substituents were present. 57 Fe Mössbauer data support the strong electron acceptor properties of the [CpFe +C 6H 5] moiety. In particular the quadrupole splitting ( QS) shows a marked increase for the 4-OMe derivative relative to the unsubstituted comples. This is in direct contrast to the aryl ferrocenes. Here, the ferrocenyl and OMe substituents are electronically and so there is no ( QS) enhancement.