Qualitative interpretation of Mössbauer data for some [1] ferrocenophanes; Fe–Pd dative bonding in [(SC5H4)2FePd (PPh3)] and Fe–Hg and Fe–H+ bonding in ferrocene

Abstract

The Mössbauer data for a number of ferrocene complexes in which the Fe is directly bonding to or interacting with an additional atom are discussed. For weak ferrocenyl Fe–X (X = Ge, Si, P, or Pd) interactions in [1]ferrocenophanes or 1,2,3-trithia[3] ferrocenophanes quadrupole splittings (Δ) around 0.4 mm s^–1 smaller than that of ferrocene are observed. These are explained by the interactions of the ferrocene electrons with the X atom. Exalted Δ values relative to ferrocene are found for ferrocenyl Fe (and substituted ferrocenophanes) bonded to mercury, and for ferrocenyl Fe bonded to H⁺. These are explained by ring tilt altering the molecular orbital bonding scheme relative to ferrocene, causing the 3d_z² orbital to partake in the bonding. This orbital makes a negative contribution to Δ so that if electron density is removed from it then Δ will increase. Compounds of Pd^II and Pt^II trichalcogeno[n] ferrocenophanes (n= 7 or 9) which show little variation in quadrupole splitting from ferrocene (but do differ by having higher isomer shifts) are also explained as having metal–iron bonds.