Systematic trends of the 57Fe Mössbauer isomer shifts in (FeOn) and (FeFn) polyhedra. Evidence of a new correlation between the isomer shift and the inductive effect of the competing bond T-X (→ Fe) (where X is O or F and T any element with a formal positive charge)

The isomer shift of 57Fe has long been known to provide information about the covalency of the iron-ligand bonds. A correlation exists between the isomer shift, δ, and the electronegativity of the ligand. A decrease of the iron coordination generally leads to a decrease of δ. Moreover, it has been suggested that the length of the iron-ligand bond has some influence on the isomer shift. To these various “rules,” however, there are many notable exceptions, a priori unexplainable. In order to try to interpret these apparent anomalies, the extent to which the second-order Doppler shift contributes to the experimental isomer shift is first briefly recalled. Then, a comparative study over a very large number of compounds is described. The main conclusions of this study are the following: (1) Some of the trends which have been accepted up to now are well verified: (a) a systematic decrease of δ with the oxidation state of iron, (b) a decrease of δ in general with the coordination of iron, (c) a decrease of δ in general with the electronegativity of the ligand. (2) The correlation between the isomer shift and the length of the iron-ligand bonds appears, in most cases, groundless. (3) The concept of the inductive effect of the antagonistic bond T-X (→ Fe) (X = O, F and T = any element with a formal positive charge), which implies an increase of the ionic character of the Fe-X bond for an increase of the covalency of the T-X bond (and vice versa) explains most of the anomalies occurring in the scale of the isomer shifts. A much closer correlation between the isomer shift and the degree of covalency of the Fe-X bonds (X = O, F) is achieved when the inductive effect of the antagonistic bond is taken into account.