Electronic Structures of the [Fe(N₂)(SiP^iPr₃)]^+1/0/–1 Electron Transfer Series: A Counterintuitive Correlation between Isomer Shifts and Oxidation States

The electronic structure analysis of the low-spin iron(II/I/0) complexes [Fe(N2)(SiPiPr3)]+/0/– (SiPiPr3 = [Si(o-C6H4PiPr2)3]−) recently published by J. Peters et al. (Nature Chem. 2010, 2, 558–565) reveals that the redox processes stringing this electron transfer series are best viewed as metal-centered reductions, i.e. FeIIN20 → FeIN20 → Fe0N20. Superficially, the interpretation seems to be incompatible with the Mössbauer measurement, because the observed isomer shifts are more negative for the lower oxidation states, whereas typically iron-based reduction tends to increase the isomer shift. To rationalize the experimental findings, we analyzed the contributions from the 1s to 4s orbitals to the charge density at the Mössbauer nucleus and found that the positive correlation between the isomer shift and the oxidation state results from an unusual shrinking of the Fe–N2 bond upon reduction due to enhanced N2 to Fe π-backbonding. The other effects of reduction arising from shielding of the nuclear potential, decreasing covalency, and changes in the 4s population would induce the usual negative correlation. The structure distortion dictates the radial distribution of the 4s orbital and the charge density at the nucleus such that a virtually linear relationship between the isomer shift and the Fe–N2 distance could be identified for this series.