for the Fe 3+/2+ couple in a four-coordinate SNS pincer complex with an NHC as the fourth ligand, and 1.4 V more negative than the analogous five-coordinate SNS scaffold with two THF molecules. The more negative reduction potential in our system may result from the negativelycharged aryl pincer in place of the pyridine pincer, and also from the coordination of three strong phosphine donors. Although the N-H BDFE of 3 is significantly lower than those of free thioamides, it is comparable to other iron systems with N-H bonds in the supporting ligand. In a recent example, the N-H BDFEs of aminobenzenethiolate iron(II) complexes supported by a TACN ligand were determined by measurement of pKa and E1/2. In the parent system, the Fe 3+/2+ redox couple was found at E1/2 = -0.84 V, and the pKa was determined to be ≥ 19.5, giving a BDFE of ≥ 62 kcal/mol; bracketing showed an exact BDFE near 68 kcal/mol. Studies of nickel, platinum, and ruthenium SCS compounds also corroborate decreased N-H bond acidity and more negative reduction potential upon metalation, though these examples do not report bond energies. There are interesting implications of our determination that deprotonating a distant thioamide site gives a highly-reducing iron(II) species, because this could be a mechanism for generating electron-rich active sites in metalloenzymes. This fits into a growing body of evidence that reduction potentials can may be modulated by the protonation state of sites outside the vicinity of metals in metallocofactors like the nitrogenase FeMoco. Electronic Structure. The EPR spectrum of 9 is notable for its high rhombicity. While the spectrum resembles a few other S = 3/2 examples of well-defined iron complexes with large E/D, the most notable comparison is to a square planar S = 3/2 cobalt(II) ONO pincer complex with a chloride fourth donor. Structurally, this dianionic complex is reminiscent of 9, with the ONO pincer coordinating through a central amido N donor and lithium counterions flanking the alkoxide arms of the pincer ligand. The EPR spectrum of this complex was also highly rhombic, with E/D ≥ 0. Fe (~10 -8 s) but slower than the ~10 -12 s that would give negligible paramagnetic broadening. The smaller line widths of 9 relative to those in 8 and 10 indicate that paramagnetic relaxation is faster, which is attributable to its larger E/D that mixes the Kramers doublets. To our knowledge, the quadrupole splitting values of 9 and 9-crown are the lowest reported for any intermediate-spin iron(III) complex, though anomalously small quadrupole splitting has also been reported in a few S = 2 iron(II) complexes with planar ligand environments. Planar ligand scaffolds can induce large positive contributions to the electric field gradient (EFG) on the axis perpendicular to the ligand plane (usually assigned as the z axis), and the Mössbauer quadrupole splitting is proportional to the z-component of the EFG. Interestingly, computational analysis of the cited systems with anomalously small ΔEQ also revealed a doubly-occupied dz 2 orbital, as we found for 9 (Figure ). Thus, the small ΔEQ values imply that the large positive contribution to the z-component of the electric field gradient by the planar ligand is counteracted by a large negative contribution from the dz 2 ground state. Double occupation of the dz 2 orbital also accounts for the decreased isomer shift (which decreases with increasing iron s-electron density) of 9 by greater mixing of the doubly-occupied 3dz 2 with the 4s orbital, which is corroborated by our DFT calculations.