To compare the electronic structures of the computational models whose Mössbauer parameters match experiment to those that do not, we plotted the frontier quasi-restricted orbitals (QROs) for models A (reproduces experimental Mössbauer parameters; all atoms optimized with K + ) and C (does not reproduce experimental Mössbauer parameters; all atoms optimized without K + ) in Figure . The QROs show that the shape of the doubly-occupied d orbital (DOMO) for model A resembles dz 2 while the doubly-occupied d orbital for model C resembles dyz. The dz 2 DOMO has 3.7% Fe s character, while the dyz DOMO has 0.0% Fe s character, explaining the significant change in the isomer shift. The ordering of the dxz and dxy orbitals is also switched, but the dx 2 -y 2 orbital is always higher in energy than the remaining four d orbitals due to its s antibonding interactions with the SCS pincer. QRO analysis of optimized 9-crown shows the same d orbital ordering as model A (all atoms optimized with K + ), though dz 2 is 0.90 eV more stabilized than dxy in 9-crown. (Figure ) Additional experiments were performed to probe the influence of the cation on the electronic structure of 9. First, we computationally sampled a wide range of potential locations for a positive charge to learn about the orientational dependence, but the results were not conclusive (Figure ). In addition, we experimentally sought to "remove" the potassium completely by reacting starting material 5-Et2O with N(TMS)2 in the presence of excess (two equiv) of for free thioamides (90-100 kcal/mol), and its pKa of ~21 indicates that the N-H proton is many orders of magnitude less acidic than free thioamides, which lie in the range 11-15 (Figure ). It is seen from the Bordwell equation that the high pKa of 2 raises the BDFE relative to organic thioamides, but the very negative reduction potential (E1/2 = -1.42 V vs. Fc + /Fc) of the iron(II) center lowers the BDFE to a much larger extent. This potential is 1.0 V more negative than E1/2