Quality of Structures
As noted in the literature, a good prediction of structures is essential for high quality predictions of Mössbauer parameters. In particular the isomer shift is sensitive to the metal-ligand bond distance.72 The geometries of all structures were optimized with the closest available crystal structures as starting points (see Table 1 for CCDC codes). Overall, when assessing the quality of optimized structures where the exact crystal structure is available, the agreement is excellent with an average absolute deviation of the bond lengths of 0.010 Å and a maximum deviation of 0.041 Å (see SI, Tables S1–S3).
We note that for complex 5 , the available crystal structure contains a phenoxy ligand with two chloride ligands inortho -position which were replaced with hydrogen atoms for the geometry optimization. Accordingly, the iron-oxygen-carbon angle changes from 132° in the crystal structure to 111° in the optimized structure, presumably due to lower steric demands of the axial ligand. The concomitant increase in equatorial bond lengths up to 0.068 Å should be viewed in light of the change in interaction with the axial ligand. For complex 6 , two of the equatorial Fe-N bond lengths change by +0.053 Å and –0.047 Å upon optimization. This is likely due to a rotation of the axial ligands about the Fe-NNCS bonds relative to the crystal structure, which shows some disorder in the positions of the axial ligand atoms. For complex 7 , only the crystal structure of the Fe(III) complex is available. While the bond lengths of iron with the equatorial ligands increase between 0.024–0.069 Å, the elongation of the distances to the axial ligands is much more pronounced with 0.108 Å and 0.112 Å. These variations are within expectations for an oxidation state change. For complexes17 and 18 , the closest available crystal structure was of the parent azide complexes, which upon photolysis yield the high-valent iron nitrido complexes utilised here as spectroscopically characterized intermediates.108 While the equatorial bond lengths are again found to agree with the crystal structure to within ca. 0.07 Å, the bond lengths to the axial ligands should obviously not be compared due to the drastic change in bonding situation.