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.