Detailed spectroscopic and computational studies of the low-spin iron complexes [Fe
III(S
2Me2N
3(Pr,Pr))(N
3)] (
1) and[Fe
III(S
2Me2N
3(Pr,Pr))]
1+ (
2) were performed to investigate the unique electronic features of these species and theirrelation to the low-spin ferric active sites of nitrile hydratases. Low-temperature UV/vis/NIR and MCD spectra of
1and
2 reflect electronic structures that are dominated by antibonding interactions of the Fe 3d manifold and theequatorial thiolate S 3p orbitals. The six-coordinate complex
1 exhibits a low-energy S
Fe 3d
xy (~13000 cm
-1)charge-transfer transition that results predominantly from the low energy of the singly occupied Fe 3d
xy orbital, dueto pure
interactions between this acceptor orbital and both thiolate donor ligands in the equatorial plane. The3d
3d
ligand-field transitions in this species occur at higher energies (>15000 cm
-1), reflecting its near-octahedral symmetry. The Fe 3d
xz,yz Fe 3d
xy (d
d
) transition occurs in the near-IR and probes the Fe
III-S
-donor bond; this transition reveals vibronic structure that reflects the strength of this bond (
e 340 cm
-1). Incontrast, the ligand-field transitions of the five-coordinate complex
2 are generally at low energy, and the S
Fecharge-transfer transitions occur at much higher energies relative to those in
1. This reflects changes in thiolatebonding in the equatorial plane involving the Fe 3d
xy and Fe 3d
x2-y2 orbitals. The spectroscopic data lead to asimple bonding model that focuses on the
and
interactions between the ferric ion and the equatorial thiolateligands, which depend on the S-Fe-S bond angle in each of the complexes. These electronic descriptions provideinsight into the unusual
S =
1/
2 ground spin state of these complexes: the orientation of the thiolate ligands inthese complexes restricts their
-donor interactions to the equatorial plane and enforces a low-spin state. Theseanisotropic orbital considerations provide some intriguing insights into the possible electronic interactions at theactive site of nitrile hydratases and form the foundation for further studies into these low-spin ferric enzymes.