To examine how small structural changes influence the reactivity and magnetic properties of biologically relevant metal complexes, the reactivity and magnetic properties of two structurally related five-coordinate Fe(III) thiolate compounds are compared. (Et,Pr)-ligated [Fe(III)((S2N3)-N-Me2(Et,Pr))]PF6 (3) is synthesized via the abstraction of a sulfur from alkyl persulfide ligated [Fe(III)((S2N3)-N-Me2(Et,Pr))-S-pers]PF6 (2) using PEt3. (Et,Pr)-3 is structurally related to (Pr,Pr)-ligated [Fe(III)((S2N3)-N-Me2(Pr,Pr))]PF6 (1), a nitrile hydratase model compound previously reported by our group, except it contains one fewer methylene unit in its ligand backbone. Removal of this methylene distorts the geometry, opens a S-Fe-N angle by similar to10degrees, alters the magnetic properties by stabilizing the S = 1/2 state relative to the S = 3/2 state, and increases reactivity. Reactivity differences between 3 and 1 were assessed by comparing the thermodynamics and kinetics of azide binding. Azide binds reversibly to both (Et,Pr)-3 and (Pr,Pr)-1 in MeOH solutions. The ambient temperature K-eq describing the equilibrium between five-coordinate 1 or 3 and azide-bound 1-N-3 or 3-N-3 in MeOH is similar to10 times larger for the (Et,Pr) system, In CH2Cl2, azide binds -3 times faster to 3 relative to 1, and in MeOH, azide dissociates 1 order of magnitude slower from 3-N-3 relative to 1-N-3. The increased on rates are most likely a consequence of the decreased structural rearrangement required to convert 3 to an approximately octahedral structure, or they reflect differences in the LUMO (vs SOMO) orbital population (i.e., spin-state differences). Dissociation rates from both 3-N-3 and 1-N-3 are much faster than one would expect for low-spin Fe-III. Most likely this is due to the labilizing effect of the thiolate sulfur that is trans to azide in these structures.