Conclusive evidence is presented for an S = 1/2 spin-coupled pair of high spin ferric and ferrous ions in the major reaction product of sulfide with the met Fe(III), Fe(III) form of the non-heme iron, oxygen-carrying protein hemerythrin (Hr). The resulting derivative is referred to as (mu)-S('2-)semi-metHr. Mossbauer and EPR spectroscopies establish that for this derivative (a) the charge and spin states of the individual iron atoms are Fe(III), S = 5/2, and Fe(II), S = 2, (b) an antiferromagnetic exchange interaction couples the two spins to a resultant spin S = 1/2 and (c) the majority ((GREATERTHEQ)90 %) of the iron centers in the protein can be stabilized at this oxidation level. Evidence for an analogous selenide derivative is also reported. Titrations of metHr with sulfide or with selenide indicate that the resulting semi-met chalcogenide derivative forms by a two-step process consisting of one-electron reduction to the semi-met Fe(III), Fe(II) oxidation level followed by incorporation of chalcogenide into the iron site.;This characterization of (mu)-S('2-)semi-metHr led to the discovery that (mu)-S('2-)metHr can be obtained by one-electron oxidation of (mu)-S('2-)semi-metHr. Chemical analyses show that (mu)-S('2-)metHr contains one sulfide per two Fe. The Mossbauer and resonance Raman spectra and the reactivity of this derivative are consistent with replacement of the (mu)-oxo bridge between the irons with a single sulfide. Resonance Raman spectroscopy strongly suggests that (mu)-S('2-)semi-metHr has the same iron site geometry as (mu)-S('2-)metHr.;The midpoint redox potentials for the (mu)-S('2-)metHr/(mu)-S('2-)semi-metHr couple are in the range of 283-312 mV versus NHE and depend on the presence or absence of perchlorate. This range is about 200 mV more positive than the met/semi-met potential where a (mu)-oxo bridge is present. Studies of the reactivities of both (mu)-S('2-)methr and (mu)-S('2-)semi-metHr under oxidizing and reducing conditions and with ligand anions such as azide are presented and compared to those of their (mu)-oxo-bridged counterparts. Comparisons of the "artificial" iron-sulfur center in the (mu)-S('2-)Hrs with naturally-occurring iron-sulfur clusters and sulfur metabolizing systems are discussed.