Spontaneous transformation of the thermally stable [HS](-)bound {Fe(NO)(2)}(9) dinitrosyl iron complex (DNIC) [(HS)(2)Fe(NO)(2)](-) (1) into [(NO)(2)Fe(mu-S)](2)(2-) (Roussin's red salt (RRS)) along with release of H2S, probed by NBD-SCN (NBD = nitrobenzofurazan), was observed when DNIC 1 was dissolved in water at ambient temperature. The reversible transformation of RRS into DNIC 1 (RRS -> DNIC 1) in the presence of H2S was demonstrated. In contrast, the thermally unstable hydrosulfide-containing mononitrosyl iron complex (MNIC) [(HS)(3)Fe-III(NO)](-) (3) and [Fe-III(SH)(4)](-) (5) in THF/DMF spontaneously dimerized into the first structurally characterized Fe-III-hydrosulfide complexes [(NO)(SH)Fe(mu-S)](2)(2-) (4) with two {Fe(NO)}(7) motifs antiferromagnetically coupled and [(SH)(2)Fe(mu-S)](2)(2-) (6) resulting from two Fe-III (S = 5/2) centers antiferromagnetically coupled to yield an S = 0 ground state with thermal occupancy of higher spin states, respectively. That is, the greater the number of NO ligands bound to [2Fe2S], the larger the antiferromagnetic coupling constant. On the basis of DFT computation and the experimental (and calculated) reduction potential (E-1/2) of complexes 1, 3, and 5, the NO-coordinate ligand(s) of complexes 1 and 3 serves as the stronger electron-donating ligand, compared to thiolate, to reduce the effective nuclear charge (Z(eff)) of the iron center and prevent DNIC 1 from dimerization in an organic solvent (MeCN).