The reversible redox transformations [(NO)(2)Fe(StBu)(2)](-) reversible arrow [Fe(mu-(SBu)-Bu-t)(NO)(2)](2-) reversible arrow [Fe(mu-(SBu)-Bu-t)(NO)(2)](2-) reversible arrow [Fe(mu-(SBu)-Bu-t)(NO)(2)](2) and [cation][(NO)(2)Fe(SEt)(2)] reversible arrow [cation](2)[(NO)(2)Fe(SEt)(2)] (cation = K+-18-crown-6 ether) are demonstrated. The countercation of the {Fe(NO)(2)}(9) dinitrosyliron complexes (DNICs) functions to control the formation of the {Fe(NO)(2)}(10){Fe(NO)(2)}(10) dianionic reduced Roussin's red ester (RRE) [PPN](2)[Fe(mu-SR)(NO)(2)](2) or the {Fe(NO)(2)}(10) dianionic reduced monomeric DNIC [K+-18-crown-6 ether](2)[(NO)(2)Fe(SR)(2)] upon reduction of the {Fe(NO)(2)}(9) DNICs [cation][(NO)(2)Fe(SR)(2)] (cation = PPN+, K+-18-crown-6 ether; R = alkyl). The binding preference of ligands [OPh](-)/[SR](-) toward the {Fe(NO)(2)}(10) {Fe(NO)(2)}(10) motif of dianionic reduced RRE follows the ligand-displacement series [SR](-) > [OPh](-). Compared to the Fe K-edge preedge energy falling within the range of 7113.6-7113.8 eV for the dinuclear {Fe(NO)(2)}(9){Fe(NO)(2)}(9) DNICs and 7113.4 -7113.8 eV for the mononuclear {Fe(NO)(2)}(9) DNICs, the {Fe(NO)(2)}(10) dianionic reduced monomeric DNICs and the {Fe(NO)(2)}(10){Fe(NO)(2)}(10) dianionic reduced RREs containing S/O/N-ligation modes display the characteristic preedge energy 7113.1-7113.3 eV, which may be adopted to probe the formation of the EPR-silent {Fe(NO)(2)}(10) -{Fe(NO)(2)}(10) dianionic reduced RREs and {Fe(NO)(2)}(10) dianionic reduced monomeric DNICs in biology. In addition to the characteristic Fe/S K-edge preedge energy, the IRv(NO) spectra may also be adopted to characterize and discriminate [(NO)(2)Fe(mu-(SBu)-Bu-t)](2) [IRv(NO) 1809 vw, 1778 s, 1753 s cm(-1) ((KBr)], [Fe(mu-S(t)Bo)(NO)(2)](2)(-) [IRv(NO) 1674 s, 1651 s cm(-1) (KBr)], [Fe(mu-(SBu)-Bu-t)(NO)(2)](2)(2-) [IRv(NO) 1637 m, 1613 s, 1578 s, 1567 s cm(-1) (KBr)], and [K-18-crown-6 ether](2)[(NO)(2)Fe(SEt)(2)] [IRv(NO) 1604 s, 1560 s cm(-1) (KBr)].