Addition and redox reactivity of hydrogen sulfides (H2S/HS⁻) with nitroprusside: new chemistry of nitrososulfide ligands.

The nitroprusside ion [Fe(CN)(5)NO](2-) (NP) reacts with excess HS(-) in the pH range 8.5-12.5, in anaerobic medium ("Gmelin" reaction). The progress of the addition process of HS(-) into the bound NO(+) ligand was monitored by stopped-flow UV/Vis/EPR and FTIR spectroscopy, mass spectrometry, and chemical analysis. Theoretical calculations were employed for the characterization of the initial adducts and reaction intermediates. The shapes of the absorbance-time curves at 535-575 nm depend on the pH and concentration ratio of the reactants, R=[HS(-)]/[NP]. The initial adduct [Fe(CN)(5)N(O)SH](3-) (AH, λ(max) ≈570 nm) forms in the course of a reversible process, with k(ad)=190±20 M(-1)s(-1) , k(-ad)=0.3±0.05 s(-1) . Deprotonation of AH (pK(a)=10.5±0.1, at 25.0 °C, I=1 M), leads to [Fe(CN)(5)N(O)S](4-) (A, λ(max)=535 nm, ε=6000±300 M(-1) cm(-1) ). [Fe(CN)(5)NO](.)(3-) and HS(2)(.)(2-) radicals form through the spontaneous decomposition of AH and A. The minor formation of the [Fe(CN)(5)NO](3-) ion equilibrates with [Fe(CN)(4)NO](2-) through cyanide labilization, and generates the "g=2.03" iron-dinitrosyl, [Fe(NO)(2)(SH)(2)](-) , which is labile toward NO release. Alternative nucleophilic attack of HS(-) on AH and A generates the reactive intermediates [Fe(CN)(5)N(OH)(SH)(2)](3-) and [Fe(CN)(5)N(OH)(S)(SH)](4-) , respectively, which decompose through multielectronic nitrosyl reductions, leading to NH(3) and hydrogen disulfide, HS(2)(-) . N(2)O is also produced at pH≥11. Biological relevance relates to the role of NO, NO(-) , and other bound intermediates, eventually able to be released to the medium and rapidly trapped by substrates. Structure and reactivity comparisons of the new nitrososulfide ligands with free and bound nitrosothiolates are provided.

Metal-catalyzed anaerobic disproportionation of hydroxylamine. Role of diazene and nitroxyl intermediates in the formation of N2, N2O, NO+, and NH3.

The catalytic disproportionation of NH(2)OH has been studied in anaerobic aqueous solution, pH 6-9.3, at 25.0 degrees C, with Na(3)[Fe(CN)(5)NH(3)].3H(2)O as a precursor of the catalyst, [Fe(II)(CN)(5)H(2)O](3)(-). The oxidation products are N(2), N(2)O, and NO(+) (bound in the nitroprusside ion, NP), and NH(3) is the reduction product. The yields of N(2)/N(2)O increase with pH and with the concentration of NH(2)OH. Fast regime conditions involve a chain process initiated by the NH(2) radical, generated upon coordination of NH(2)OH to [Fe(II)(CN)(5)H(2)O](3)(-). NH(3) and nitroxyl, HNO, are formed in this fast process, and HNO leads to the production of N(2), N(2)O, and NP. An intermediate absorbing at 440 nm is always observed, whose formation and decay depend on the medium conditions. It was identified by UV-vis, RR, and (15)NMR spectroscopies as the diazene-bound [Fe(II)(CN)(5)N(2)H(2)](3)(-) ion and is formed in a competitive process with the radical path, still under the fast regime. At high pH's or NH(2)OH concentrations, an inhibited regime is reached, with slow production of only N(2) and NH(3). The stable red diazene-bridged [(NC)(5)FeHN=NHFe(CN)(5)](6)(-) ion is formed at an advanced degree of NH(2)OH consumption.