Kinetics and mechanism of the interaction of nitric oxide with pentacyanoferrate(II). Formation and dissociation of [Fe(CN)5NO]3-
The interaction of NO with [Fe(CN)5H2O]3- (generated by aquation of the corresponding ammine complex) to produce [Fe(CN)5NO]3- was studied by UV-vis spectrophotometry. The reaction product is the well characterized nitrosyl complex, described as a low-spin Fe(II) bound to the NO radical. The experim...
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2003
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| Acceso en línea: | https://bibliotecadigital.exactas.uba.ar/collection/paper/document/paper_00201669_v42_n13_p4179_Roncaroli http://hdl.handle.net/20.500.12110/paper_00201669_v42_n13_p4179_Roncaroli |
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| Sumario: | The interaction of NO with [Fe(CN)5H2O]3- (generated by aquation of the corresponding ammine complex) to produce [Fe(CN)5NO]3- was studied by UV-vis spectrophotometry. The reaction product is the well characterized nitrosyl complex, described as a low-spin Fe(II) bound to the NO radical. The experiments were performed in the pH range 4-10, at different concentrations of NO, temperatures and pressures. The rate law was first-order in each of the reactants, with the specific complex-formation rate constant, kf = 250 ± 10 M-1 s-1 (25.4 °C, I = 0.1 M, pH 7.0), ΔHf‡ = 70 ± 1 kJ mol-1, ΔSf‡ = +34 ± 4 J K-1 mol-1, and ΔVf‡ = +17.4 ± 0.3 cm3 mol-1. These values support a dissociative mechanism, with rate-controlling dissociation of coordinated water, and subsequent fast coordination of NO. The complex-formation process depends on pH, indicating that the initial product [Fe(CN)5NO]3- is unstable, with a faster decomposition rate at lower pH. The decomposition process is associated with release of cyanide, further reaction of NO with [Fe(CN)4NO]2-, and formation of nitroprusside and other unknown products. The decomposition can be prevented by addition of free cyanide to the solutions, enabling a study of the dissociation process of NO from [Fe(CN)5NO]3-. Cyanide also acts as a scavenger for the [Fe(CN)5]3- intermediate, giving [Fe(CN)6]4- as a final product. From the first-order behavior, the dissociation rate constant was obtained as kd = (1.58 ± 0.06) × 10-5 s-1 at 25.0 °C, I = 0.1 M, and pH 10.2. Activation parameters were found to be ΔHd‡ = 106.4 ± 0.8 kJ mol-1, ΔSd‡ = +20 ± 2 J K-1 mol-1, and ΔVd‡ = +7.1 ± 0.2 cm3 mol-1, which are all in line with a dissociative mechanism. The low value of kd as compared to values for the release of other ligands L from [FeII(CN)5L]n- suggests a moderate to strong σ-π interaction of NO with the iron(II) center. It is concluded that the release of NO from nitroprusside in biological media does not originate from [Fe(CN)5NO]3- produced on reduction of nitroprusside but probably proceeds through the release of cyanide and further reactions of the [Fe(CN)4NO]2- ion. |
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