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)(5)H2O](3-) (generated by aquation of the corresponding ammine complex) to produce [Fe(CN)(5)NO](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, k(f) = 250 +/- 10 M-1 s(-1) (25.4 degreesC, / = 0.1 M, pH 7.0), DeltaH(f)double dagger = 70 +/- 1 kJ mol(-1), DeltaS(f)double dagger = +34 +/- 4 J K-1 mol(-1), and DeltaV(f)double dagger = +17.4 +/- 0.3 cm(3) 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)(5)NO](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)(4)NO](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)(5)NO](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 k(d) = (1.58 +/- 0.06) x 10(-5) s(-1) at 25.0 degreesC, I = 0.1 M, and pH 10.2. Activation parameters were found to be DeltaH(d)double dagger = 106.4 +/- 0.8 kJ mol(-1), DeltaS(d)double dagger = +20 +/- 2 J K-1 mol(-1), and DeltaV(d)double dagger = +7.1 +/- 0.2 cm(3) mol(-1), which are all in line with a dissociative mechanism. The low value of k(d) as compared to values for the release of other ligands L from [Fe-II(CN)(5)L](n-) suggests a moderate to strong sigma-pi 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)(5)NO](3-) produced on reduction of nitroprusside but probably proceeds through the release of cyanide and further reactions of the [Fe(CN)(4)NO](3-) ion.