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 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 (plus-minus) 10 M^-1 s^-1 (25.4 °C, I = 0.1 M, pH 7.0), (delta)Hf (double dagger) = 70 (plus-minus) 1 kJ mol^-1, (delta)Sf(double dagger) = +34 (plus-minus) 4 J K^-1 mol^-1, and (delta)Vf(double dagger) = +17.4 (plus -minus) 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)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 (plus-minus) 0.06) × 10^-5 s^-1 at 25.0 °C, I = 0.1 M, and pH 10.2. Activation parameters were found to be (delta)Hd(double dagger) = 106.4 (plus-minus) 0.8 kJ mol^-1, (delta)Sd (double dagger)= +20 (plus-minus) 2 J K^-1 mol^-1, and (delta)Vd (double dagger)= +7.1 (plus-minus) 0.2 cm^3 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 (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) 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.