Kinetics, Mechanism, and Spectroscopy of the Reversible Binding of Nitric Oxide to Aquated Iron(II). An Undergraduate Text Book Reaction Revisited

Abstract

A detailed kinetic and mechanistic analysis of the classical "brown-ring" reaction of [Fe(H(2)O)(6)](2+) with NO was performed using stopped-flow and laser flash photolysis techniques at ambient and high pressure. The kinetic parameters for the "on" and "off" reactions at 25 degrees C were found to be k(on) = 1.42 x 10(6) M(-1) s(-1), DeltaH(++)(on) = 37.1 +/- 0.5 kJ mol(-1), DeltaS(++)(on) = -3 +/- 2 J K(-1) mol(-1), DeltaV(++)(on) = +6.1 +/- 0.4 cm(3) mol(-1), and k(off) = 3240 +/- 750 s(-1), DeltaH(++)(off) = 48.4 +/- 1.4 kJ mol(-1), DeltaS(++)(off) = -15 +/- 5 J K(-1) mol(-1), DeltaV(++)(off) = +1.3 +/- 0.2 cm(3) mol(-1). These parameters suggest that both reactions follow an interchange dissociative (I(d)) ligand substitution mechanism, which correlates well with the suggested mechanism for the water exchange reaction on [Fe(H(2)O)(6)](2+). In addition, Mössbauer spectroscopy and EPR measurements were performed on the reaction product [Fe(H(2)O)(5)(NO)](2+). The Mössbauer and EPR parameters closely resemble those of the [FeNO](7) units in any of the other well-characterized nitrosyl complexes. It is concluded that its electronic structure is best described by the presence of high-spin Fe(III) antiferromagnetically coupled to NO(-) (S = 1) yielding the observed spin quartet ground state (S = (3)/(2)), i.e., [Fe(III)(H(2)O)(5)(NO(-))](2+), and not [Fe(I)(H(2)O)(5)(NO(+))](2+) as usually quoted in undergraduate text books.

Elucidation of inorganic reaction mechanisms in ionic liquids: the important role of solvent donor and acceptor properties

“…The low-field part of the spectrum together with the signal at g = 2.04 could be assigned to an iron nitrosyl complex with g values of 4.05, 4.05 and 2.04, which are typical for a spin quartet species (S = 3/2) with dominating zero-field splitting, D ≫ hν (0.3 cm −1 at X-band), and vanishing rhombicity, E/D = 0. Similar spectra have been observed for solutions of [Fe(edta) 57 Therefore, the subspectrum assigned to the nitrosylated iron-dca complex confirms the S = 3/2 ground state of the {FeNO} 7 core.…”

“…Although these absorbance changes are shifted to longer wavelengths, as compared to aqueous solution (viz., 451 and 585 nm), the typical greenish-brown color of the resulting solution indicates the formation of an Fe-NO adduct similar to the brown-ring-test. 57 Bubbling an inert gas through the solution leads to reversed spectral changes indicating a complete reversibility of reaction (3).…”

“…The excess of Fe II is necessary to avoid the formation of higher nitrosyl-adducts. 57 The observed rate constants k obs resulted in linear concentration dependences which allowed a determination of k on and k off from slope and intercept, respectively. The appropriate rate constants and activation parameters ΔH ≠ , ΔS ≠ and ΔV ≠ are summarized in Table 5 and compared with the data derived from aqueous solution.…”

Nitric oxide binding at the mononuclear active site of reduced Pyrococcus furiosus superoxide reductase

“…3). Analogous absorption bands have been reported for the S ϭ 3͞2 NO adducts of a wide range of nonheme proteins and inorganic complexes (22,28,31,34,35) and have been attributed to charge transfer (CT) transitions associated with the{FeNO} 7 unit. The 4.2 K MCD spectrum of the NO adduct of ascorbatereduced P. furiosus SOR in the 14,000 -29,000 cm Ϫ1 region correlates closely with the absorption spectrum and is dominated by a positive MCD C term centered at 21,050 cm Ϫ1 , with less pronounced positive C terms apparent at 28,000 cm Ϫ1 (357 cm Ϫ1 ) and as a shoulder at Ϸ23,500 cm Ϫ1 (425 nm; see Fig.…”

Comment on “Role of (NO)2 Dimer in Reactions of Fe+ with NO and NO2 Studied by ICP-SIFT Mass Spectrometry”

“…This in agreement with many others whom deal with nitrogen oxides. [13][14][15][16][17][18][19][20][21] It would be a direct test of our hypothesis for the Bohme group to employ a baked Ascarite trap, or other suitable trap, and repeat their measurements.…”