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

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Abstract
A detailed kinetic and mechanistic analysis of the classical “brown-ring” reaction of [Fe(H2O)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 °C were found to be kon = 1.42 × 106 M-1 s-1, ΔH⧧on = 37.1 ± 0.5 kJ mol-1, ΔS⧧on = −3 ± 2 J K-1 mol-1, ΔV⧧on = +6.1 ± 0.4 cm3 mol-1, and koff = 3240 ± 750 s-1, ΔH⧧off = 48.4 ± 1.4 kJ mol-1, ΔS⧧off = −15 ± 5 J K-1 mol-1, ΔV⧧off = +1.3 ± 0.2 cm3 mol-1. These parameters suggest that both reactions follow an interchange dissociative (Id) ligand substitution mechanism, which correlates well with the suggested mechanism for the water exchange reaction on [Fe(H2O)6]2+. In addition, Mössbauer spectroscopy and EPR measurements were performed on the reaction product [Fe(H2O)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 FeIII antiferromagnetically coupled to NO- (S = 1) yielding the observed spin quartet ground state (S = 3/2), i.e., [FeIII(H2O)5(NO-)]2+, and not [FeI(H2O)5(NO+)]2+ as usually quoted in undergraduate text books.