title: 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.
authors.affiliation: Institute for Inorganic Chemistry, University of Erlangen-Nürnberg, Egerlandstrasse 1, 91058 Erlangen, Germany.
authors.forename: Alicja
authors.initials: A
authors.lastname: Wanat
authors.forename: Thorsten
authors.initials: T
authors.lastname: Schneppensieper
authors.forename: Grazyna
authors.initials: G
authors.lastname: Stochel
authors.forename: Rudi
authors.initials: R
authors.lastname: van Eldik
authors.forename: Eckhard
authors.initials: E
authors.lastname: Bill
authors.forename: Karl
authors.initials: K
authors.lastname: Wieghardt
mesh_terms: D001711:Biotransformation; D002384:Catalysis; D002412:Cations / Q000737:chemistry; D055598:Chemical Phenomena; D002627:Chemistry, Physical; D005296:Ferrous Compounds / Q000737:chemistry*; D007700:Kinetics; D008956:Models, Chemical; D009569:Nitric Oxide / Q000737:chemistry* / Q000378:metabolism; D019906:Nuclear Magnetic Resonance, Biomolecular; D010084:Oxidation-Reduction; D010777:Photochemistry; D021241:Spectrometry, Mass, Electrospray Ionization; D013055:Spectrophotometry, Infrared; D013056:Spectrophotometry, Ultraviolet
chemical_list: D002412:Cations; D005296:Ferrous Compounds; D009569:Nitric Oxide