- Kinetic and thermodynamic properties of chromium(III) complexes containing pyrazine radical ligands
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Green complexes of 1:1 stoichiometry are formed when solutions of chromous ion and pyrazine are mixed; complexation is accompanied by proton binding at the remote nitrogen site on the ring. The association constant for monoprotonated pyrazine binding is K1 = 1.5 × 104 M-1 at 20.6°C, μ = 0.10 M. The association - dissociation kinetics were studied by using combined stopped-flow relaxation techniques; a single relaxation curve was obtained for which τ-1 = kf[Cr(II)][pyrazine] + kr; the upper limit for coordination by monoprotonated pyrazine, k1 ≤ 3.6 × 106 M-1 s-1 at 28°C, μ = 0.10 M, was about 102-fold slower than anticipated for rate-limiting substitution on chromous ion. In the presence of excess pyrazine the green complex disappears in a biphasic manner; the fast bleaching reaction gave rise to a 69-line EPR signal, identified as the dihydropyrazine radical cation. The reaction was described by the equilibrium (where pyz = pyrazine) Cr(pyzH)3+ + pyzH+ ?k-2k2 pyzH2+ + CrIII(pyz)3+ for which K2 = 1.3, k2 ? 14 M-1 s-1, and k-2 ? 10 M-1 s-1 at 20.6°C, μ = 0.10 M. A transitory species with spectral features nearly identical with the Cr(pyzH)3+ ion was formed when solutions containing chromous and pentaammine-(pyrazine)cobalt(III) ions were mixed; the colored species decayed rapidly (t1/2 ? 100 ms). Application of temperature-jump perturbation immediately after mixing yielded a single relaxation curve which could not be fitted to a simple mechanism. Evidence favoring formulation of the green species as chromium(III)-bound ligand radicals is discussed.
- Dunne, Thomas G.,Hurst, James K.
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- Evidence for a Bond Homolysis Pathway for O2 Release in the Decomposition of Superoxochromium(III) Ion CrO22+
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The cation (H2O)5CrO22+ has a considerable lifetime in aqueous solution in the absence of Cr2+.One pathway for decomposition consists of bond homolysis, CrO22+ -> Cr2+ + O2, with k = 2.5 x 10E-4 s-1 at 25.0 deg C (ΔH = 25.1 +/- 1.1 kcal mol-1, ΔS = 9.7 +/- 3.6 cal mol-1 K-1).The resulting Cr2+ then reacts very rapidly with CrO22+, with k = 8 x 10E8 M-1 s-1, estimated by kinetic modeling of the effect of .A secondary pathway, which yields HCrO4-, proceeds by a bimolecular reaction between two CrO22+ ions.It is characterized by a rate constant 6.0 +/- 0.9 M-1 s-1 at 25 deg C (ΔH = 12.0 +/- 1.7 kcal mol-1, ΔS = -14 +/- 6 cal mol-1 K-1).A structure of the activated complex for this pathway is proposed.The equilibrium constant for homolysis of CrO22+ is Khom = 1.6 x 10E-12 M and that calculated from the thermodynamic data for heterolysis (CrO22+ Cr3+ + O2-) is Khet = 3 x 10E-8 M.The latter, although kinetically unimportant, is suitable for thermodynamic analysis.The Lewis basicity of O2- for Cr3+ is discussed on the basis of comparisons to cobalt(III)-ammine complexes, other (H2O)5CrX2+ complexes, and acid ionization constants.The data do not require a description of CrO22+ as other than a superoxochromium(III) ion.
- Brynildson, Mark E.,Bakac, Andreja,Espenson, James H.
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- The solid-state electrochemistry of metal octacyanomolybdates, octacyanotungstates, and hexacyanoferrates explained on the basis of dissolution and reprecipitation reactions, lattice structures, and crystallinities
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The electrochemical behavior of solid microparticles of metal (Ag+, Cd2+, Co2+, Cr2+, Cu2+, Fe2+, Mn2+, Ni2+, Pb2+, and Zn2+) octacyanomolybdates, octacyanotungstates, and hexacyanoferrates has been studied by voltammetry, electrochemical quartz crystal microbalance, and microscopic diffuse reflectance spectroelectrochemical measurements. The solid microparticles have been immobilized on the surface of graphite electrodes prior to the electrochemical measurements. A comparative study of the cyclic oxidation and reduction of these compounds in the presence of potassium ions revealed that any interpretation of the electrochemistry requires the solubility equilibria of the reduced compounds to be taken into account, such as in the case of the silver salts {Ag3K[X]} and {Ag4[X]} (with X = Fe(II)(CN)6/4-, M(IV)(CN)8/4- (M = Mo, W)). Because {Ag4[X]} has a lower solubility than {Ag3K[X]}, the electrochemistry is accompanied by a conversion of solid {Ag3K[X]} into solid {Ag4[X]}. Two distinct voltammetric signal systems are generated by these two compounds according to {Ag3K[X]} ? {Ag3-[X]} + K+ + e- and {Ag4[X]} ? {Ag3[X]} + Ag+ + e-. When silver ions are present in the solution adjacent to the microparticles, the silver octacyanometalates and silver hexacyanoferrate show a chemically reversible and very stable voltammetric behavior. Despite the fact that the electrochemistry is based upon a single-electron/single-ion transfer reaction ({Ag4[X]} ? {Ag3[X]} + Ag+ + e-), more than one electrochemical signal is observed because of the simultaneous presence of amorphous and crystalline particles. This study shows that the interplay of solubility equilibria and electrochemical equilibria is generally observed for the other metal octacyanomolybdates, octacyanotungstates, and hexacyanoferrates as well.
- Schr?der, Uwe,Scholz, Fritz
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p. 1006 - 1015
(2008/10/08)
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- Reaction of aquachromium(II) ions with 2,2'-bipyridine. A pulse radiolysis study
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Chromium(II) ion was generated from Cr(NH3)63+ and hydrated electrons in the presence of 2,2'-bipyrimidine (bpm).The reaction of Cr(NH3)63+ with e-(aq) has k = 4.9 * 1010 dm3 mol1- s-1 at 25 deg C.The Cr(NH3)62- so produced loses all the ammonia ligands with k > 105 s-1.A product complex with an absorption maximum at around 350 nm was formed in the reaction of chromium(II) ion and bpm.In the presence of a moderate excess of bpm an equilibrium is established on the 0.1 ms timescale with Kapp = kf /kb.Both the forward (kf) and back (kb) reactions are pH dependent in the range 3.5 to 8.6.At pH 3.5 and 8.6 kf has the values of 1.6 * 108 and 4.4 * 107 dm3 mol-1 s-1 and kb of 4.3 * 104 and 3.2 * 103 s-1, respectively.The UV spectra of the reaction product(s) indicate that the absorbing product contains chromium(III) and bipyrimidine radical ligand.The first acid dissociation constant of chromium(II) ion was determined as 1.4 * 10-5 mol dm-3.At pH 2+ + bpm (H2O)5CrII(bpm)2+ (H2O)4CrII(bpm)2+ (H2O)4CrIII(bpm.-)2+ where II(bpm)>2+ and II(bpm)>2+ represent steady-state intermediates present in stoichiometrically negligible concentrations.On the 0.1 ms timescale, III(bpm.-)>2+ is stable but undergoes changes at longer times.The protonated bpm molecule, H+bpm, is reduced by 1-hydroxy-1-methylethyl radicals, k = 5 * 109 dm3 mol-1 s-1, yielding a diprotonated bpm radical, H2bpm.+.No reaction was observed between Cr(H2O)63+ and H2bpm.+ in pulse radiolysis experiments, which places the rate constant for this reaction at 6 dm3 mol-1 s-1.
- Bonifacic, Marija,Lovric, Jasna,Orhanovic, Mato
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p. 2879 - 2886
(2007/10/03)
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- Catalytic oxidation of the (hydroxymethyl)chromium(III) ion by the superoxochromium(III) ion
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The superoxochromium(III) ion, CrO22+, is an efficient catalyst for autoxidation of the (hydroxymethyl)chromium(III) ion, CrCH2OH2+, in aqueous solution. The reaction involves one-electron oxidation of CrCH2OH2+ by CrO22+, k = 137 M-1 s-1, to yield Cr2+, CH2O, and the novel (hydroperoxo)chromium(III) ion, CrO2H2+. The Cr2+ produced reacts rapidly with O2 to regenerate the catalyst, CrO22+. When oxygen is depleted, the Cr2+ reacts instead with CrO22+ to produce the chromyl(IV) ion, CrO2+. This initiates a chain reaction that rapidly consumes the remaining CrO22+ and a stoichiometric amount of CrCH2OH2+.
- Scott, Susannah L.,Bakac, Andreja,Espenson, James H.
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p. 4112 - 4117
(2008/10/08)
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- Comparison of the Formation Rate Constants of some Chromium(II) and Copper(II) Complexes
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The equilibrium kinetics in aqueous solutions of the chromium(II)-ethylenediamine (en) and -iminodiacetate (ida) complexes has been studied using a n.m.r. relaxation method.The paramagnetic relaxation rate and shift of the CH2 protons of the ligands were
- Micskei, Karoly,Nagypal, Istvan
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p. 1301 - 1304
(2007/10/02)
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- Comparative study of structures, including Jahn-Teller effects, in the saccharinate complexes, [M(C7H4NO3S)2(H2O) 4]·2H2O, of chromium and zinc, as well as other divalent metal ions
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The chromium(II) and zinc(II) complexes of saccharine, [M(C7H4NO3S)2(H2O) 4]·2H2O, have been prepared and their structures determined by X-ray crystallography. The chromium compound shows clear evidence of Jahn-Teller distortion. When the results for the two new compounds are combined with results in the literature for analogous compounds of MnII, FeII, CoII, NiII, and CuII, a broad picture of structural properties vs. d-orbital population for high-spin divalent ions for the first transition series is provided. For the series Mn, Fe, Co, Ni, and Zn the expected trends are seen in the bond lengths as the high-spin configurations change, with maxima at Mn and Zn and a minimum at Ni. The bond lengths for the Cu compound deviate enormously (one Cu-O distance being greater, the other Cu-O and the Cu-N distances being less) from the values interpolated between the Ni and Zn data. For the Cr compound, the Jahn-Teller distortions take the same form as those for Cu, but they are only about half as large. The chromium and zinc compounds are isomorphous, crystallizing in space group P21/c with Z = 2. The cell constants for ZnC14H20O12N2S2 are a = 7.934 (1) A?, b = 16.118 (3) A?, c = 7.691 (1) A?, and β = 99.90 (1)° and for CrC14H20O12N2S2 are a = 8.042 (4) A?, b = 16.032 (8) A?, c = 7.804 (4) A, and β = 100.90 (4)°.
- Cotton, F. Albert,Lewis, Gregg E.,Murillo, Carlos A.,Schwotzer, Willi,Valle, Grettel
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p. 4038 - 4041
(2008/10/08)
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- Reactions of hydroxyl and other free radicals with organopentaaquochromium(2+) ions
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Complexes of the family (H2O)5CrR2+ (R = CH2OCH3, CH(CH3)OC2H5, CH(CH3)2, and CH2C6H5) react with hydroxyl radicals, with accompanying cleavage of the chromium-carbon bond. Decomposition of the CrR2+ complexes in the presence of H2O2 occurs by self-sustaining chain reactions, because the reactive intermediates from the HO? reactions, Cr2+ and R?, react with H2O2 to form HO? again. The overall chain length is not high because of efficient chain-terminating reactions. An estimate of ~1.6 × 109 M-1 s-1 is made for the total rate constant representing the reactions of HO? with CrR2+. Less detailed studies show that Br2- reacts with CrCH(CH3)OC2H52+ and CrCH(CH3)22+ and that R? also reacts with the latter.
- Baka?, Andreja,Blau, Reed J.,Espenson, James H.
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p. 3789 - 3793
(2008/10/08)
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- Homolytic decomposition of tertiary organochromium(III) complexes and evidence for their decomposition via reactions with aliphatic free radicals. A pulse radiolysis study
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The rates of homolytic decomposition of Cr-C(CH3)2OH2+-, Cr-C(CH3)2CO2H2+, and Cr-C(CH3)2CN2+ are 0.15 s-1, 4 s-1, and 104 -6 s-1, respectively. The reaction of some aliphatic free radicals,·R with CrIII-R complexes is very fast, K3 ≥ 108 M-1 S-1. The rate of reaction of Cr2+(aq) with H2O2 is k16 = (3.7 ±0.7) × 104 M-1 s-1 in good accord with the literature value. These results are discussed in detail.
- Mulac, William A.,Cohen, Haim,Meyerstein, Dan
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p. 4016 - 4020
(2008/10/08)
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