Reaction of [RuIII(edta)(H2O)]- with H2O2 in aqueous solution. Kinetic and mechanistic investigation

The reaction of [Ru(III)(edta)(H(2)O)](-) (1) (edta = ethylenediaminetetraacetate) with hydrogen peroxide was studied kinetically as a function of [H(2)O(2)], temperature (5-35 degrees C) and pressure (1-1300 atm) at a fixed pH of 5.1 using stopped-flow techniques. The reaction was found to consist of two steps involving the rapid formation of a [Ru(III)(edta)(OOH)](2-) intermediate which subsequently undergoes parallel heterolytic and homolytic cleavage to produce [(edta)Ru(V)=O](-) (45%) and [(edta)Ru(IV)(OH)](-) (55%), respectively. The water soluble trap, 2,2'-azobis(3-ethylbenzithiazoline-6-sulfonate) (ABTS), was employed to substantiate the mechanistic proposal. Reactions were carried out under pseudo-first conditions for [ABTS] > [HOBr] > [1], and were monitored as a function of time for the formation of the one-electron oxidation product ABTS* (+). A detailed mechanism in agreement with the rate and activation parameters is presented, and the results are discussed with reference to data reported for the corresponding [Fe(III)(edta)(H(2)O)](-)/H(2)O(2) system.     

Kinetics and mechanism of O-O bond cleavage in the reaction of [RuIII(edta)(H2O)]- with hydroperoxides in aqueous solution

The reactions of [Ru(III)(edta)(H(2)O)](-) (1) (edta = ethylenediaminetetraacetate) with tert-butylhydroperoxide ((t)BuOOH) and potassium hydrogenpersulfate (KHSO(5)) were studied kinetically as a function of oxidant concentration and temperature (10-30 degrees C) at a fixed pH of 6.1 using stopped-flow techniques. Kinetic results were analyzed by using global kinetic analysis techniques. The reaction was found to consist of two steps involving the rapid formation of a [Ru(III)(edta)(OOR)](2-) intermediate, which subsequently undergoes heterolytic cleavage to form [(edta)Ru(V)=O](-). Since [(edta)Ru(V)=O](-) was produced almost quantitatively in the reaction of 1 with the hydroperoxides (t)BuOOH and KHSO(5), the common mechanism is one of heterolytic scission of the O-O bond. The water soluble and easy to oxidize substrate 2,2'-azobis(3-ethylbenzithiazoline-6-sulfonate (ABTS), was employed to substantiate the mechanistic proposal. Reactions were carried out under pseudo-first order conditions for [ABTS] > [hydroperoxide] > [1], and were monitored as a function of time for the formation of the one-electron oxidation product ABTS (*+). The detailed suggested mechanism is consistent with the reported rate and activation parameters, and discussed in reference to the results reported for the reaction of [Ru(II)(edta)(H(2)O)](-) with H(2)O(2).     

Remarkably high catalytic activity of the Ru(III)(edta)/H2O2 system towards degradation of the azo-dye Orange II

The Ru(III)(edta)/H(2)O(2) system (edta(4-) = ethylenediaminetretaacetate) was found to degrade the azo-dye Orange II at remarkably high efficiency under ambient conditions. Catalytic degradation of the dye was studied by using rapid-scan spectrophotometry as a function of [H(2)O(2)], [Orange II] and pH. Spectral analyses and kinetic data point towards a catalytic pathway involving the rapid formation of [Ru(III)(edta)(OOH)](2-) followed by the immediate subsequent degradation of Orange II prior to the conversion of [Ru(III)(edta)(OOH)](2-) to [Ru(IV)(edta)(OH)](-) and [Ru(V)(edta)(O)](-)via homolysis and heterolysis of the O-O bond, respectively. The higher oxidation state Ru(IV) and Ru(V) complexes react three orders of magnitude slower with Orange II than the Ru(III)-hydroperoxo complex. In comparison to biological oxygen transfer reactions, the Ru(edta) complexes show the reactivity order Compound 0 ? Compounds I and II.     

Equilibrium, kinetic and solvent effect studies on the reactions of [RuIII(Hedta)(H2O)] with thiols

The interaction of [Ru(III)(edta)(H(2)O)](-) with a series of selected thiols having extra functional groups was investigated potentiometrically and kinetically. The pK(a) values of the uncoordinated carboxylic acid group and coordinated water molecule are 3.12 and 7.41, respectively, in aqueous solution at 25 degrees C and 0.1 M ionic strength. The formation constants of the complexes were determined in the pH range 3-9, and the concentration distribution of the various complex species was evaluated as a function of pH. The effect of dioxane on the pK(a) values of [Ru(III)(Hedta)(H(2)O)] and the formation constants of the corresponding thiol complexes is presented. The study also provides mechanistic information on the reaction of [Ru(III)(edta)(H(2)O)](-) with the thiols. The low values of DeltaH(not equal) and negative values of DeltaS(not equal) and DeltaV(not equal) for the substitution reactions of [Ru(III)(edta)(H(2)O)](-) clearly support the associative character of the substitution process.     

Kinetics and mechanism of the reaction of [RuIII(edta)(H2O)]- with HOBr to form an intermediate RuV=O complex in aqueous solution

The interaction of [Ru(III)(edta)(H(2)O)](-) (1) (edta = ethylenediaminetetraacetate) with the oxygen transfer agent HOBr, was studied kinetically as a function of [HOBr] and temperature (5-35 degrees C) at a fixed pH of 6.2. Spectroscopic evidence is reported for the formation of a high valent intermediate (edta)Ru(V)=O complex. Water soluble 2,2'-azobis(3-ethylbenzithiazoline-6-sulfonate) (ABTS) was employed as a trap for this intermediate in order to gain further mechanistic information. Reactions were carried out under pseudo-first conditions for [ABTS] > [HOBr] > [1], and were monitored as a function of time for the formation of the one-electron oxidation product ABTS(*+). The reported kinetic data are interpreted in terms of a suggested reaction mechanism and discussed in reference to data reported before.     

Kinetics and mechanism of NO production in the Ru(III)-(edta) mediated oxidation of L-arginine with H(2)O(2)

The kinetics of the Ru(III)-(edta) (edta(4-) = ethylenediaminetetraacetate) catalyzed oxidation of l-arginine by H(2)O(2) mimicking the action of nitric oxide synthases (NOSs) has been studied spectrophotometrically. The time course of the reaction of [Ru(V)(edta)O](-) with l-arginine was followed at 390 nm under catalytic turn-over conditions. Formation of NO in the reacting system has been confirmed with an isolated nitric oxide free radical analyzer. A detailed reaction mechanism in agreement with the spectral and kinetic data is presented.     

Substitution reactions of [Ru(dppe)(CO)(H(2)O)(3)][OTf](2)

The labile nature of the coordinated water ligands in the organometallic aqua complex [Ru(dppe)(CO)(H(2)O)(3)][OTf](2) (1) (dppe = Ph(2)PCH(2)CH(2)PPh(2); OTf = OSO(2)CF(3)) has been investigated through substitution reactions with a range of incoming ligands. Dissolution of 1 in acetonitrile or dimethyl sulfoxide results in the facile displacement of all three waters to give [Ru(dppe)(CO)(CH(3)CN)(3)][OTf](2) (2) and [Ru(dppe)(CO)(DMSO)(3)][OTf](2) (3), respectively. Similarly, 1 reacts with Me(3)CNC to afford [Ru(dppe)(CO)(CNCMe(3))(3)][OTf](2) (4). Addition of 1 equiv of 2,2'-bipyridyl (bpy) or 4,4'-dimethyl-2,2'-bipyridyl (Me(2)bpy) to acetone/water solutions of 1 initially yields [Ru(dppe)(CO)(H(2)O)(bpy)][OTf](2) (5a) and [Ru(dppe)(CO)(H(2)O)(Me(2)bpy)][OTf](2) (6a), in which the coordinated water lies trans to CO. Compounds 5a and 6a rapidly rearrange to isomeric species (5b, 6b) in which the ligated water is trans to dppe. Further reactivity has been demonstrated for 6b, which, upon dissolution in CDCl(3), loses water and coordinates a triflate anion to afford [Ru(dppe)(CO)(OTf)(Me(2)bpy)][OTf] (7). Reaction of 1 with CH(3)CH(2)CH(2)SH gives the dinuclear bridging thiolate complex [[(dppe)Ru(CO)](2)(mu-SCH(2)CH(2)CH(3))(3)][OTf] (8). The reaction of 1 with CO in acetone/water is slow and yields the cationic hydride complex [Ru(dppe)(CO)(3)H][OTf] (9) via a water gas shift reaction. Moreover, the same mechanism can also be used to account for the previously reported synthesis of 1 upon reaction of Ru(dppe)(CO)(2)(OTf)(2) with water (Organometallics 1999, 18, 4068).     

Reactions of [Ru(H(2)O)(6)](2+) with water-soluble tertiary phosphines

In aqueous solutions under mild conditions, [Ru(H(2)O)(6)](2+) was reacted with various water-soluble tertiary phosphines. As determined by multinuclear NMR spectroscopy, reactions with the sulfonated arylphosphines L =mtppms, ptppms and mtppts yielded only the mono- and bisphosphine complexes, [Ru(H(2)O)(5)L](2+), cis-[Ru(H(2)O)(4)L(2)](2+), and trans-[Ru(H(2)O)(4)L(2)](2+) even in a high ligand excess. With the small aliphatic phosphine L = 1,3,5-triaza-7-phosphatricyclo-[3.3.1.1(3,7)]decane (pta) at [L]:[Ru]= 12:1, the tris- and tetrakisphosphino species, [Ru(H(2)O)(3)(pta)(3)](2+), [Ru(H(2)O)(2)(pta)(4)](2+), [Ru(H(2)O)(OH)(pta)(4)](+), and [Ru(OH)(2)(pta)(4)] were also detected, albeit in minor quantities. These results have significance for the in situ preparation of Ru(II)-tertiary phosphine catalysts. The structures of the complexes trans-[Ru(H(2)O)(4)(ptaMe)(2)](tos)(4)x2H(2)O, trans-[Ru(H(2)O)(4)(ptaH)(2)](tos)(4)[middle dot]2H(2)O, and trans-mer-[RuI(2)(H(2)O)(ptaMe)(3)]I(3)x2H(2)O, containing protonated or methylated pta ligands (ptaH and ptaMe, respectively) were determined by single crystal X-ray diffraction.     

Immobilization of the [RuII(edta)NO+] Ion on the surface of functionalized silica gel

The reaction of NO and the immobilized dimer complex (edta)(2)Ru(2)(III(1/2),III(1/2)) on silica gel chemically modified with [3-(2-aminoethyl)aminopropyl]trimethoxysilane (AEATS) produces the corresponding immobilized nitrosyl complex AEATS/Ru(II)NO(+). This compound, a monomer, was obtained by reducing the immobilized ruthenium dimer either electrochemically or with Eu(II) and reacting this species with NO(2)(-) ions. The properties of [Ru(edta)NO](-) in solution and anchored (AEATS/Ru(II)NO(+)) on silica were compared using electrochemical (DPV, CV) and spectroscopic (IR, UV-vis, and ESR) techniques. The results indicate that immobilization does not alter the reactivity of the ruthenium complex and confirm that [Ru(edta)(H(2)O)](2)(-) may be used, either in solution or immobilized, as a catalyst for the conversion of NO(2)(-) to NO(+). Both the anchored nitrosyl complex AEATS/Ru(II)NO(+) and the [Ru(edta)NO](-) species in solution, upon one-electron reduction, liberate NO at comparable rates.     

Dimerization of mono-ruthenium substituted alpha-Keggin-type tungstosilicate [alpha-SiW11O39RuIII(H2O)]5- to micro-oxo-bridged dimer in aqueous solution: synthesis, structure, and redox studies

We report the dimerization of a mono-ruthenium(III) substituted alpha-Keggin-type tungstosilicate [alpha-SiW(11)O(39)Ru(III)(H2O)](5-) to a micro-oxo-bridged dimer [{alpha-SiW(11)O(39)Ru(m)}2O](n-) (m = III, n = 12; m = IV/III, n = 11; m = IV, n = 10). Single crystal X-ray structure analysis of Rb(10)[{alpha-SiW(11)O(39)Ru(IV)}2O].9.5H2O (triclinic, P1, with a = 12.7650(6) A, b = 18.9399(10) A, c = 20.2290(10) A, alpha = 72.876(3) degrees, beta = 88.447(3) degrees, gamma = 80.926(3) degrees, V = 4614.5(4) A(3), Z = 2) reveals that two mono-ruthenium substituted tungstosilicate alpha-Keggin units are connected through micro-oxo-bridging Ru-O-Ru bonds. Solution (183)W-NMR of [{SiW(11)O(39)Ru(IV)}2O](10-) resulted in six peaks (-63, -92, -110, -128, -132, and -143 ppm, intensities 2 : 2 : 1 : 2 : 2 : 2) confirming that the micro-oxo bridged dimer structure is maintained in aqueous solution. The dimerization mechanism is presumably initiated by deprotonation of the aqua-ruthenium complex [alpha-SiW(11)O(39)Ru(III)(H2O)](5-) leading to a hydroxy-ruthenium complex [alpha-SiW(11)O(39)Ru(III)(OH)](6-). Dimerization of two hydroxy-ruthenium complexes produces the micro-oxo bridged dimer [{alpha-SiW(11)O(39)Ru(III)}2O](12-) and a water molecule. The Ru(III) containing dimer is oxidized by molecular oxygen to produce a mixed valence species [{alpha-SiW(11)O(39)Ru(IV-III)}2O](11-), and further oxidation results in the Ru(IV) containing [{alpha-SiW(11)O(39)Ru(IV)}2O](10-).     

Influence of fluoride on the reversible binding of NO by [Fe(II)(EDTA)(H2O)]2-. Inhibition of autoxidation of [Fe(II)(EDTA)(H2O)]2-

The promising BioDeNO(x) process for NO removal from gaseous effluents suffers from an unsolved problem that results from the oxygen sensitivity of the Fe(II)-aminopolycarboxylate complexes used in the absorber unit to bind NO(g). The utilized [Fe(II)(EDTA)(H2O)](2-) complex is extremely oxygen sensitive and easily oxidized to give a totally inactive [Fe(III)(EDTA)(H2O)](-) species toward the binding of NO(g). We found that an in situ formed, less-oxygen-sensitive mixed-ligand complex, [Fe(II)(EDTA)(F)](3-), still reacts quantitatively with NO(g). The formation constant for the mixed ligand complex was determined spectrophotometrically. For [Fe(III)(EDTA)(F)](2-) we found log K(MLF)(F) = 1.7 +/- 0.1. The [Fe(II)(EDTA)(F)](3-) complex has a smaller value of log K(MLF)(F) = 1.3 +/- 0.2. The presence of fluoride does not affect the reversible binding of NO(g). Even over extended periods of time and fluoride concentrations of up to 1.0 M, the nitrosyl complex does not undergo any significant decomposition. The [Fe(III)(EDTA)(NO(-))](2-) complex releases bound NO on passing nitrogen through the solution to form [Fe(II)(EDTA)(H2O)](2-) almost completely. A reaction cycle is feasible in which fluoride inhibits the autoxidation of [Fe(II)(EDTA)(H2O)](2-) during the reversible binding of NO(g).     

Synthesis and characterization of a twin Cubane-type molybdenum-rhodium-sulfur cluster, [{Mo3RhCp*S4(H2O)7(O)}2]8+. X-ray structure of [{Mo3RhCpS4(H2O7O}2](CH3C6H4SO3)(8).14H2O

The reaction of [Mo(3)S(4)(H(2)O)(9)](4+) (1) with [(CpRhCl(2))(2)] afforded a novel rhodium-molybdenum cluster, [{Mo(3)RhCpS(4)(H(2)O)(7)(O)}(2)](8+) (2). X-ray structure analysis of [2](pts)(8).14H(2)O (pts(-) = CH(3)C(6)H(4)SO(3)(-)) has revealed the existence of a new oxo-bridged twin cubane-type core, (Mo(3)RhCpS(4))(2)(O)(2). The high affinity of the CpRh group for sulfur atoms in 1 seems to be the main driving force for this reaction. The strong Lewis acidity of the CpRh group in intermediate A, [Mo(3)RhCpS(4)(H(2)O)(9)](6+), caused a release of proton from one of the water molecules attached to the molybdenum atoms to give intermediate B, [Mo(3)RhCpS(4)(H(2)O)(8)(OH)](5+). The elimination of two water molecules from two intermediate B molecules, followed by the deprotonation reaction of hydroxo bridges, generated the twin cubane-type cluster 2. The formal oxidation states of rhodium and molybdenum atoms are the same before and after the reaction (i.e., Mo(IV)(3), Rh(III)). The Mo-O-Mo moieties in [2](pts)(8).14H(2)O are nearly linear with a bond angle of 164.3(3) degrees, and the basicity of the bridging oxygen atoms seems to be weak. For this reason, protonation at the bridging oxygen atoms does not occur even in a strongly acidic aqueous solution. The binding energy values of Mo 3d(5/2), Rh 3d(5/2), and C 1s obtained from X-ray photoelectron spectroscopy measurements for [2](pts)(8).14H(2)O are 229.8, 309.3, and 285 eV, respectively. The XPS measurements on the Rh 3d(5/2) binding energy indicate that the oxidation state of Rh is 3+. The binding energy of Mo 3d(5/2) (229.8 eV) compares with that observed for [1](pts)(4).7H(2)O (230.7 eV, Mo 3d(5/2)). A lower energy shift (0.9 eV) is observed in the binding energy of Mo 3d(5/2) for [2](pts)(8).14H(2)O. This energy shift may correspond to the coordination of an oxygen atom having a negative charge to the molybdenum atom.     

Elucidation of the solution structure and water-exchange mechanism of paramagnetic [Fe(II)(edta)(H(2)O)](2-)

The lability and structural dynamics of [Fe(II)(edta)(H(2)O)](2-) (edta = ethylenediaminetetraacetate) in aqueous solution strongly depend on solvent interactions. To study the solution structure and water-exchange mechanism, (1)H, (13)C, and (17)O NMR techniques were applied. The water-exchange reaction was studied through the paramagnetic effect of the complex on the relaxation rate of the (17)O nucleus of the bulk water. In addition to variable-temperature experiments, high-pressure NMR techniques were applied to elucidate the intimate nature of the water-exchange mechanism. The water molecule in the seventh coordination site of the edta complex is strongly labilized, as shown by the water-exchange rate constant of (2.7 +/- 0.1) x 106 s(-1) at 298.2 K and ambient pressure. The activation parameters DeltaH(not equal), DeltaS(not equal), and DeltaV(not equal) were found to be 43.2 +/- 0.5 kJ mol(-1), +23 +/- 2 J K(-1) mol(-1), and +8.6 +/- 0.4 cm(3) mol(-1), respectively, in line with a dissociatively activated interchange (Id) mechanism. The scalar coupling constant (A/h) for the Fe(II)-O interaction was found to be 10.4 MHz, slightly larger than the value A/h = 9.4 MHz for this interaction in the hexa-aqua Fe(II) complex. The solution structure and dynamics of [Fe(II)(edta)(H(2)O)](2-) were clarified by (1)H and (13)C NMR experiments. The complex undergoes a Delta,Lambda-isomerization process with interconversion of in-plane (IP) and out-of-plane (OP) positions. Acetate scrambling was also found in an NMR study of the corresponding NO complex, [Fe(III)(edta)(NO(-))](2-).     

Hydrothermal and solid-state transformation of ruthenium-supported Keggin-type heteropolytungstates [XW11O39{Ru(II)(benzene)(H2O)}]n- (X = P (n = 5), Si (n = 6), Ge (n = 6)) to ruthenium-substituted Keggin-type heteropolytungstates

A new pathway for the preparation of mono-ruthenium (Ru)(iii)-substituted Keggin-type heteropolytungstates with an aqua ligand, [PW(11)O(39)Ru(iii)(H(2)O)](4-) (1a), [SiW(11)O(39)Ru(iii)(H(2)O)](5-) (1b) and [GeW(11)O(39)Ru(iii)(H(2)O)](5-) (1c), using [Ru(ii)(benzene)Cl(2)](2) as a Ru source was described. Compounds 1a-1c were prepared by reacting [XW(11)O(39)](n-) (X = P, Si and Ge) with [Ru(ii)(benzene)Cl(2)](2) under hydrothermal condition and were isolated as caesium salts. Ru(benzene)-supported heteropolytungstates, [PW(11)O(39){Ru(ii)(benzene)(H(2)O)}](5-) (2a), [SiW(11)O(39){Ru(ii)(benzene)(H(2)O)}](6-) (2b) and [GeW(11)O(39){Ru(ii)(benzene)(H(2)O)}](6-) (2c), were first produced in the reaction media, and then transformed to 1a, 1b and 1c, respectively, under hydrothermal conditions. Calcination of Ru(benzene)-supported heteropolytungstates, 2a, 2b and 2c, in the solid state produced mixtures of 1a, 1b and 1c with CO (carbon monoxide)-coordinated complexes, [PW(11)O(39)Ru(ii)(CO)](5-) (4a), [SiW(11)O(39)Ru(ii)(CO)](6-) (4b) and [GeW(11)O(39)Ru(ii)(CO)](6-) (4c), respectively. From comparison of their catalytic activities in water oxidation reaction, it was indicated that ruthenium should be incorporated in the heteropolytungstate in order to promote catalytic activity.     

Olefin epoxidation with hydrogen peroxide catalyzed by lacunary polyoxometalate [gamma-SiW10O34H2O2]4-

The tetra-n-butylammonium (TBA) salt of the divacant Keggin-type polyoxometalate [TBA](4)[gamma-SiW(10)O(34)(H(2)O)(2)] (I) catalyzes the oxygen-transfer reactions of olefins, allylic alcohols, and sulfides with 30 % aqueous hydrogen peroxide. The negative Hammett rho(+) (-0.99) for the competitive oxidation of p-substituted styrenes and the low value of (nucleophilic oxidation)/(total oxidation), X(SO)=0.04, for I-catalyzed oxidation of thianthrene 5-oxide (SSO) reveals that a strongly electrophilic oxidant species is formed on I. The preferential formation of trans-epoxide during epoxidation of 3-methyl-1-cyclohexene demonstrates the steric constraints of the active site of I. The I-catalyzed epoxidation proceeds with an induction period that disappears upon treatment of I with hydrogen peroxide. (29)Si and (183)W NMR spectroscopy and CSI mass spectrometry show that reaction of I with excess hydrogen peroxide leads to fast formation of a diperoxo species, [TBA](4)[gamma-SiW(10)O(32)(O(2))(2)] (II), with retention of a gamma-Keggin type structure. Whereas the isolated compound II is inactive for stoichiometric epoxidation of cyclooctene, epoxidation with II does proceed in the presence of hydrogen peroxide. The reaction of II with hydrogen peroxide would form a reactive species (III), and this step corresponds to the induction period observed in the catalytic epoxidation. The steric and electronic characters of III are the same as those for the catalytic epoxidation by I. Kinetic, spectroscopic, and mechanistic investigations show that the present epoxidation proceeds via III.     

Peroxydisulfate activation by [RuII(tpy)(pic)(H2O)]+. Kinetic, mechanistic and anti-microbial activity studies

The oxidation of [Ru(II)(tpy)(pic)H(2)O](+) (tpy = 2,2',6',2'-terpyridine; pic(-) = picolinate) by peroxidisulfate (S(2)O(8)(2-)) as precursor oxidant has been investigated kinetically by UV-VIS, IR and EPR spectroscopy. The overall oxidation of Ru(II)- to Ru(IV)-species takes place in a consecutive manner involving oxidation of [Ru(II)(tpy)(pic)H(2)O](+) to [Ru(III)(tpy)(pic)(OH)](+), and its further oxidation of to the ultimate product [Ru(IV)(tpy)(pic)(O)](+) complex. The time course of the reaction was followed as a function of [S(2)O(8)(2-)], ionic strength (I) and temperature. Kinetic data and activation parameters are interpreted in terms of an outer-sphere electron transfer mechanism. Anti-microbial activity of Ru(II)(tpy)(pic)H(2)O](+) complex by inhibiting the growth of Escherichia coli DH5α in presence of peroxydisulfate has been explored, and the results of the biological studies have been discussed in terms of the [Ru(IV)(tpy)(pic)(O)](+) mediated cleavage of chromosomal DNA of the bacteria.     

Further mechanistic information on the reaction between FeIII(edta) and hydrogen peroxide: observation of a second reaction step and importance of pH

A detailed study of the effect of buffer, temperature, and pressure on the reaction of hydrogen peroxide with [Fe(III)(edta)H(2)O](-) was performed using stopped-flow techniques. The reaction was found to consist of two steps and resulted in the formation of the already characterized high-spin Fe(III) side-on bound peroxo complex. The second step of the reaction was found to be independent of the hydrogen peroxide concentration. Formation of the purple peroxo complex is only observable above pH 7.5. Both reaction steps are affected by specific and general acid-catalysis. Five different buffer systems were used to clarify the role of general acid-catalysis in these reactions. Both reaction steps reveal an element of reversibility, which disappears on decreasing the acid concentration. The positive volumes of activation for both the forward and reverse reactions of the first step suggest a dissociative interchange substitution process for the reversible end-on binding of hydrogen peroxide to [Fe(III)(edta)H(2)O](-). The small negative volume of activation for the second reaction step suggests an associative interchange mechanism for the formation of the side-on bound peroxo complex that is accompanied by dissociation of one of the four carboxylates of edta. A detailed mechanism in agreement with all the reported kinetic data is presented.     

Oxidative homolysis of a nitrosylchromium complex

Metal(III)-polypyridine complexes [M(NN)(3)](3+) (M = Ru or Fe; NN = bipyridine (bpy), phenanthroline (phen), or 4,7-dimethylphenanthroline (Me(2)-phen)) oxidize the nitrosylpentaaquachromium(III) ion, [Cr(aq)NO](2+), with an overall 4:1 stoichiometry, 4 [Ru(bpy)(3)](3+) + [Cr(aq)NO](2+) + 2 H(2)O --> 4 [Ru(bpy)(3)](2+) + [Cr(aq)](3+) + NO(3)(-) + 4 H(+). The kinetics follow a mixed second-order rate law, -d[[M(NN)(3)](3+)]/dt = nk[[M(NN)(3)](3+)][[Cr(aq)NO](2+)], in which k represents the rate constant for the initial one-electron transfer step, and n = 2-4 depending on reaction conditions and relative rates of the first and subsequent steps. With [Cr(aq)NO](2+) in excess, the values of nk are 283 M(-1) s(-1) ([Ru(bpy)(3)](3+)), 7.4 ([Ru(Me(2)-phen)(3)](3+)), and 5.8 ([Fe(phen)(3)](3+)). In the proposed mechanism, the one-electron oxidation of [Cr(aq)NO](2+) releases NO, which is further oxidized to nitrite, k = 1.04x10(6) M(-1) s(-1), 6.17x10(4), and 1.12x10(4) with the three respective oxidants. Further oxidation yields the observed nitrate. The kinetics of the first step show a strong correlation with thermodynamic driving force. Parallels were drawn with oxidative homolysis of a superoxochromium(III) ion, [Cr(aq)OO](2+), to gain insight into relative oxidizability of coordinated NO and O(2), and to address the question of the "oxidation state" of coordinated NO in [Cr(aq)NO](2+).     

Spectroelectrochemical studies on mixed-valence states in a cyanide-bridged molecular square, [Ru(II)(2)Fe(II)(2)(mu-CN)(4)(bpy)(8)](PF6)(4).CHCl(3).H(2)O

A cyanide-bridged molecular square of [Ru(II) (2)Fe(II) (2)(mu-CN)(4)(bpy)(8)](PF(6))(4).CHCl(3).H(2)O, abbreviated as [Ru(II) (2)Fe(II) (2)](PF(6))(4), has been synthesised and electrochemically generated mixed-valence states have been studied by spectroelectrochemical methods. The complex cation of [Ru(II) (2)Fe(II) (2)](4+) is nearly a square and is composed of alternate Ru(II) and Fe(II) ions bridged by four cyanide ions. The cyclic voltammogram (CV) of [Ru(II) (2)Fe(II) (2)](PF(6))(4) in acetonitrile showed four quasireversible waves at 0.69, 0.94, 1.42 and 1.70 V (vs. SSCE), which correspond to the four one-electron redox processes of [Ru(II) (2)Fe(II) (2)](4+) right arrow over left arrow [Ru(II) (2)Fe(II)Fe(III)] (5+) right arrow over left arrow [Ru(II) (2)Fe(III) (2)](6+) right arrow over left arrow [Ru(II)Ru(III)Fe(III) (2)](7+) right arrow over left arrow [Ru(III) (2)Fe(III) (2)](8+). Electrochemically generated [Ru(II) (2)Fe(II)Fe(III)](5+) and [Ru(II) (2)Fe(III) (2)](6+) showed new absorption bands at 2350 nm (epsilon =5500 M(-1) cm(-1)) and 1560 nm (epsilon =10 500 M(-1) cm(-1)), respectively, which were assigned to the intramolecular IT (intervalence transfer) bands from Fe(II) to Fe(III) and from Ru(II) to Fe(III) ions, respectively. The electronic interaction matrix elements (H(AB)) and the degrees of electronic delocalisation (alpha(2)) were estimated to be 1090 cm(-1) and 0.065 for the [Ru(II) (2)Fe(II)Fe(III) (2)](5+) state and 1990 cm(-1) and 0.065 for the [Ru(II) (2)Fe(III) (2)](6+) states.     

Diruthenium complexes [((acac)2RuIII)2(mu-OC2H5)2], [((acac)(2RuIII)2(mu-L)](ClO4)2, and [((bpy)(2RuII)2(mu-L)](ClO4)4 [L = (NC5H4)2-N-C6H4-N-(NC5H4)2, acac = acetylacetonate, and bpy = 2,2'-bipyridine]. synthesis, structure, magnetic, spectral, and photophysical aspects

Paramagnetic diruthenium(III) complexes (acac)(2)Ru(III)(mu-OC(2)H(5))(2)Ru(III)(acac)(2) (6) and [(acac)(2)Ru(III)(mu-L)Ru(III)(acac)(2)](ClO(4))(2), [7](ClO(4))(2), were obtained via the reaction of binucleating bridging ligand, N,N,N',N'-tetra(2-pyridyl)-1,4-phenylenediamine [(NC(5)H(4))(2)-N-C(6)H(4)-N-(NC(5)H(4))(2), L] with the monomeric metal precursor unit (acac)(2)Ru(II)(CH(3)CN)(2) in ethanol under aerobic conditions. However, the reaction of L with the metal fragment Ru(II)(bpy)(2)(EtOH)(2)(2+) resulted in the corresponding [(bpy)(2)Ru(II) (mu-L) Ru(II)(bpy)(2)](ClO(4))(4), [8](ClO(4))(4). Crystal structures of L and 6 show that, in each case, the asymmetric unit consists of two independent half-molecules. The Ru-Ru distances in the two crystallographically independent molecules (F and G) of 6 are found to be 2.6448(8) and 2.6515(8) A, respectively. Variable-temperature magnetic studies suggest that the ruthenium(III) centers in 6 and [7](ClO(4))(2) are very weakly antiferromagnetically coupled, having J = -0.45 and -0.63 cm(-)(1), respectively. The g value calculated for 6 by using the van Vleck equation turned out to be only 1.11, whereas for [7](ClO(4))(2), the g value is 2.4, as expected for paramagnetic Ru(III) complexes. The paramagnetic complexes 6 and [7](2+) exhibit rhombic EPR spectra at 77 K in CHCl(3) (g(1) = 2.420, g(2) = 2.192, g(3) = 1.710 for 6 and g(1) = 2.385, g(2) = 2.177, g(3) = 1.753 for [7](2+)). This indicates that 6 must have an intermolecular magnetic interaction, in fact, an antiferromagnetic interaction, along at least one of the crystal axes. This conclusion was supported by ZINDO/1-level calculations. The complexes 6, [7](2+), and [8](4+) display closely spaced Ru(III)/Ru(II) couples with 70, 110, and 80 mV separations in potentials between the successive couples, respectively, implying weak intermetallic electrochemical coupling in their mixed-valent states. The electrochemical stability of the Ru(II) state follows the order: [7](2+) < 6 < [8](4+). The bipyridine derivative [8](4+) exhibits a strong luminescence [quantum yield (phi) = 0.18] at 600 nm in EtOH/MeOH (4:1) glass (at 77 K), with an estimated excited-state lifetime of approximately 10 micros.