A kinetic study of the complex formation between aquachromium(III) ions and L-iso-leucine

Summary The kinetics of complex formation between aquachromium(III) ions and L-iso-leucine have been studied spectrophotometrically. Effects of varying the total chromium(III), total amino acid and H⁺ concentrations, ionic-strength, temperature and % EtOH on the k_ohs were determined. The results are best accounted for by outer-sphere complexation equilibria involving HL (the amino acid zwitterion) and [Cr(H₂O)₆]³⁺/[Cr(H₂O)₅OH]²⁺ which precede anations. A rate-equation is established which involves K_os1, K_os2, k₁, k₂ (the respective outer-sphere complexation and interchange rate constants with [Cr(H₂O)₆]³⁺ and [Cr(H₂O)₅OH]²⁺), K_a and K_h (the acid-dissociation constants of H₂L⁺HL and [Cr(H₂O)₆]³⁺ [Cr(H₂O)₅OH]²⁺ pairs). The proposed mechanism is I_a for the path involving hexaaqua- and I_d for that involving hydroxopentaaquachromium(III).     

Mechanism of anation of chromium (III) by L-lysine

The kinetics of L-lysine anation of aquachromium(III) ions have been investigated in the acidity range 5.6 ≤ 10⁵[H⁺] ≤ 31.6 mol dm⁻. The reaction takes place with outer-sphere association between Cr³⁺/CrOH²⁺ and H₂L⁺ (L =⁺HGCH (⁺NH₃)(CO ₂ ^t- ), G being the side chain) followed by transformation of the outer-into an inner-sphere complex by slow interchange. The results are discussed in relation to the data of analogous systems and it is concluded that anation of [Cr(H₂O)₆]^{3 +} follows anI _a path whereas that of [Cr(H₂O)₅OH]_{2 +} follows anI _d path.     

Methionine anation of aquachromium(III)

The kinetics of anation of chromium(III) species, [Cr(H₂O)₆]³⁺ and [Cr(H₂O)₅OH]²⁺, by DL-methionine have been studied spectrophotometrically. Effects of varying [methionine]_T, [H⁺], and temperature were investigated. The results are in accord with a mechanism involving a fast 11 outer-sphere association between chromium(III) species and amino acid zwitterion, followed by transformation of the outer-into inner-sphere complex by slow interchange. The rate law consistent with the mechanism is as follows:

Kinetics and mechanism of oxidation of the binary and ternary complexes of chromium(III) involving inosine and glycine by N -bromosuccinimide

The kinetics of oxidation of the chromium(III) complexes, [Cr(Ino)(H₂O)₅]³⁺ and [Cr(Ino)(Gly)(H₂O)₃]²⁺ (Ino?=?Inosine and Gly?=?Glycine) involving a ligands of biological significance by N-bromosuccinimide (NBS) in aqueous solution to chromium(VI) have been studied spectrophotometrically over the 25–45°C range. The reaction is first order with respect to both [NBS] and [Cr], and increases with pH over the 6.64–7.73 range in both cases. The experimental rate law is consistent with a mechanism in which the hydroxy complexes [Cr(Ino)(H₂O)₄(OH)]²⁺ and [Cr(Ino)(Gly)(H₂O)₂(OH)]⁺ are significantly more reactive than their conjugate acids. The value of the intramolecular electron transfer rate constant, k ₁, for the oxidation of the [Cr(Ino)(H₂O)₅]³⁺ (6.90?×?10^?4?s^?1) is lower than the value of k ₂ (9.66?×?10^?2?s^?1) for the oxidation of [Cr(Ino)(Gly)(H₂O)₂]²⁺ at 35°C and I?=?0.2?mol?dm^?3. The activation parameters have been calculated. Electron transfer apparently takes place via an inner-sphere mechanism.     

Speciation of the Vanadium (III) Complexes with 6-Methylpicolinic Acid,Salicylic Acid and Phthalic Acid

The complex species formed in aqueous solutions (25 °C, I=3.0 mol⋅dm⁻³ KCl ionic medium) between the V(III) cation and the ligands 6-methylpicolinic acid (MePic, HL), salicylic acid (H₂Sal, H₂L) and phthalic acid (H₂Phtha, H₂L) have been studied by potentiometric and spectrophotometric measurements. Application of the least-squares computer program LETAGROP to the experimental emf(H) data, taking into account the hydrolytic species and hydrolysis constants of V(III), indicates that under the employed experimental conditions the complexes [VL]²⁺, [V(OH)L]⁺, [V(OH)₂L], [V(OH)₃L]⁻, [VL₂]⁺, [VL₃] and [V₂OL₄] form in the vanadium(III)–MePic system. Were observed the complexes [VL]⁺, [VL₂]⁻, [V(OH)L₂]²⁻ and [VL₃]³⁻ in the vanadium(III)–H₂Sal system, and the species [VHL]²⁺, [VL]⁺, [V(OH)L], [VHL₂], [VL₂]⁻, [V(OH)L₂]²⁻, [V(OH)₂L₂]³⁻ and [VL₃]³⁻ in the vanadium(III)–H₂Phtha system. The stability constants of these complexes were determined by potentiometric measurements, and spectrophotometric measurements were made in order to perform a qualitative characterization of the complexes formed in aqueous solution.     

Über Reaktionen oxygenierter Kobalt(II)-Komplexe. IX. Oxydative Eigenschaften von Tetrakis(äthylendiamin)-μ-peroxo-μ-hydroxo-dikobalt(III)

Reactions of oxygenated cobalt(II) complexes. IX. Oxidative properties of tetrakis(ethylenediamine)-μ-peroxo-μ-hydroxo-dicobalt(III)

1 VIII s. [1].

[(en)₂Co(O₂, OH)Co(en)₂]³⁺ ( a ) reacts with I^? in acidic aqueous solution according to: Co^III(O₂, OH)Co^III + 21^? + 5H⁺ ? 2Co^III + 3H₂O + I₂. Using I^? in excess first order rate constants are obtained which, to a first approximation, are independent of [I^?]. Comparison with kinetic data of deoxygenation of [(en)₂Co(O₂, OH)Co(en)₂]³⁺ under analogous conditions suggests that both reactions have the same rate determining step. The singly bridged species [(en)₂(H₂O)CoO₂Co(H₂O) (en)₂]⁴⁺ is shown to be the reactive intermediate in the iodide oxidation (Schema 2).     

Reactions of 2-(1-carboxyl-2-hydroxyphenyl)thiazolidine with chromium(III) salts leading to newer synthesis of chromium(III) complexes

Reactions of 2-(L-carboxyl-2-hydroxyphenyl)thiazolidine with different chromium(III) salts [CrCl₃?·?6H₂O, K₃[Cr(SCN)₆], NH₄[Cr(NH₃)₂(SCN)₄]?·?H₂O, [Cr(urea)₆]Cl₃?·?3H₂O and [Cr(CH₃COO)₂H₂O]₂] under varied reaction conditions afforded many new mixed-ligand chromium(III) complexes. The ligand is a tridentate dibasic NSO donor except for complexes 1 and 4 where two moles of the ligand are present for each molecule of complex, one functioning as a dibasic tridentate (NSO) and the other as a monobasic bidentate (NS) (phenolic OH and carboxylic COOH groups remaining uncoordinated). The complexes have been characterized by elemental analyses, magnetic susceptibilities, molar conductances, molecular weights and spectroscopic (IR, Uv-vis) data. The ligand field parameters and NSH Hamiltonian parameters suggest tetragonal geometries of the complexes.     

Dioxygen binding and activation by a highly reactive Cr(II) compound containing S,N-donors derived from o-aminothiophenol

We report the synthesis, characterization, and reactivity of a Cr(II) complex, [Cr(H₂O)(L^ISQ)₂] (1) [(L^ISQ)^1? is o-iminothionebenzosemiquinonate(1?) π-radical], that is highly stable in solid state in the presence of air but undergoes spontaneous change in solution, both in the presence and absence of air. Physicochemical studies in solution show that a superoxo-Cr^III species, [Cr(O₂)(OH)(L^ISQ)₂]^? is generated initially in DMF solution of 1 in the presence of air owing to its immediate deprotonation followed by O₂ binding to the deprotonated species. The formation of this superoxo-Cr^III species is prominent and gradual in the presence of CH₃OH, a scavenger of CrO²⁺ species. This Cr(O₂)²⁺ species in turn is converted to another highly reactive O=Cr(IV) intermediate [O=Cr(OH)(L^ISQ)₂]^? which undergoes disproportionation producing an unstable O=Cr(V) species, [O=Cr(OH)(L^ISQ)₂] and a stable Cr(III) compound, [Cr(OH)(DMF)(L^ISQ)₂] (2). The rate of this disproportionation is enhanced in the presence of MnCl₂, [N(n-Bu)₄]PF₆ and KSCN. The generated O=Cr(IV) species interacts with DNA with complete cleavage. The O=Cr(V) species slowly disappears from solution as revealed from EPR studies.     

Kinetics and mechanism of the reaction between dimethylformamide and chromium(VI)

The kinetics of oxidation of N,N‐dimethylformamide by chromium(VI) has been studied spectrophotometrically in aqueous perchloric acid media at 20°C. The rate showed a first‐order dependence on both [Cr(VI)] and [DMF], and increased markedly with increasing [H⁺]. The order with respect to [HClO₄] was found to lie between 1 and 2. The rate was found to be independent of ionic strength as well as of any inhibition effect of Mn(II). The formation of superoxochromium(III) ion was detected in an aerated solution of chromium(VI), DMF and HClO₄. The proposed mechanism, involving two reaction pathways, leads to the rate law, rate = K_a1 [HCrO₄⁻] [DMF] (k_I K_a2 [H⁺]²+k_II[H⁺]). The first pathway, with rate constant k_I, involves the formation of chromium(V) and a free radical. The second pathway, with rate constant k_II, involves the formation of Cr(IV), CO₂ and dimethylamine. © 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 409–415, 1999     

Strukturchemische Verwandtschaft von Kaliumhexahydroxoscandat(III), K3[Sc(OH)6] mit den isotypen Hydroxometallaten Rb3[Sc(OH)6], K3[Cr(OH)6] und Rb3[Cr(OH)6]

Structural Relationship of Potassium Hexahydroxoscandate(III), K₃[Sc(OH)₆] with the Isotypic Hydroxometallates Rb₃[Sc(OH)₆], K₃[Cr(OH)₆], and Rb₃[Cr(OH)₆] Ternary hydroxides M}M^III(OH)₆{ with M^I ? K, Rb and M^III ? Sc, Cr were obtained in the same way as K₃[Cr(OH)₆] [1] from alkali metal amides and d-metal nitrates by a comproportionation reaction of amide and nitrate ions in supercritical ammonia to elementary nitrogen and hydroxide ions at 523 K and 3 ≤ p(NH₃) ≤ 6 kbar within 1 to 3 months. Their structures were determined by single crystal x-ray methods inclusive the positions of the hydrogen atoms. The ratio of size of r(M^I)/r(M^III) is related to the symmetry of these hydroxometallates. Structural relationships between K₃[Sc(OH)₆] and Rb₃[Sc(OH)₆], K₃(Cr(OH)₆], Rb₃[Cr(OH)₆]) and K₄[CdCl₆] [4] are discussed.     

Kinetics and mechanism of the complexation between hydroxopentaaquochromium(III) and octacyanomolybdate(IV) ions in acidic medium

Summary The reaction between hydroxopentaaquochromium(III) and octacyanomolybdate(IV) was investigated spectrophotometrically and obeyed a 2:1 reactant stoichiometry with respect to formation of the [Cr(H₂O)₄OH]₂ Mo(CN)₈ complex. Kinetic studies reveal that the reaction is first order with respect to hydroxopentaaquochromium(III) in the presence of an excess of octacyanomolybdate(IV). The reaction rate increased with an increase in the ionic strength and temperature, and decreased with an increase in hydrogen ion concentration. A mechanism has been proposed based upon ion-pair formation. The results are best accounted for by the Eigen-Tamm mechanism. Anation of [Cr(H₂O)₅OH]²⁺ is discussed in terms of an associative interchange (I _a) where bond breaking and bond making are equally important. The activation parameters were calculated using Arrhenius's equation.     

Kinetics of the heterobimetallic complexes formation in the reactions between chromium(III) macrocyclic species and [Fe(CN)<Subscript>6</Subscript>]<Superscript>4−</Superscript> ions

Formation of a singly bridged heterobimetallic Cr^III–NC–Fe^II anation product of the cis − [Cr(cycb)(H₂O)₂]³⁺ and trans − [Cr(cyca)(H₂O)₂]³⁺ complexes, where cyca and cycb are meso- and rac-5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane respectively, by [Fe(CN)₆]⁴⁻ ions is accompanied by an intensive absorbance increase within 390–470 nm due to an intermetal electron transition. A bell-shape of the pseudo-first order rate constants/pH profile observed for the reactions which have been studied under a large excess of the iron(II) complex is in accordance with the highest reactivity of the chromium(III) complexes in their monohydroxomonoaqua forms. The reaction mechanism has been discussed based on the determined rate law.     

Crystallization method controlled trinuclear or ion-pair Manganese(III)-porphyrin-based heterobimetallic complexes: Synthesis,spectra characterization,and crystal structure

Cyanide-bridged trinuclear heterometallic Ag(I)-Mn(III) complex {[Mn(TClPP)(H₂O)]₂[Ag(CN)₂]}₂ · 2Br · 2C₃H₆O · 3H₂O (I) and ion-pair complex {[Mn(TClPP)(CH₃OH)₂][Ag(CN)₂]} · 0.5H₂O (II) have been synthesized with [Mn(TClTPP)(H₂O)₂]Br (H₂TClTPP = meso-tetra(4-chlorophenyl)porphyrin) as assembling segment and K[Ag(CN)₂] as building block by using different crystallization method. These two complexes have been characterized by elemental analysis, IR spectroscopy and X-ray structure determination. In the trinuclear complex I, [Ag(CN)₂]^? as bidentate ligand coordinates with the two central Mn(III) atom of [Mn(TClPP)(H₂O)₂]⁺ through its two trans cyanide groups to form the complex cation of [Mn(TClPP)(H₂O)]₂[Ag(CN)₂]⁺, which further constructs the neutral complexes with the help of one Br^? as balanced anion. For the ion-pair complex II composed by free [Mn(TClPP)(CH₃OH)₂]⁺ cation and free [Ag(CN)2]^? anion, it can be linked into one-dimensional supramolecular structure with the dependence of the intermolecular O-H...N and O-H...O hydrogen bond interactions.     

Kinetics and mechanism of oxidation of cis-diaquabis(glycinato)chromium(III) by periodate ion in aqueous solutions

The kinetics of oxidation of cis-[Cr^III(gly)₂(H₂O)₂]⁺ (gly = glycinate) by $ {\text{IO}}_{ 4}^{ - } $ has been studied in aqueous solutions. The reaction is first order in the chromium(III) complex concentration. The pseudo-first-order rate constant, k _obs, showed a small change with increasing $ \left[ {{\text{IO}}_{ 4}^{ - } } \right] $ . The pseudo-first-order rate constant, k _obs, increased with increasing pH, indicating that the hydroxo form of the chromium(III) complex is the reactive species. The reaction has been found to obey the following rate law: $ {\text{Rate}} = 2k^{\text{et}} K_{ 3} K_{ 4} \left[ {{\text{Cr}}\left( {\text{III}} \right)} \right]_{t} \left[ {{\text{IO}}_{ 4}^{ - } } \right]/\left\{ {\left[ {{\text{H}}^{ + } } \right] + K_{ 3} + K_{ 3} K_{ 4} \left[ {{\text{IO}}_{ 4}^{ - } } \right]} \right\} $ . Values of the intramolecular electron transfer constant, k ^et, the first deprotonation constant of cis-[Cr^III(gly)₂(H₂O)₂]⁺, K ₃ and the equilibrium formation constant between cis-[Cr^III(gly)₂(H₂O)(OH)] and $ {\text{IO}}_{ 4}^{ - } $ , K ₄, have been determined. An inner-sphere mechanism has been proposed for the oxidation process. The thermodynamic activation parameters of the processes involved are reported.     

Kinetics and mechanism of oxidation of tris(ethylenediamine)-chromium(III) by N-bromosuccinimide

Summary In aqueous solutions, [Cr(en)₃]³⁺ aquates to [Cr(en)₂-(H₂O) ₂]³⁺. A kinetic study of the oxidation of [Cr(en)₃]³⁺ by N-bromosuccinimide (NBS) in aqueous solutions and water-alcohol solvent mixtures was performed. The reaction is first order with respect to both total [Cr^III] and [NBS]. The rate is inversely dependent upon [H⁺] in the 7.0–7.9 pH range, and varies with the co-solvent according to the order: MeOH > EtOH > PrOH. An appropriate mechanism, in which the deprotonated [Cr(en)₂(OH)(H₂O)]²⁺ is the reactive species, is suggested. Thermodynamic activation parameters have been calculated.Abstracted from the PhD thesis (Ain Shams University) of A. E.- D. M. Abdel-Hady.     

Photosubstitution in some cyanide complexes of chromium(III)(1)

Photosubstitution by OH^? ligand was concluded from a photochemical study of the [Cr(CN)₆]^3? and [Cr(CN)₅OH]^3? complexes in alkaline medium. Photoaccelerated aquation was found to proceed in the case of aquocyanochromates(III): [Cr(CN)₅H₂O]^2? and [Cr(CN)₃(H₂O)₃].     

State of Rh(III) in Hydrofluoric Acid Solutions

A simple method is developed for synthesizing [Rh(H₂O)₆]F₃. 3H₂O with a yield of 80–90%. ¹⁹F, ¹⁰³Rh, and ¹⁷О NMR spectroscopic studies show that the following three processes simultaneously run in the Rh(III)–HF/K–H₂O system via parallel routes: the formation of mononuclear aquafluoro complexes [Rh(H₂O)₆]³⁺ + F–→ [Rh(H₂O)₅F]²⁺ + H₂O; the formation of aquahydrofluoro complexes [Rh(H₂O)₆]³⁺ + HF₂^-→ [Rh(H₂O)₅HF₂]²⁺ + H₂O; and hydrolysis of the aqua ion followed by coordination of fluoride ion and condensation of the hydroxo species [Rh(H₂O)₆]³⁺ + 2F^– → [Rh(H₂O)₄(OH)F]⁺ + HF → condensation. [Rh(H₂O)₆]³⁺ and [Rh(H2O)5F]²⁺ are the two species making a major contribution to the material balance at high acidity under equilibrium conditions. Parameters of the ¹⁹F NMR spectra of individual complex species are presented.     

Kinetics and mechanisms of oxidation by metalions. Part XI(1). Kinetics of the osmium(VIII)-catalysed oxidation of glycols by alkaline hexacyanoferrate(III)

Summary The kinetics of the Os^VIII-catalysed oxidation of glycols by alkaline hexacyanoferrate(III) ion exhibits zerothorder dependence in [Fe(CN) ₆ ^3– ] and first-order dependence in [OsO₄]. The order with respect to glycols is less than unity, whereas the rate dependence on [OH^–] is a combination of two rate constants; one independent of and the other first-order in [OH^–]. These observations are commensurate with a mechanism in which two complexes, [OsO₄(H₂O)G]^– and [OsO₄(OH)G]^2–, are formed either from [OsO₄(H₂O)(OH)]^– or [OsO₄(OH)₂]^2– and the glycol GH, or by [OsO₄(H₂O)₂] and [OsO₄(H₂O)(OH)]^– and the glycolate ion, G^–, which is in equilibrium with the glycol GH through the reaction between GH and OH^–. Hence there is an ambiguity about the true path for the formation of the two Os^VIII-glycol complexes. A reversal in the reactivity order of glycols in the two rate-determining steps, despite the common attack of OH^– ion on the two species of Os^VIII-complexes, indicates that the two complexes are structurally different because S changes from the negative (corresponding to k₁₁) to positive (related to k₂).     

Hexamolybdoaluminates and Hexamolybdochromates of Some Trivalent Lanthanides and Americium

The reactions of some rare-earth elements (La, Ce, Sm, Lu, Gd) and Am(III) with heteropolymolybdate anions are studied. The complexation rate constants of Am(III) in a solution with Al(OH)₆Mo₆O₁₈ ^3- and Cr(OH)₆MoO₁₈ ^3- ₆ (₁ = 18 ± 6 and 25 ± 5, respectively) are determined by spectral methods. The Ln[Al(OH)₆Mo₆O₁₈] · nH₂O, Eu[Cr(OH)₆Mo₆O₁₈] · nH₂O, and Am[Al(OH)₆Mo₆O₁₈] · nH₂O isostructural compounds are synthesized. The crystal structure of the Sm[Al(OH)₆Mo₆O₁₈] · 11H₂O complex was determined by the X-ray diffraction analysis. The results of spectroscopic and thermogravimetric studies of the obtained compounds are presented.