New chromium(III)–picolinamide complexes. Kinetics and mechanism of picolinamide liberation in HClO4 solutions

Two new chromium(III) complexes with picolinamide (pica) and oxalates, [Cr(C₂O₄)₂(N,N′-pica)]²⁻ and [Cr(C₂O₄)₂(N,O-pica)]⁻, were obtained and the kinetics of their aquation in HClO₄ solutions were studied. The aquation leads to pica liberation and proceeds in two stages: (i) the chelate-ring opening at the Cr–amide bond and (ii) the Cr–N-pyridine bond breaking, which gives free pica and cis-[Cr(C₂O₄)₂(H₂O₂)₂]⁻. In the case of N,N′-bonded pica the kinetics of both stages was determined and in the case of the N,O-bonded pica only the second stage was investigated. The following rate laws were established: (k _obs)₁ = k ₀ + k ₁ Q ₁[H⁺] and (k _obs)₂ = k ₂ Q ₂[H⁺], where k ₀ and k ₁ are the rate constants of the chelate-ring opening in the unprotonated and protonated starting complex, and k ₂ is the rate constant of the pica liberation from the protonated intermediate. Kinetic parameters are calculated and the aquation mechanism is discussed.     

Kinetics and mechanism of aquation of chromium(III) complexes with 3-hydroxypicolinic and 2-hydroxynicotinic acids in HClO<Subscript>{4{</Subscript> solutions

Two complexes, [Cr(3-hpic)_{3}]^{0} and [Cr(2-hnic)_{3}]^{0} (where 3-hpic = hydroxypicolinic acid and 2-hnic = 2-hydroxynicotinic acid anions), were prepared and characterized in solution. The 3-hpic ligand forms a 5-membered chelate ring via pyridine nitrogen and carboxlate oxygen atoms, whereas the 2-hnic ligand forms a 6-membered chelate ring via carboxylate and phenolate oxygen atoms. The kinetics of the acid-catalyzed aquation were studied spectrophotometrically in the 0.1–1.0 HClO_{4} range, at I = 1.0 . The rate equations for the first aquation step – the chelate-ring opening – was determined and a mechanism was proposed. In the case of [Cr(3-hpic)_{3}]^{0}, the reversible chelate-ring opening at the Cr—N bond precedes much slower than the second aquation step – a one-end bonded ligand liberation. The equation rate is of the form: k _{obs} = k _{1} + k _{-1} /Q _{1}[H^{+}], where k _{1} and k _{-1} are the rate constants for the forward and the reverse processes in the unprotonated substrate and Q _{1} is the protonation constant of the non-bonded pyridine nitrogen atom. In the case of [Cr(2-hnic)₃]^{0}, the chelate-ring opening at the Cr—O (phenolate) bond is the rate-determining step. The observed pseudo-first order rate constant increases as [H^{+}] increases: k _{obs} = k _{0} + k _{H} Q _{H}[H^{+}], where k _{0} and k _{H} are the rate constants of the spontaneous and acid catalyzed processes and Q _{H} is the protonation constant of the coordinated phenolate oxygen atom. The results lead to the conclusion that an aquation mechanism depends on the coordination mode of the ligand.     

Chromium(III)-isocinchomeronato and quinolinato complexes: kinetic studies in NaOH solutions and effect on 3T3 fibroblast proliferation

The aquation of chromium(III)-isocinchomeronato and quinolinato complexes, mer-[Cr(icaH)₃]⁰ and mer-[Cr(quinH)₃]⁰ (where icaH⁻ and quinH⁻ are N,O-bonded isocinchomeronic and quinolinic acid anion, respectively) was studied in NaOH solutions. The process leads to successive ligand liberation in the fully deprotonated species. The kinetics of the first ligand liberation were studied spectrophotometrically in the visible region. A mechanism is proposed in which the rate of the chelate-ring opening at the Cr–N bond is much faster than the rate of the Cr–O bond breaking. The rate-determining step is described by the rate law: k _obs1 = k _OH(1) + k _O Q ₂ [OH⁻], where k _OH(1) and k _O are rate constants of the first ligand liberation from the hydroxo- and oxo-forms of the intermediate, respectively, and Q ₂ is an equilibrium constant between these two protolytic forms. The first pseudo-first-order rate constants (k _obs1) were calculated using SPECFIT software for an A → B → C reaction pattern. The results are compared with those determined in acidic medium. Kinetics of the second and third ligand liberation were also studied and values of successive pseudo-first-order rate constants (k _obs2, k _obs3) are [OH⁻] independent. Effect of chromium(III)-quinolinato and isocinchomeronato complexes on 3T3 fibroblast proliferation was evaluated. Cytotoxicity of these complexes is low, suggesting they may be promising candidates as novel dietary supplements.     

Kinetics and mechanism of base hydrolysis of chromium(III) complexes with oxalates and quinolinic acid

Base hydrolysis of [Cr(ox)₂(quin)]³⁻ (where quin²⁻ is N,O-bonded 2,3-pyridinedicarboxylic acid dianion) causes successive ligand dissociation and leads to a formation of a mixture of oligomeric chromium(III) species, known as chromates(III). The reaction proceeds through [Cr(ox)(quin)(OH)₂]³⁻ and [Cr(quin)(OH)₄]³⁻ formation. Dissociation of oxalato ligands is preceded by the opening of the Cr-quin chelate-ring at the Cr–N bond. The kinetics of the chelate-ring opening and the first oxalate dissociation were studied spectrophotometrically, within the lower energy d–d band region at 0.4–1.0 M NaOH. The pseudo-first-order rate constants (k _obs0 and k _obs1) were calculated using SPECFIT software for an A → B → C reaction pattern. Additionally, kinetics of base hydrolysis of [Cr(ox)(quin)(OH)₂]³⁻ and cis-[Cr(ox)₂(OH)₂]³⁻ were studied. The determined pseudo-first-order rate constants were independent of [OH⁻]. A mechanism is postulated that the reactive intermediate with the one-end bonded quin ligand, [Cr(ox)₂(O-quin)(OH)]⁴⁻, formed in the first reaction stage, subsequently undergoes oxalates substitution. Kinetic parameters for the chelate-ring opening and the first oxalate dissociation were determined.     

Synthesis and kinetic studies in aqueous solution on chromium(III) complexes with isocinchomeronic acid—potential new biochromium sources

The chromium(III) complexes with a new potential chromium transporting ligand—2,5-pyridinedicarboxylic acid (isocinchomeronic acid, icaH₂):[Cr(icaH)₃]⁰, [Cr(icaH)₂ (H₂O)₂]⁺ and [Cr(icaH)(H₂O)₄]²⁺ (where icaH = N,O-bonded isocinchomeronic acid anion), have been obtained and characterized in solution. The [Cr(icaH)₃]⁰ complex undergoes aquation in acidic media to the diaqua-product. Kinetics of this process was studied spectrophotometrically in the 0.1–1.0 M HClO₄ range, at I = 1.0 M. The first aquation stage, the chelate-ring opening at the Cr–N bond, is a much faster than the second one. The rate laws are of the form: k _obs = k ₁ + k ₋₁/Q ₁[H⁺] and k _obs = k ₂ Q ₂[H⁺]/(1 + Q ₂[H⁺]), where k ₁ and k ₂ are the rate constants for the chelate-ring opening and the ligand liberation, respectively, k ₋₁ is the rate constant of the chelate-ring closure, Q ₁ and Q ₂ are the protonation constants of the pyridine nitrogen and 5-carboxylate group in the one-end bonded intermediate, respectively. The results are discussed in terms of potential pharmaceutical application of the complex.     

Chromium(III)-isonicotinate Complexes. Kinetics and Mechanism of Isonicotinate Ligand Liberation in HClO4 Solutions

Chromium(III)-isonicotinate complexes, cis-[Cr(C₂O₄)₂(N-inic)(H₂O)]^- and [Cr(C₂O₄)(H₂O)₃-OH-Cr(C₂O₄)₂(O-inic)]^-(N-inic)(H₂ (N-inic = N-bonded and O-inic = O-bonded isonicotinic acid) were obtained and characterized in solution. Kinetics of acid-catalyzed isonicotinate ligand liberation were studied spectrophotometrically in the 0.1–1.0 m HClO₄ range, at I=1.0 m. The dependencies of the pseudo-first order rate constant on [H⁺] were established: k_obs = k₀+k_HQ_H[H⁺] and k_obs = k_HQ_H[H⁺] for the N-inic and O-inic complex, respectively, where k₀ and k_H are the rate constants of the spontaneous and the acid-catalyzed reaction paths, and Q_H is the protonation constant of the carboxylic group in isonicotinic ligand. The obtained results indicate that N-bonded isonicotinic acid liberation occurs mainly via a spontaneous reaction path and is much slower than O-bonded inic liberation. The mechanisms for these processes are proposed.     

Kinetic studies on chromium-glycinato complexes in acidic and alkaline media

Two solid complexes, fac–[Cr(gly)₃] and [Cr(gly)₂(OH)]₂, (where gly is glycinato ligand) were prepared and their acid-catalysed aquation products were identified. The structure of [Cr(gly)₃] was solved by X-ray diffraction, revealing a cationic 3D sublattice with perchlorate anions inside its cavities. Acid-catalysed aquation of [Cr(gly)₃] and [Cr(gly)₂(OH)]₂ leads to the same inert product, [Cr(gly)₂(H₂O)₂]⁺, in a two-stages process. At the first stage, intermediate complexes, [Cr(gly)₂(O–glyH)(H₂O)]⁺ and [Cr(gly)₂(H₂O)–OH–Cr(gly)₂(H₂O)]⁺, are formed respectively. Kinetics of the first aquation stage of [Cr(gly)₃] were studied in HClO₄ solutions. The dependencies of the pseudo first-order rate constants on [H⁺] are as follows: k _obs1H = k ₀ + k ₁ K _p1[H⁺], where k ₀ and k ₁ are rate constants for the chelate-ring opening via spontaneous and acid-catalysed reaction paths, respectively, and K _p1 is the protonation constant. The proposed mechanism assumes formation of the reactive intermediate as a result of proton addition to the coordinated carboxylate group of the didentate ligand. Some kinetic studies on the second reaction stage, the one-end bonded glycine liberation, were also done. The obtained results were analogous to those for stage I. In this case, the proposed reactive species are intermediates, protonated at the carboxylate group of the monodentate glycine. Base hydrolysis of two complexes, [Cr(gly)₂(O–gly)(OH)]⁻ and [Cr(gly)₂(OH)₂]⁻, was studied in 0.2–1.0 M NaOH. The pseudo first-order rate constants, k _obsOH, were [OH⁻] independent in the case of [Cr(gly)₂(O–gly)(OH)]⁻, whereas those for [Cr(gly)₂(OH)₂]⁻ linearly depended on [OH⁻]. The reaction mechanisms were proposed, where the OH⁻ -catalysed reaction path was rationalized in terms of formation of the reactive conjugate base, [Cr(gly)₂(OH)(O)]²⁻, as a result of OH⁻ ligand deprotonation. Activation parameters were determined and discussed.     

Kinetics and mechanism of tris-quinolinatochromium(III) aquation in HClO<Subscript>4</Subscript> media

The chromium(III)-quinolinato complexes, [Cr(quinH)₃]⁰, [Cr(quinH)₂(H₂O)₂]⁺ and [Cr(quinH)(H₂O)₄]²⁺ (where quinH = N,O-bonded quinolinic acid anion), were obtained and characterized in solution. The tris-quinolinato complex undergoes acid-catalyzed aquation to give the diaqua-product, whereas subsequent ligand liberation processes are exceptionally slow. Kinetics of the aquation were studied spectrophotometrically over the 0.1–1.0 M HClO₄ range, at I = 1.0 M. The first aquation stage, the chelate-ring opening at the Cr-N bond, is much faster than the second one. The following rate laws were established: k _obs = k ₁ + k ₋₁/Q ₁[H⁺] and k _obs = k ₂ Q ₂[H⁺]/(1 + Q ₂[H⁺]), where k ₁ and k ₂ are the rate constants for the chelate-ring opening and the ligand liberation, respectively, k ₋₁ is the rate constant of the chelate-ring closure, Q ₁ and Q ₂ are the protonation constants of the pyridine nitrogen and 3-carboxylate group in the one-end bonded intermediate, respectively. Kinetic parameters have been determined and the mechanism has been discussed.     

Chromium(III) complexes with lutidinic acid: kinetic studies in HClO<Subscript>4</Subscript> and NaOH solutions

Chromium(III)-lutidinato complexes of general formula [Cr(lutH) _n (H₂O)_6−2n ]³⁻ⁿ (where lutH⁻ is N,O-bonded lutidinic acid anion) were obtained and characterized in solution. Acid-catalysed aquation of [Cr(lutH)₃]⁰ leads to only one ligand dissociation, whereas base hydrolysis produces chromates(III) as a result of subsequent ligand liberation steps. The kinetics of the first ligand dissociation were studied spectrophotometrically, within the 0.1–1.0 M HClO₄ and 0.4–1.0 M NaOH range. In acidic media, two reaction stages, the chelate-ring opening and the ligand dissociation, were characterized. The dependencies of pseudo-first-order rate constants on [H⁺] are as follows: k _obs1 = k ₁ + k ₋₁/K ₁[H⁺] and k _obs2 = k ₂ K ₂[H⁺]/(1 + K ₂[H⁺]), where k ₁ and k ₂ are the rate constants for the chelate-ring opening and the ligand dissociation, respectively, k ₋₁ is the rate constant for the chelate-ring closure, and K ₁ and K ₂ are the protonation constants of the pyridine nitrogen atom and coordinated 2-carboxylate group in the one-end bonded intermediate, respectively. In alkaline media, the rate constant for the first ligand dissociation depends on [OH⁻]: k _obs1 = k _OH(1) + k _O[OH⁻], where k _OH(1) and k _O are rate constants of the first ligand liberation from the hydroxo- and oxo-forms of the intermediate, respectively, and K ₂ is an equilibrium constant between these two protolytic forms. Kinetic parameters were determined and a mechanism for the first ligand dissociation is proposed. The kinetics of the ligand liberation from [Cr(lut)(OH)₄]³⁻ were also studied and the values of the pseudo-first-order rate constants are [OH⁻] independent.     

Kinetic studies on H+-catalysed aquation of some chromium(III)-oxalato-amino acid complexes

The following chromium(III) complexes with serine (Ser) and aspartic acid (Asp) were obtained and characterized in solution: [Cr(ox)₂(Aa)]²⁻ (where Aa = Ser or Asp), [Cr(AspH₋₁)₂]⁻ and [Cr(ox)(Ser)₂]⁻. In acidic solutions, [Cr(ox)₂(Aa)]²⁻ undergoes acid-catalysed aquation to cis-[Cr(ox)₂(H₂O)₂]⁻ and the appropriate amino acid. [Cr(ox)(Ser)₂]⁻ undergoes consecutive acid-catalysed Ser liberation to give [Cr(ox)(H₂O)₄]⁺, and the [Cr(Asp)₂]⁻ ion is converted into [Cr(Asp)(H₂O)₄]²⁺. Kinetics of these reactions were studied under isolation conditions. The determined rate expressions for all the reactions are of the form: k _obs = a + b[H⁺]. Reaction mechanisms are proposed, and the meaning of the determined parameters has been established. Evidence for the formation of an intermediate with O-monodentate amino acid is given. The effect of the R-substituent at the α-carbon atom of the amino acid on the complex reactivity is discussed.     

Kinetic studies on H<Superscript>+</Superscript>-catalysed aquation of chromium(III)-oxalato-asparaginato and chromium(III)-oxalato-histidinato complexes

Three chromium(III) complexes with asparagine (Asn) and histidine (His) of the [Cr(ox)₂(Aa)]²⁻ type, where Aa = N,O–Asn, N,O–His or N,N′–His, were obtained and characterized in solution. The complexes with N,O–Aa undergo acid-catalysed aquation to give a free amino acid and cis-[Cr(ox)₂(H₂O)₂]⁻, whereas the complex with N,N′–His undergoes parallel reaction paths: (1) isomerization to the N,O–His complex and (2) liberation of an oxalate ligand. Kinetics of the N,O–Aa complexes in HClO₄ media were studied spectrophotometrically under pseudo-first-order conditions. The absorbance changes were attributed to the chelate ring opening at the Cr–N bond. The linear dependence of rate constants on [H⁺] was established, and a mechanism for the chelate ring cleavage was postulated. The existence of a metastable intermediate with O-monodentate Aa ligand was proved experimentally. Effect of [Cr(ox)₂(Aa)]²⁻ on 3T3 fibroblasts proliferation was studied. The tests revealed low cytotoxicity of the complexes. Complexes with Ala, His and Cys are good candidates for biochromium sources.     

Mixed-ligand chromium(III)-oxalate-pirydinedicarboxylate complexes: potential biochromium sources: kinetic studies in NaOH solutions and effect on 3T3 fibroblasts proliferation

Base hydrolysis of [Cr(ox)₂(pda)]³⁻ (where pda is N,O-bonded 2,4- and 2,5- pyridinedicarboxylic acid dianion) causes successive ligand dissociation and leads to formation of a mixture of oligomeric chromium(III) species, known as chromates(III). The main reaction path proceeds through [Cr(ox)(pda)(OH)₂]³⁻ and [Cr(pda)(OH)₄]³⁻ complexes. The kinetics of the first oxalate dissociation was studied spectrophotometrically, within the lower energy d–d band region, at 0.4–1.0 M NaOH. The character of spectroscopic changes was consistent with a consecutive reaction model, where the chelate-ring opening and the one-end bonded oxalato liberation are the first and the second reaction stages. The pseudo-first order rate constants (k _obs0 and k _obs1) were calculated using SPECFIT software for an A → B → C reaction pattern. Additionally, kinetics of base hydrolysis of [Cr(ox)₃]³⁻ were studied. The calculated rate constants were independent of [OH ⁻ ]. Kinetic parameters for the chelate-ring opening and the first oxalate dissociation were determined. Effect of the [Cr(ox)₂(pda)]³⁻ and [Cr(2,4-pda)₃]³⁻ complexes on 3T3 fibroblasts proliferation was studied. The results manifested low cytotoxicity of these complexes, which makes them promising candidates for dietary supplements.     

Mixed chromium(III) complexes with pyridinedicarboxylato and oxalato ligands: kinetic studies in HClO<Subscript>4</Subscript> solutions

Three chromium(III) complexes of general formula [Cr(ox)₂(pdaH)]²⁻ (where ox = C₂O₄ ²⁻ and pdaH⁻ is N,O-bonded 2,3-, 2,4- or 2,5-pyridinedicarboxylic acid anion) were obtained and characterized in solution. Acid-catalysed aquation of [Cr(ox)₂(pdaH)]²⁻ gave two products: [Cr(ox)(pdaH)(H₂O)₂]⁰ (P₁) and cis-[Cr(ox)₂(H₂O)₂]²⁻ (P₂). The kinetics of these reactions were studied spectrophotometrically, within the 0.1–1.0 M HClO₄ range, and the pseudo-first-order rate constants for the oxalato (k _obs1) and pdaH⁻ (k _obs2) ligands dissociation were calculated based on the determined pseudo-first-order rate constants (k _obs) and P₁:P₂ molar ratio. The dependencies of the pseudo-first-order rate constants on [H⁺] are as follows: k _obs1 = b ₁[H⁺] and k _obs2 = b ₂[H⁺], where b ₁ and b ₂ are the second-order rate constants for the oxalato and pdaH⁻ ligands dissociation, respectively. Kinetic parameters were determined and the mechanism of the pdaH⁻ ligand dissociation is proposed.     

Kinetics and mechanism of base hydrolysis of mer-[Cr(pic)3]0 and [Cr(ox)2(pic)]2? (pic?=?picolinate, ox?=?oxalate)

Mer-[Cr(pic)₃]⁰ and [Cr(ox)₂(pic)]²⁻ undergo successive base hydrolysis to give chromates(III). Dissociation of the first ligand, pic from [Cr(pic)₃]⁰ and ox from [Cr(ox)₂(pic)]²⁻, proceeds in two stages, namely initial chelate-ring opening followed by slower liberation of the monodentate ligand. Kinetics of both the stages were studied spectrophotometrically in 0.2–0.9 M NaOH solution, under pseudo-first-order conditions. The calculated values of k _obs were independent of [OH⁻]. A mechanism is proposed, where the formation of intermediates in the hydroxo form prevents the monodentate ligand from undergoing chelate-ring closure. Evidence for the formation of an intermediate with O-bonded picolinate is given. The effects of pH and the complex composition on the reactivity are discussed.     

New chromium(III)–nicotinate complexes. Kinetics and mechanism of nicotinate ligand liberation in acidic media

Two new chromium(III)–nicotinate complexes, cis-[Cr(C₂O₄)₂(O-nic)(H₂O)]⁻ and cis-[Cr(C₂O₄)₂(N-nic)(H₂O)]⁻, were obtained and characterized in solution (where O-nic=O-bonded and N-nic=N-bonded nicotinic acid). The kinetics of nicotinate ligand liberation were studied spectrophotometrically in the 0.1–1.0 m HClO₄ range, at I=1.0 m. The rate equations were determined and a mechanism is proposed. The rate of Cr–O bond breaking is [H⁺] dependent: k_obs=k_HQ_H[H⁺], where k_H is the acid-catalyzed rate constant and Q_H is the protonation constant of the nonbonded oxygen atom in the O-coordinated ligand. The Cr–N bond breaking proceeds via two paths: spontaneous and acid-catalyzed; k_obs=k₀ + k_HQ_H[H⁺], where k₀ and k_H are the spontaneous and acid catalyzed rate constants and Q_H is the protonation constant of the carboxylic group in the N-bonded nicotinic acid. The results demonstrate by comparison that Cr–N bond breaking is a much slower process than Cr–O bond fission.     

A kinetic study on the iron(III)-promoted aquation of [Cr(C2O4)2(picolinato)]2− and [Cr(C2O4)2(2-pyridineacetato)]2− complexes

Two [Cr(C₂O₄)₂(AB)]^2– type complexes, obtained from the reaction of cis-[Cr(C₂O₄)₂(H₂O)₂]^– with the AB ligand, [AB = picolinic (pyac) or 2-pyridine-ethanoic acid (pyeac) anions], were converted into [Cr(C₂O₄)(pyac)(H₂O)₂]⁰ and [Cr(C₂O₄)(pyeac)(H₂O)₂]⁰ compounds, respectively via Fe^III-induced substitution of the oxalato ligand. The aquation products were separated chromatographically and their spectral characteristics and acid dissociation constants determined. The kinetics of the oxalato ligand substitution were studied with a 10–40 fold excess of Fe^III over [Cr^III] at [H⁺] = 0.2 M and at constant ionic strength 1.0 M (Na⁺, H⁺, Fe³⁺, ClO^– ₄). The reaction rate law is of the form: r = k _obs[Cr^III], where k _obs = kQ[Fe^III]/(1 + Q[Fe^III]). The first-order rate constants (k), preequilibria quotients (Q) and activation parameters derived from the k values have been determined. The reaction mechanism is discussed in terms of a Lewis acid catalyzed (induced) ligand substitution.     

Kinetics of oxidation of a 2-aminomethylpyridinechromium(III) complex by periodate

The oxidation kinetics of the 2-aminomethylpyridineCr^III complex with periodate in aqueous solution were studied and found to obey the rate law:Rate = [Cr^III]_T [IO₄ ^-]{k¹K² + k₂ K₁ K₃/[H⁺]}/{1+K₁/[H⁺] + k₂[IO₄ ^-]+K₁K₃/[H⁺][IO₄ ^-]} where K ₁, K ₂ and K ₃ are the deprotonation of [Cr(L)₂(H₂O)]³⁺ and pre-equilibrium formation constants for [(L)₂—Cr—OIO₃]²⁺ and [(L)₂—Cr—OH—OIO₃]⁺ precursor complexes respectively. An inner-sphere mechanism was proposed. The effect of Cu²⁺ on the oxidation rate was studied over the (1.0–9.0) × 10⁻⁵ mol dm⁻³ range. The reaction rate was found to be inversely proportional to the Cu²⁺ concentration over the range studied. This revised version was published online in June 2006 with corrections to the Cover Date.     

Chromium(III) complexes ofd(−)tartaric andl(−) mandelic acids

The binary complexes of anhydrous chromium(III) chloride withd(−) tartaric acidl(−) mandelic acids have been characterized by elemental analyses, magnetic susceptibility, vibrational, electronic and circular dichroism spectra. The magnetic susceptibility data are close to the spin only value for a d³ chromium(III) ion. Three (Cr−Cl) vibrational modes in the region 420–290 cm⁻¹ are observed for the formed complexes indicatingC ₂ local symmetry of ligand atoms around the chromium(III) rather thanC ₃, which would allow two modes. In the visible spectra, two peaks in the 21052–22222 and 15384–16129 cm⁻¹ range are observed and are assigned to the⁴ A _2g → ⁴ T _1g (F) and⁴ A _2g → ⁴ T _2g transitions. The parameters (Dq, B,β ₃₅) place the ligands in the higher end of the spectrochemical series and provide reassurance that the hydroxy acid oxygen complexes to chromium(III) ion. The Cotton effects observed in the spin-forbidden band are assigned to the² E(² E _g ),² A ₂(² T _1g ) and² E(² T _1g ), while that in the spin-allowed band are a results of the splitting of the⁴ A _2g (⁴ T _2g ) to⁴ A ₁(⁴ T _2g) and⁴ E(⁴ T _2g ) transitions. The tartaric acid chelates are likely to befac in terms of ligand carboxylate and/or hydroxy groups since stronger and better defined Cotton effects are observed while mandelic acid chelates are weak suggesting formation of themer structure. TMC 2633     

Synthesis and structure of bismuth(III) oxohydroxocarboxylates

The structure and thermal transformation of bismuth(III) oxohydroxocarboxylates Bi₆O₄(OH)₄(C _n H_{2n − 1}O₂)₆, where (C _n H_{2n − 1}O₂) is a carboxylate ion and n = 2 (2–9, 11), were studied by X-ray powder diffraction, thermogravimetry, IR spectroscopy, and chemical analysis. The conditions of precipitation of bismuth carboxylates from perchlorate solutions were determined. The compounds have a layered structure and undergo the same phase transformations on heating.     

Kinetic and mechanistic studies on the formation of bis(dicarboxylato)carbonatochromate(III) complexes

The reaction of the Cr(xx)₂(H₂O)₂ ⁻ (xx = oxalate, malonate and methylmalonate) complexes with dissolved CO₂ was studied by stopped-flow spectrophotometry in the 7 < pH < 9 range and between 20 to 30°C at an ionic strength of 0.5 mol dm⁻³ (NaCl). Under the experimental conditions the aqua complex ion consists of a pH-dependent mixture of Cr(xx)₂(H₂O)₂ ⁻, Cr(xx)₂(OH) (H₂O)²⁻ and Cr(xx)₂(OH)₂ ³⁻. The monohydroxo and dihydroxo species undergo CO₂ uptake and subsequent intramolecular carbonate ligand chelation independently, at rates which are readily distinguishable and are governed by the uptake rate constants k ₁ and k ₂ and chelation rate constants k ₃ and k ₄, respectively. Only the k ₁ values for oxalato, malonato and methylmalonato complexes could be calculated; k ₁ = 1084 and 1333 and 1650 mol⁻¹ dm³ s⁻¹, respectively. The results obtained were compared with those obtained from other systems that have either cobalt(III), iridium(III) or rhodium(III) as central atoms. This revised version was published online in June 2006 with corrections to the Cover Date.