Kinetics and Mechanism of Chromium(III)-Picolinato and Chromium(III)-Dipicolinato Complexes Aquation in HNO3 Solutions

The acid-catalyzed aquation of [Cr(pic)(H₂O)₄]₂ ²⁺ and [Cr(dpic)(H₂O)₃]⁺(pic = picolinic acid anion, dpic = dipicolinic acid dianion) in nitrate(V) media was studied. The reaction is reversible in the case of the pic-complex and practically irreversible in the case of the dpic-complex. It is assumed that the reactive form of the substrate undergoes fast chelate ring-opening followed by protolytic equilibria, followed by the rate of the Cr—O bond breaking of the monodentate bonded ligand which is the rate-determining step. The kinetics of pic/dpic ligand liberation were followed spectrophotometrically in the 0.4–2.0 M HNO₃ range at I= 2.0 M. The following dependences of the pseudo-first order rate constants on [H⁺] have been established:k _obs=a+b[H⁺](where b and a are apparent rate constants for the forward and the reverse reaction of the pic-complex) and k _obs=b[H⁺]+c[H⁺]²(where b and c are apparent rate constants for the dpic liberation). Fast protolytic pre-equilibria, leading to protonation of the carboxylic oxygen atom on the monodentate bonded ligand, preceeds ligand liberation.     

Initial state and transition state contributions to reactivity in mercury(II)-catalysed aquation of thetrans-[Rh(en)2Cl2]+, [Cr(NH3)5Cl]2+, andcis-[Cr(NH3)4(OH2)Cl]2+ cations in binary aqueous solvent mixtures

Summary Rate constants are reported for mercury(II)-catalysed aquation of thetrans-[Rh(en)₂Cl₂]⁺, [Cr(NH₃)₅Cl]²⁺, andcis-[Cr(NH₃)₄(OH₂)Cl]²⁺ cations in water and in methanol-, ethanol-, and acetonitrile-water solvent mixtures. In the case oftrans-[Rh(en)₂Cl₂]⁺, the dependence of rate constants on mercury(II) concentration indicates reaction through a binuclear (Rh-Cl-Hg bridged) intermediate. The dependence of the equilibrium constant for the formation of this intermediate and of its rate constant for dissociation (loss of HgCl⁺) on solvent composition have been established. With the aid of measured solubilities, published ancillary thermodynamic data, and suitable extrathermodynamic assumptions, the observed reactivity trends for these mercury(II)-catalysed aquations are dissected into initial state and transition state components. The reactivity patterns for these three complexes are compared with those for mercury(II)-catalysed aquation of other chloro-transition metal complexes, particularlycis-[Rh(en)₂Cl₂]⁺, [Co(NH₃)₅Cl]²⁺, and [ReCl₆]^2–.     

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.     

Synthesis and kinetics of the acid hydrolysis of the monothiooxalato-S,O-bis(ethylenediamine)chromium(III) complex

Cis-[Cr(en)₂(SC₂O₃)]Cl·H₂O has been synthesized by a new method involving the reaction of cis-[Cr(en)₂Cl₂]Cl with K₂SC₂O₃ at 60°C and the kinetics of the acidic aquation of this complex have been studied. The complex has been characterized by IR and electronic spectroscopic techniques. The aquation of the purified product was carried out at 30, 40 and 50°C in acidic media at constant ionic strength. The experimental result indicates a pH-dependent aquation of the compound in HCl solutions to yield H₂S and ethylenediamine. The plot of the rate of the reaction versus the square of the proton concentration, [H⁺]², produces a straight line giving the pseudo-first-order rate constant, k_obs. Activation energies were found to be proportional with the entropies in various acidic solutions. The “isokinetic temperature” of 285 K and the free energy of activation, ΔG‡ = 94.7 kJ mol⁻¹, were determined from the linear relationship. The results of experiments for the aquation of this compound suggest a mechanism through formation of a conjugate acid of an asymmetric atom, and the Cr---S bond cleavage is considered as the rate determining step.     

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.     

Kinetics and mechanism of hydrolysis ofcis-bis(ethylenediamine)imidazole(salicylato)cobalt(III) ion

Summary The kinetics of aquation and base hydrolysis reactions ofcis-[(en)₂Co(imH)O₂CC₆H₄OH-o-o]²⁺ (imH = imidazole) have been investigated in a medium of 1.0 M ionic strength, In the 0,1–1,0 M [H⁺] range (60–70°) aquation proceedsvia spontaneous and acid catalysed paths . In the 0,05–1.0 M [OH^–] range (30–40°), the complex exists predominantly as the bis-deprotonated species,cis-[(en)₂Co(im)O₂CC₆H₄O-o], and the pseudo-first-order rate constant fits the relationship k_obs = k_b + k_b° [OH^–] satisfactorily. The labilizing action of coordinated imidazolate anion(im^–) on the cobalt(III)-bound salicylate is 10³ times stronger than that of imidazole. The mechanism is essentially I_d in the aquation paths and SN₁cb (Co-O bond fission) in the alkali independent and dependent paths respectively.     

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 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.     

Hydrolysis of [Pt(dien)H2O]2+ and [Pd(dien)H2O]2+ complexes in water

The hydrolysis of the [Pt(dien)H₂O]²⁺ and [Pd(dien)H₂O]²⁺ complexes has been investigated by potentiometry at 298 K, in 0.1 mol dm^–3 aqueous NaClO₄. Least-squares treatment of the data obtained indicates the formation of mononuclear and -hydroxo-bridged dinuclear complexes with stability constants: log ₁₁ = –6.94 for [Pt(dien)OH]⁺, log ₁₁ = –7.16 for [Pd(dien)OH]⁺, and also log ₂₂ = –9.37 for [Pt₂(dien)₂(OH)₂]²⁺ and log ₂₂ = –10.56 for [Pd₂(dien)₂(OH)₂]²⁺. At pH values > 5.5, formation of the dimer becomes significant for the Pt^II complex, and at pH > 6.5 for the Pd^II complex. These results have been analyzed in relation to the antitumor activity of Pt^II complexes.     

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 Chromium(III)-oxalates-2-hydroxynicotinate and Chromium(III)-oxalates-3-hydroxypicolinate Aquation in HClO4 Solutions

New chromium(III) complexes, [Cr(C₂O₄)₂(2-hnic)]²⁻ and [Cr(C₂O₄)₂(3-hpic)]²⁻ (where 2-hnic = O,O′-bonded 2-hydroxynicotinic acid and 3-hpic = N,O-bonded 3-hydroxypicolinic acid), were obtained and characterized in solution. The acid-catalyzed aquation of the both complexes leads to liberation of the appropriate pyridinecarboxylic acid and formation of cis-[Cr(C₂O₄)₂(H₂O)₂]⁻. Kinetics of these reactions were studied spectrophotometrically in the 0.1–1.0 M HClO₄ range, at I = 1.0 M. In the case of [Cr(C₂O₄)₂(2-hnic)]²⁻, a slow chelate-ring opening at the Cr–O (phenolate) bond is followed by a fast Cr–O (carboxylate) bond breaking. The rate law: k_obs = k_HQ_H[H⁺] was established, where k_H is the acid-catalyzed rate constant and Q_H is the protonation constant of the coordinated phenolate oxygen atom. In the case of [Cr(C₂O₄)₂(3-hpic)]²⁻, the reversible chelate-ring opening at Cr–N bond is followed by the rate determining step – the one-end bonded ligand liberation. The rate law for the first step was determined: k_obs = k₁+k₋₁/Q₁[H⁺], where k₁ and k₋₁ are the rate constants of the chelate-ring opening and closure and Q₁ is the protonation constant of the pyridine nitrogen atom. The aquation mechanisms are proposed and the effect of ligand coordination mode on complex reactivity is discussed.     

Kinetic,solvation and reactivity studies of iron(II) complexes of monoxime and dioxime ligands

Complexes of Fe^II with monoxime and dioxime ligands have been isolated and characterised. Kinetic results and rate laws are reported for acid aquation and base hydrolysis of these complexes in H₂O and in MeOH–H₂O mixtures. Kinetics of acid catalysed aquation of Fe^II–monoxime complexes follow a rate law with k_obs = k₂[H⁺] + k₃[H⁺]², while kinetics of acid dissociation and base hydrolysis of the Fe^II–dioxime complex follow rate laws with k_obs = k₂[H⁺] and k_obs = k₂[OH⁻]. Acid aquation and base hydrolysis mechanisms are proposed. The solubilities of Fe^II–monoxime and –dioxime complex salts are reported and transfer chemical potentials of their complex cations are calculated. Solvent effects on reactivity trends have been analysed into initial and transition state components. These are determined from transfer chemical potentials of reactant and kinetic data. Rate constant trends from these complexes are compared and discussed in terms of ligand structure and solvation properties. Our kinetic results give information relevant to the application of these ligands as analytical reagents for trace Fe^II in acidic and neutral media, in water and in aqueous alcohols.     

Acid-catalyzed Hydrolysis of the cis-[Cr(1,10-phenanthroline)2(O2CO)]+ Ion Studied by the u.v.–vis Stopped Flow Method

The cis-[Cr(phen)₂(O₂CO)]⁺ ion was prepared through the displacement of two molecules of water from the cis-[Cr(phen)₂(OH₂)₂]³⁺ by the bidentate carbonate anion. It underwent two-phase hydrolysis reactions under acidic conditions (0.1 < [H⁺] < 2.7 m) at 5, 10, 15, 20 and 25 °C. Via slow carbonato chelate ring opening (first step k₁^slow) and a second fast decarboxylation(k₂^fast value). The first step was preceded by protonation of the coordinate bidentate carbonate ligand. The second step exhibited no pH dependence, while k₁^slow values increased with acid concentration that suggested the presence of both protonated and deprotonated reactant species. Based on these observations we have proposed a hydrolysis mechanism featuring H₂O-induced ring-opening of the coordinate CO₃²⁻ group in the first step k₁^slow followed by loss of CO₃²⁻ from two intermediates, [Cr(phen)₂(O₂COH)]²⁺ (k₁^slow) and [Cr(phen)₂(OH₂)(O₂COH)]²⁺ (k₂^fast).     

Kinetics and mechanism of acid and base hydrolysis ofcis-chloro-(benzotriazole)bis(ethylenediamine)cobalt(III) andcis-chloro-(N-methylbenzotriazole)bis(ethylenediamine)cobalt(III) cations

Summary The kinetics of acid hydrolysis ofcis-[CoCl(btzH)(en)₂]²⁺ andcis-[CoCl(btzMe)(en)₂]²⁺ complexes (where btzH = benzotriazole, btzMe =N-methylbenzotriazole and en = ethylenediamine) have been investigated in HClO₄ at ionic strength 1 = 0.25 mol dm^–3 in the 30–40° range. In the 1.0 x 10^–1 to 1.0 X 10^–3 mol dm^–3 acid strength range, the rate of aquation of the [CoCl(btzH)(en)₂]²⁺ cation follows the relationship:-d ln[complex]/dt = k₁ + k₂K_NH[^H+]^–1, where k₁ and k₂ are aquation rate constants of the acid independent and acid dependent steps respectively, and K_NH is the acid dissociation constant of the coordinated benzotriazole.cis-[CoCl(btzMe)-(en)₂]²⁺ undergoes acid independent hydrolysis presumably due to the absence of a labile N-H proton. The base hydrolysis could be followed for thecis-[CoCl(btzMe)(en)₂]²⁺ complex only by measuring hydrolysis rates at 0°.     

Kinetic studies on H+-catalyzed aquation and hydrogen peroxide oxidation of tris-asparaginatochromium(III)

Tris-asparaginatochromium(III), [Cr(Asn)₃]⁰ (where Asn forms a 5-membered chelate ring via amine nitrogen and α-carboxylate oxygen atoms) and its mono- and diaqua-derivatives were obtained, and their acid-catalyzed aquation was studied. The first reaction for [Cr(Asn)₃]⁰ and [Cr(Asn)₂(H₂O)₂]⁺ is the chelate ring opening at the Cr-NH₂ bond, leading to metastable intermediates. Kinetics of these processes were studied spectrophotometrically in 0.1–1.0 M HClO₄ at 303 and 333 K, respectively. A linear dependence of k _obs on [H⁺], k _obs = a + b[H⁺] was determined for both the complexes. Additionally, oxidation of chromium(III) to chromate(VI) by hydrogen peroxide was studied. The process proceeds through a chromium(V) intermediate, which is next transformed, in faster parallel steps into CrO₄ ^2? and [Cr(O₂)₂]^3? anions. The latter species, a chromium(V)-peroxo complex, is metastable under a large excess of H₂O₂. Kinetics of oxidation of [Cr(Asn)₃]⁰ were studied at 298 K, at constant [OH^?], within 0.2–1.0 M H₂O₂ range. A linear dependence of k _obs on H₂O₂ was established. A mechanism is proposed, where the rate-determining step is an inner sphere 2-electron transfer within a precursor chromium(III) complex with coordinated O₂H^? anion of the [Cr(Asn)₂(OH)(HO₂)]^? formula. EPR results provided clear evidence for formation of a relatively stable tetrakis(η ²-peroxo)chromate(V) complex, [Cr(O₂)₄]^3?.     

Kinetics and mechanism of aqua ligand substitution reactions from cis-diaqua-bis(bipyridine)ruthenium(II) ion by pyridine-2-aldoxime in aqueous medium

Summary The kinetic behaviour of cis-[Ru(bipy)₂(H₂O)₂]²⁺ towards the anating ligand pyridine-2-aldoxime as a function of temperature, ligand concentration, substrate complex concentration and pH is reported and the rate expression Rate = k ₁ k ₂[Ru(bipy)₂(H₂O)₂]²⁺ [LL]/(k _-1 + k ₂[LL]) is established where k ₁ is the water dissociation rate constant for the slow step, k _-1 is the rate constant for the aquation, k ₂ is the ligand-capturing rate constant of the five-coordinate intermediate [Ru(bipy)₂(H₂O)]²⁺ and LL is pyridine-2-aldoxime. The reaction is pH-dependent in the pH range 3.65–5.50. The enthalpy and entropy of activation were obtained using Eyring plots. The results are in conformity with a dissociative mechanism.     

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.     

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.     

Kinetics of base hydrolysis of trans-{Cr([15]aneN4)Cl2}+ ([15]aneN4 = 1,4,8,12-tetra-azacyclopentadecane)

Summary Reaction of CrCl₃(DMF)₃ with [15]aneN₄ (L; L = 1,4,8,12-tetra-azacyclopentadecane) gives the green trans-{Cr([15]-aneN ₄)Cl₂}Cl in high yield. The base hydrolysis kinetics of the cations [CrLCl₂]⁺ and [CrLCl(OH)] ⁺ have been investigated over a temperature range. For the dichloro complex, k _OH = 1.03 dm³ mol^–1 s^–1] at 25° C with H =30.4 kJmol^–1 and S _inf298 ^sup = -143 JK^–1 mol^–1. The substantial negative entropy of activation implies more association of water in the loss of Cl^– from the conjugate base in a D_CB mechanism. The kinetic parameters for the chlorohydroxo complex are k _OH = 1.9 × 10^–2dm³mol^–1 s^–1 at 25°C with H = 78.3kJmol^–1 and H _inf298 ^sup = -15 J K^–1 mol ^–1. The chlorohydroxo complex probably has the trans VI configuration with the chloride ligand on the same side of the equatorial plane as the four chiral sec-NH groups. The visible spectra of a variety of complexes trans-[Cr(L)XY] ⁿ⁺ (X = Y = Cl, OH, OH₂; X = Cl, Y = OH) have been determined.     

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.