Acid-catalysed Aquation Research Articles

Hydrolytic and redox transformations of chromium(III) bis-oxalato complexes with glutaminic acid and glutamine: a kinetic, UV–Vis and EPR, study

The title complexes have been synthesized, chromatographically isolated and characterized by their ligands to metal ratio determinations and spectroscopic analyses. The kinetics of the first aquation stage, i.e., the amino acid chelate ring opening via the Cr–N bond cleavage, has been studied spectrophotometrically in acidic and alkaline media. Hydrogen peroxide oxidizes the complexes in alkaline media to CrO 4 2− anion and a relatively stable Cr(V) complex. Consecutive biphasic kinetics through two first-order steps were observed for the base hydrolysis and the oxidation process, whereas the acid-catalyzed aquation obeys a simple first-order pattern. Based on the kinetic and spectroscopic data, mechanisms of the coordinated amino acid liberation and chromium(III) oxidation are discussed.

Retraction: Acid-Catalyzed Aquation of Ni(II)-Hydrazone Complexes: Kinetics and Solvent Effect

Complexes of the type [Ni(L)(H2O)]Cl2·nH2O, where L = 2-pyridyl-3-isatinbishydrazone ligands, have been synthesized and characterized on the bases of elemental analysis, molar conductance, IR, electronic spectra, and thermal analysis (TGA and DTA). Acid-catalyzed aquation of the Ni(II) isatin-bishydrazone complexes was followed spectrophotometrically in various water–methanol and water–acetone mixtures at temperature 298 K. Kinetic behavior of the acid aquation is a linear rate law, indicating that the acid-catalyzed aquation of these complexes in water–methanol and water–acetone mixtures follows a rate law with kobs = k2[H+]. The effect of the mole fraction of the ganic solvent, i.e., methanol and acetone, on the acid aquation has been analyzed; the decrease in the rate constant values with increasing of the methanol or acetone ratios is attributable to the effect of the co-organic solvent on the initial states of the acid aquation by the destabilization of the H+ ion.

Kinetic studies on acid- and base-catalyzed aquation of bis-alaninatochromium(III) and on oxidation of tris- and bis-Aa–chromium(III) complexes (Aa = Gly, Ala, Asn) by H2O2

Two aqua derivatives of [Cr(Ala)3] were characterized in solution. Acid-catalyzed aquation of cis-[Cr(Ala)2(H2O)2]+ leads to inert [Cr(Ala)(H2O)4]2+, whereas base hydrolysis of cis-Cr(Ala)2(OH)2]− causes dissociation of both the Ala ligands and formation of chromates(III). Kinetics of these processes have been studied spectrophotometrically in both 0.1–1.0 M HClO4 and 0.2–0.9 M NaOH under first-order conditions. A linear dependence of the kobs,H on [H+] and a small dependence of the (kobs)OH on [OH−] were established. In the proposed mechanism, the rate determining step is Cr–N bond breaking in the reactive form of the substrate, i.e., in the protonated aqua- or dihydroxo complex. The effect of pH on the complex reactivity is discussed. The kinetic results are compared with those determined previously for analogous glycine and asparagine complexes. Additionally, oxidation of tris- and bis-Aa–chromium(III) complexes, where Aa = Gly, Ala or Asn, by hydrogen peroxide in alkaline medium was studied. Two reaction products were detected: thermodynamically stable CrO42− and [Cr(O2)4]3− that under a large excess of hydrogen peroxide is metastable. The rate-limiting stage of this process is an inner sphere two-electron transfer within the peroxido intermediate.

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 kobs on [H+], kobs = 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 CrO42− 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 kobs 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(η2-peroxo)chromate(V) complex, [Cr(O2)4]3−.

Kinetic studies on H+-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.

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: kobs = 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 acid-catalysed aquation of some [Cr(ox)2(amino acid)]2− complexes

Three chromium(III) complexes of general formula [Cr(ox)2(Aa)]2− (Aa is an α-amino acid, namely alanine, valine or cysteine) were obtained and characterized in solution. In acidic solutions, [Cr(ox)2(Aa)]2− undergo acid-catalysed aquation to cis-[Cr(ox)2(H2O)2]− and the appropriate amino acid. The process goes through a metastable intermediate with monodentate amino acid coordinated via the carboxylate oxygen atom. The kinetics of the chelate ring opening were studied under isolation conditions. The determined pseudo-first-order rate constants were linearly dependent on [H+]. A mechanism is proposed, in which the reactive form of substrate is in the form of the conjugate acid.

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.

Mixed chromium(III) complexes with pyridinedicarboxylato and oxalato ligands: kinetic studies in HClO4 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.

Chromium(III) complexes with lutidinic acid: kinetic studies in HClO4 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: kobs1 = k1 + k−1/K1[H+] and kobs2 = k2K2[H+]/(1 + K2[H+]), where k1 and k2 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 K1 and K2 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−]: kobs1 = kOH(1) + kO[OH−], where kOH(1) and kO are rate constants of the first ligand liberation from the hydroxo- and oxo-forms of the intermediate, respectively, and K2 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 and mechanism of tris-quinolinatochromium(III) aquation in HClO4 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: kobs = k1 + k−1/Q1[H+] and kobs = k2Q2[H+]/(1 + Q2[H+]), where k1 and k2 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, Q1 and Q2 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.

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: kobs = kHQH[H+] was established, where kH is the acid-catalyzed rate constant and QH 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: kobs = 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.

Kinetics and mechanism of aquation of chromium(III) complexes with 3-hydroxypicolinic and 2-hydroxynicotinic acids in HClO{4{ 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 Chromium(III)-Picolinato and Chromium(III)-Dipicolinato Complexes Aquation in HNO3Solutions

The acid-catalyzed aquation of [Cr(pic)(H2O)4]22+ 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:kobs=a+b[H+](where b and a are apparent rate constants for the forward and the reverse reaction of the pic-complex) and kobs=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.

Tris-[3-hydroxy-1,2-dimethyl-pyridin-4(1H)-onato]cobalt(III)

The preparation and properties of tris-[3-hydroxy-1,2-dimethyl-pyridin-4(1H)-onato]cobalt(III) are described. The structure of the fac isomer has been established by X-ray diffraction techniques. Solvation characteristics of this complex have been assessed through solubility measurements in methanol–water mixtures. The complex is very inert in neutral solution but undergoes acid-catalysed aquation at lower pHs.

Thermal and photochemical reactions of bis(diamine)(sulfito)cobalt(III) complexes: effect of chelate-ring size

The kinetics of formation of cis-[Co(tn) 2 (OH 2 )(OSO 2 -O )] + (tn = 1,3-diaminopropane), its acid-catalysed aquation to the parent diaqua complex, anation of trans-[Co(tn) 2 (OH 2 )(SO 3 - S)] + by N 3 - , NCS - , SO 3 2- –HSO 3 - , anation of trans-[Co(tn) 2 (OH)(SO 3 -S )] by SO 3 2- and acid-catalysed aquation of trans-[Co(tn) 2 (SO 3 -S) 2 ] - to the corresponding (aqua)(sulfito-S) complex were investigated and the results compared with analogous data for the corresponding 1,2-diaminoethane (en) complexes. Expanding the chelate-ring size from five to six had virtually no effect on the rate of formation of the sulfito-O complex, but retarded its acid-catalysed aquation. The latter effect was attributed to a pK perturbation; intramolecular hydrogen bonding between the co-ordinated H 2 O and sulfite in cis-[Co(tn) 2 (OH 2 )(OSO 2 -O )] + hindered the protonation pre-equilibrium of the sulfito-O complex involved in the acid-catalysed aquation. This is further supported by the fact that there was no ring-size effect on the acid-catalysed aquation of trans-[Co(L–L) 2 (OH 2 )(OSO 2 ) ] + (L–L = tn or en). The strong labilising action due to chelate-ring expansion is remarkably attenuated by the trans effect of S-bonded sulfite as observed in the anation of trans-[Co(L–L) 2 (OH 2 /OH)(SO 3 -S)] +/0 . However, trans-[Co(tn) 2 (OH 2 )(SO 3 - S)] + was found to be prone to intramolecular electron transfer between Co III and S IV under thermal conditions unlike its en analogue, further reflecting the ring-size effect. Flash photolysis of trans-[Co(L–L) 2 (OH 2 )(SO 3 -S)] + (L–L = en or tn) generated the transient trans-[Co(L–L) 2 (OH 2 )(OSO 2 )] + . The photochemical ligand isomerisation of both complexes (Co III –SO 3 + → Co III –OSO 2 + ) also occurred at comparable rates [k iso = (4.1 ± 0.8) × 10 4 and (3.2 ± 1.3) × 10 4 s -1 at 25 °C for the en and tn complexes respectively]. Steady-state photolysis at 254 nm indicated that trans-[Co(en) 2 (OH 2 )(SO 3 -S )] + underwent photoaquation and photoreduction. Strikingly photoreduction could not be detected for this complex at pH > 8.

Ligand control on the reactions of oxygen- and sulphur-bonded (sulphito)pentaminecobalt(III) complexes in solution

The kinetics of formation, acid-catalysed aquation, ligand isomerisation (CoIIIOSO2+→CoIIISO3+), intramolecular electron-transfer, and base-catalysed hydrolysis and isomerisation of O-bonded sulphito complexes, cis-[Co(en)2(B)(OSO2–O)]+[B=benzimidazole (bzimH), N-methylimidazole (N-meim)] and the anation of cis-[Co(en)2(B)OH)]2+ [B=bzimH, N-meim and imH (imidazole)] by oxSO2−3 are reported. Steric acceleration is observed in the formation and acid-catalysed aquation of the O-sulphito complexes. The ligand isomerisation leads to loss of the monodentate amine with the formation of trans-[Co(en)2(SO3–S)2]− (in an excess of sulphite). Steric acceleration is more pronounced in the isomerisation and base hydrolysis than in the redox process. The results indicate cis labilisation of the coordinated O-sulphite. The [(tetraethylenepentamine)Co(OSO2–O)]+ cation undergoes base hydrolysis 103 times faster than the corresponding (en)2(B) complexes; base-catalysed ligand isomerisation for the former is not observed unlike in the latter. The anation of cis-[Co(en)2(B)OH]2+ (B=imH, bzimH, N-meim) by SO2−3 in a mild alkali pH range (pH=7.9–9.6) and in an excess of SO2−3, yields exclusively trans-[Co(en)2(SO3–S)2]− with no evidence for the formation of the cis-[Co(en)2(B)(SO3–S)]+ or its O-sulphito analogue. The intramolecularly generated amido conjugate base of the sulphite ion-pair, {cis-[Co(en)2(B)OH]2+,SO32−}.41cm{cis-[Co(en)(en-H) (B)- OH2]2+,SO32−}, is believed to generate a five-coordinate intermediate (TBP) that captures the S-end of SO2−3 selectively from a site trans-to the amine B so that the amine is labilised by the trans effect of the sulphite. The NH-deprotonated coordinated imidazolate or benzimidazolate species, cis-[Co(en)2(bzm/im)OH]+, do not undergo anation by SO2−3.

REACTIONS OF CIS-(AQUA)(AMINE)BIS(ETHYLENEDIAMINE)COBALT(III) COMPLEXES WITH SULFUR(IV) IN AQUEOUS MEDIUM

The kinetics of the formation/acid-catalysed aquation (SO2 elimination), CoIII-OSO2+→CoIII-SO3+ isomerization, intramolecular reduction and base hydrolysis of the O-bonded sulfito complexes, cis-[Co(en)2(RNH2) (OSO2)]+ (en=ethylenediamine; R=H, Me, Et, PhCH2 and C6H11) have been investigated. The spontaneous and base-catalysed isomerization involve loss of monodentate amine ligands from the CoIII centre to give trans-[Co(en)2(SO3-S)2]– (in excess SO32–) or trans-[Co(en)2(OH)(SO3-S)] (under base hydrolysis conditions). This result is presumably associated with a cis-labilizing action of the O-bonded sulfite. Steric retardation is observed for the formation and acid-catalysed aquation of the O-sulfito complex, the effect being relatively larger for the latter reaction. Steric acceleration is observed in the isomerization and intramolecular reduction and base hydrolysis of the O-sulfito complexes.¶ The trans-S-disulfito complex is prone to fast H+-catalysed aquation {k=(1.52±0.06)×104 dm3 mol–1 s–1, ΔH‡=81 ± 2 kJ mol–1, ΔS‡= 108 ± 5 J K–1 mol–1 at 25°C, I = 1.0 mol dm−3} yielding trans-[Co(en)2 (SO3-S)(OH2)]+.

Steric effects on complex formation between nickel(II) and (2-imidazoleazo) benzene, 2,2?-biimidazole and 2,2?-bibenzimidazole

Summary The kinetics and mechanism of reversible complexation of Ni 11 with (2-imidazoleazo)benzene (lAB), 2,2'-biimidazole (Biim) and 2,2'-bibenzimidazole (Bibzm) have been investigated at 15-35 ~ I = 0.30 mol din- 3. The stability constants, KM, of the [NIL] z+ species vary in the sequence: [Ni(IAB)] z + [Ni(Biim)]2 + > [Ni(Bibzm)]Z +. The aquation of [Ni(IAB)] z+ is insensitive to acid catalysis, whilst [Ni(Biim)] z+ is relatively more susceptible towards acid-catalysed aquation than [Ni(Bibzm)] 2 +. The chelate ring in [NiL] z+ (L =IAB, Biim or Bibzm) is sterically strained. The formation of [Ni(IAB)] 2+ and [Ni(Bibzm)] z+ may be chelation controlled while the normal I d mechanism is supported by our data for [Ni(Biim)] 2+ .

Hydrolysis Kinetics of trans(O)-Bromo, Chloro or Nitro-(3,6,9-Triazaundecanedioato)cobalt(III) Complex

Abstract For the acid-uncatalyzed aquation of trans(O)-[Co(taud)X] at the ionic strength (μ) of 0.1 (NaClO4), the first order rate constant (kaq) is 9.6 × 10−5 s−1 at 25 °C, ΔH‡ = 93 kJ mol−1, ΔS‡ = −10 J K−1 mol−1, ΔV‡ = −3.2 cm3 mol−1 with X = Cl−; and kaq is 4.6 × 10−4s−1 at 25 °C, ΔH‡ = 91 kJ mol−1, ΔS‡ = −4 J K−1 mol−1, ΔV‡ = −2.3 cm3 mol−1 with X = Br−. Here taud stands for the quinquedentate −OOCCH2NH(CH2)2NH(CH2)2NHCH2COO−. These magnitudes of ΔV‡ are slightly different from the expected values. For the acid-catalyzed aquation of trans-(O)-[Co(taud)NO2], kaq/s−1 depends on the [H+]/M of the solution: kaq = 1.3 × 10−4[H+] + 3.09 × 10−3[H+]2 at 45.6 °C and μ = 0.5 (M = mol dm−3). For the base hydrolysis of trans(O)-[Co(taud)X] at μ = 1.0 (NaClO4), the second order rate constant (k2) is 104 M−1 s−1 at 25 °C, ΔH‡ = 77 kJ mol−1, ΔS‡ = 12 J K−1 mol−1 with X = Cl−; and k2 is 760 M−1 s−1 at 25 °C, ΔH‡ = 79 kJ mol−1, ΔS‡ = 36 J K−1 mol−1 with X = Br−. The ΔV‡ ’s at μ ≈ 0 are 19.1 cm3 mol−1 with X = Cl− and 19.7 cm3 mol−1 with X = Br−. In the base hydrolysis with excess NaOH at μ = 1.0, the pseudo-first-order rate constant (kobsd) departs slightly positively from the proportionality to the [OH−] at 0.3 M < [OH−]. This type of nonlinearity has rarely been reported for the base hydrolysis of a cobalt(III) complex. The magnitudes of the activation volume for the base hydrolysis are in line with the SN1 CB mechanism. Therefore, the mechanism may be the SN1 CB mechanism, where an additional minor pathway via the second deprotonation from the taud ligand or via a hydroxide adduct formation to the carbonyl group of the taud ligand also participates.