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.     

New Bimetallic Ni–Rh Carbonyl Clusters: Synthesis and X-ray Structure of the [Ni7Rh3(CO)18]3?, [Ni3Rh3(CO)13]3? and [NiRh8(CO)19]2? Cluster Anions

The reaction of the [Ni₆(CO)₁₂]²⁻ dianion with [Rh(COD)Cl]₂ (COD = cyclooctadiene) in acetone affords a mixture of bimetallic Ni–Rh clusters, mainly consisting of the new [Ni₇Rh₃(CO)₁₈]³⁻ and [Ni₈Rh(CO)₁₈]³⁻ trianions. A study of the reactivity of [Ni₇Rh₃(CO)₁₈]³⁻ led to isolation of the new [Ni₃Rh₃(CO)₁₃]³⁻ and [NiRh₈(CO)₁₉]²⁻ anions. All these new bimetallic Ni–Rh carbonyl clusters have been isolated in the solid state as tetrasubstituted ammonium salts and have been characterised by elemental analysis, X-ray diffraction studies, ESI-MS and electrochemistry. The unit cell of the [NEt₄]₃[Ni₇Rh₃(CO)₁₈] salt contains two orientationally-disordered ν₂-tetrahedral [Ni₇Rh₃(CO)₁₈]³⁻ trianions with occupancy factors of 0.75 and 0.25. Besides, their inner Ni₃Rh₃ octahedral moieties show two cis sites purely occupied by Rh atoms, two trans sites purely occupied by Ni atoms and the remaining two cis sites are disordered Ni and Rh sites with respective occupancy fraction of 0.5. At difference from the parent [Ni₇Rh₃(CO)₁₈]³⁻, the octahedral [Ni₃Rh₃(CO)₁₃]³⁻ displays an ordered distribution of Ni and Rh atoms in two staggered triangles. The [NiRh₈(CO)₁₉]²⁻ dianion adopts an isomeric metal frame with respect to that of the [PtRh₈(CO)₁₉]²⁻ congener. As a fallout of this work, new high-yield synthesis of the known [Ni₆Rh₃(CO)₁₇]³⁻ and [Ni₆Rh₅(CO)₂₁]³⁻, as well as other currently-investigated bimetallic Ni–Rh clusters have been obtained.     

Thermal Studies of N-phenylethane-1,2-diamine Complexes of Nickel(II) in the Solid State

[NiL₃]X₂ (where L=N-phenylethane-1,2-diamine and X=I⁻ and ClO₄ ⁻), [NiL₂X₂] (X is Cl⁻, Br⁻, NCS⁻, 0.5SO₄ ²⁻ or 0.5SeO₄ ²⁻) and [NiL₂(H₂O)₂](NO₃)₂ have been synthesized from solution and their thermal study has been carried out in the solid phase. [NiL₂Cl₂] upon heating undergoes irreversible endothermic phase transition (142–152°C, ΔH=0.35 kJ mol⁻¹) without showing any visual colour change. This phase transition is assumed to be due to conformation changes of the diamine chelate rings. NiLCl₂ and NiL_2.5I₂ have been prepared pyrolytically from [NiL₂Cl₂] and [NiL₃]I₂ respectively in the solid state. [NiL₂(H₂O)₂](NO₃)₂ upon heating undergoes deaquation-anation reaction without showing any visual colour change. [NiL₂X₂] (X is Cl⁻, Br⁻, NCS⁻), [NiL₂(H₂O)₂](NO₃)₂ and [NiL₂(NO₃)₂] possess trans-octahedral configuration, whereas, [NiL₂X₂] (X is 0.5SO₄ ²⁻ or 0.5SeO₄ ²⁻) are having cis-octahedral configuration. Amongst the complexes, only NiLCl₂ shows unusually high (5.1 BM at 27°C) magnetic susceptibility value. This revised version was published online in July 2006 with corrections to the Cover Date.     

The Effect of Temperature on the Equivalent Conductivities and Ion-Association Constants of Some Tris-(ethylenediamine)chromium(III) Complexes in N,N-dimethylformamide and N,N-dimethylacetamide

The equivalent conductivities of tris-(ethylenediamine)chromium complexes, [Cr(en)₃]X₃ (where X⁻= Cl⁻, Br⁻, I⁻; en = ethylenediamine) were measured as functions of temperature (278.15 to 328.15 K) and concentration [(1.948 ×10⁻⁴ to 10.728 ×10⁻⁴ mol⋅dm⁻³) and (2.282 ×10⁻⁴ to 11.246 ×10⁻⁴ mol⋅dm⁻³)] in N,N-dimethylformamide (DMF) and N,N-dimethylacetamide (DMAC), respectively. Equivalent conductivity values for [Cr(en)₃]X₃ in DMF were found to be higher than those in DMAC. The conductivity data were analyzed with the Robinson-Stokes equations. For [Cr(en)₃]X₃, the limiting equivalent ionic conductivities of [Cr(en)₃]³⁺ and the ion-association constants (K _A) of the ion-pair between [Cr(en)₃]³⁺ and the monovalent halide anions were determined in DMF and DMAC. The values of K _A for three complex salts in DMF were higher than those in DMAC. This can be ascribed to an increase of the ion-association constants with a decrease of the relative permittivity of the solvents. The values of K _A at 298.15 K decreased in the order Cl⁻> Br⁻> I⁻ in DMF and Cl⁻> I⁻> Br⁻ in DMAC. The K _A values for [Cr(en)₃]Cl₃ increased with increasing temperature in both DMF and DMAC. For [Cr(en)₃]X₃(X⁻= Br⁻, I⁻) in both solvents, this indicates increasing disorder occurs with increasing temperature. Thermodynamic parameters (standard Gibbs energy, enthalpy and entropy changes) were determined from the temperature dependence of K _A in DMF and DMAC. These parameters were inter-compared in their dependences on temperature and solvent.     

Electronic Structure and Molecular Properties of the Mixed Rhenium–Molybdenum [Re6−x Mo x S8(CN)6]q−, (x = 0 to 6) Clusters

Relativistic density functional calculations including scalar and spin-orbit effects via the ZORA approximation and including solvent effects were carried out on the [Re₆S₈(CN)₆]⁴⁻, [Re₅MoS₈(CN)₆]⁵⁻, [Re₄Mo₂S₈(CN)₆]⁵⁻, [Re₃Mo₃S₈(CN)₆]⁵⁻, [Re₂Mo₄S₈(CN)₆]⁵⁻, [ReMo₅S₈(CN)₆]⁵⁻ and [Mo₆S₈(CN)₆]⁶⁻ clusters. By increasing the replacement of each Re atom with Mo atoms we find that for x > 2 the HOMO–LUMO gap decreases significantly. The calculated gap of the [Re₃Mo₃S₈(CN)₆]⁵⁻, [Re₂Mo₄S₈(CN)₆]⁵⁻ and [ReMo₅S₈(CN)₆]⁵⁻ clusters is similar to the calculated and observed gap of the superconducting PbMo₆S₈ Chevrel phases. The current calculations also indicates that the electronic similarities of the lowest excited states of the semiconducting 24e [Re₅MoS₈(CN)₆]⁵⁻ and 23e [Re₄Mo₂S₈(CN)₆]⁵⁻ clusters with the strongly luminescent 24e [Re₆S₈(CN)₆]⁴⁻ cluster, suggest that these mixed metal clusters might be luminescent.     

Kinetic and mechanistic studies on the electron-transfer reactions between diaquabisethylenediaminecobalt(III) and ethylenediaminetetraacetatocobaltate(III) with promazine in acidic aqueous media

The kinetics of the electron-transfer reactions between promazine (ptz) and [Co(en)₂(H₂O)₂]³⁺ in CF₃SO₃H solution ([Co^III] = (2–6) × 10⁻³ m, [ptz] = 2.5 × 10⁻⁴ m, [H⁺] = 0.02 − 0.05 m, I = 0.1 m (H⁺, K⁺, CF₃SO ₃ ⁻ ), T = 288–308 K) and [Co(edta)]⁻ in aqueous HCl ([Co^III] = (1 − 4) × 10⁻³ m, [ptz] = 1 × 10⁻⁴ m, [H⁺] = 0.1 − 0.5 m, I = 1.0 m (H⁺, Na⁺, Cl⁻), T = 313 − 333 K) were studied under the condition of excess Co^III using u.v.–vis. spectroscopy. The reactions produce a Co^II species and a stable cationic radical. A linear dependence of the pseudo-first-order rate constant (k _obs) on [Co^III] with a non-zero intercept was established for both redox processes. The rate of reaction with the [Co(en)₂(H₂O)₂]³⁺ ion was found to be independent of [H⁺]. In the case of the [Co(edta)]⁻ ion, the k _obs dependence on [H⁺] was linear and the increasing [H⁺] accelerates the rate of the outer-sphere electron-transfer reaction. The activation parameters were calculated as follows: ΔH ^≠ = 105 ± 4 kJ mol⁻¹, ΔS ^≠ = 93 ± 11 J K⁻¹mol⁻¹ for [Co(en)₂(H₂O)₂]³⁺; ΔH ^≠ = 67 ± 9 kJ mol⁻¹, ΔS ^≠ = − 54 ± 28 J K⁻¹mol⁻¹ for [Co(edta)]⁻.     

Outer-Sphere electron-transfer and the effect of linkage isomerism in the titanium(III) reduction of nitro-pentacyanocobaltate(III) and nitro-, and nitrito-pentaamminecobalt(III) complexes in aqueous acidic media

The reductions of [Co(CN)₅NO₂]³⁻, [Co(NH₃)₅NO₂]²⁺ and [Co(NH₃)₅ONO]²⁺, by Ti^III in aqueous acidic solution have been studied spectrophotometrically. Kinetic studies were carried out using conventional techniques at an ionic strength of 1.0 mol dm⁻³ (LiCl/HCl) at 25.0 ± 0.1 °C and acid concentrations between 0.015 and 0.100 mol dm⁻³. The second-order rate constant is inverse—acid dependent and is described by the limiting rate law:- k₂ ≈ k₀ + k[H⁺]⁻¹,where k=k′K_a and K_a is the hydrolytic equilibrium constant for [Ti(H₂O)₆]³⁺. Values of k₀ obtained for [Co(CN)₅NO₂]³⁻, [Co(NH₃)₅NO₂]²⁺ and [Co(NH₃)₅ONO]²⁺ are (1.31 ± 0.05) × 10⁻² dm³ mol⁻¹ s⁻¹, (4.53 ± 0.08) × 10⁻² dm³ mol⁻¹ s⁻¹ and (1.7 ± 0.08) × 10⁻² dm³ mol⁻¹ s⁻¹ respectively, while the corresponding k′ values from reductions by TiOH²⁺ are 10.27 ± 0.45 dm³ mol⁻¹ s⁻¹, 14.99 ± 0.70 dm³ mol⁻¹ s⁻¹ and 17.93 ± 0.78 dm³ mol⁻¹ s⁻¹ respectively. Values of K _a obtained for the three complexes lie in the range (1–2) × 10⁻³ mol dm⁻³ which suggest an outer-sphere mechanism.     

Base hydrolysis ofcis-(methyl/ethylamine)bis(ethylenediamine) (salicylato)cobalt(III) complexes

The kinetics of the base hydrolysis ofcis-[Co(en)₂(RNH₂)-(SalH)]²⁺ (R=Me or Et; SalH=HOC₆H₄CO ₂ ⁻ ) were investigated in aqueous ClO ₄ ⁻ in the 0.004–0.450 mol dm⁻³ [OH⁻] range, I=0.50 mol dm⁻³ at 30–40°C. The phenoxide species is hydrolysed via [OH⁻]-independent and [OH⁻]-dependent paths, the latter being first order in [OH⁻]. The high rate of alkali-independent hydrolysis of the phenoxide species is associated with high ΔH^≠ and ΔS^≠ values, in keeping with the SNICB mechanism involving an amido conjugate base generated by the phenoxide-assisted NH-deprotonation of the coordinated amine. The [OH⁻]-dependent path also involves the conventional SN₁ CB mechanism. The rate constant, k₁, for the SNICB path exhibits a steric acceleration with the increasing size of the non-labile alkylamine, whereas the rate constant, k₂, for the SN₁CB path shows a reverse trend. TMC 2578     

Singlet quenching of tetraphenylporphyrin and its metal derivatives by iron(III) coordination compounds

The quenching of fluorescence of the free-base tetraphenylporphyrin, H₂TPP, and its metal derivatives, MgTPP and ZnTPP by diverse iron(III) complexes, [Fe(CN)₆]³⁻, Fe(acac)₃, [Fe(mnt)₂]⁻, Fe(Salen)Cl, [Fe₄S₄(SPh)₄]²⁻·, FeTPPCl and [Fe(Cp)₂]⁺ has been studied both in homogeneous medium (CH₃CN) and micellar media, SDS., CTAB and Triton X-100. The quenching efficiencies are analysed in terms of diffusional encounters and it has been possible to separate static quenching components. The quenching constants are dependent on the nature of the ligating atoms around iron(III) and also on the extent of π-conjugation of the ligands. The quenching mechanism has been investigated using steady-state irradiation experiments. Evidence for oxidative quenching by iron(III) complexes was obtained, though the spin multiplicities of the excited electronic states of iron(III) complexes permit both energy and electron transfer mechanisms for quenching of the singlet excited state of the porphyrins.     

Ion-selective electrodes for the determination of <Emphasis Type="Italic">closo</Emphasis>borate anions

Ion-selective electrodes are proposed on the basis of tetradecylammonium salts for determining [B₁₂H₁₂]²⁻, [B₁₀H₁₀]²⁻, and [B₁₀Cl₁₀]²⁻ closoborate anions. Their basic electroanalytical parameters, selectivity, linear response range, detection limit, potential stability, and the pH effect on electrode indications are estimated. The closoborate anions can be arranged in the following series by the selectivity of their determination: [B₁₀Cl₁₀]²⁻ ≫ [B₁₂H₁₂]²⁻ > [B₁₀H₁₀]²⁻.     

Nature of the silver precipitation obtained by cementation from thiosulphate solutions

The morphology and composition of the deposits formed on the surface of magnesium disk during cementation from thiosulphate solutions (0.0025–0.1M) [Ag(S₂O₃)₂]³⁻ + 0.5M S₂O₃ ²⁻ have been studied. A porous deposit with low adhesion is formed on the surface of the magnesium metal substrate. Within a wide range of [Ag(S₂O₃)₂]³⁻ ion concentrations, sulfur as well as silver are constituents of the deposit at the initial stages of cementation and at the end of the reaction. This is attributed to the electrochemical behaviour of magnesium in thiosulphate solutions resulting in the exceeding of current limit on cathode for pure silver reduction. Hence, parallel electrochemical reactions take place that are very close in their values to the standard redox potentials of reduction of [Ag(S₂O₃)₂]³⁻ ions to Ag⁰ and S₂O₃2− ions to S²⁻. Sulfur content in the cement deposits increases with the decrease in [Ag(S₂O₃)₂]³⁻ ion concentration and increase in cementation time. This tendency is also observed with the decreasing solution temperature.     

Kinetics of the Reactions between [(RS)2Fe(µ-S)2M(µ-S)2Fe(SR)2] n− (M = Mo, R = Ph, n = 2; M = V, R = Et, n = 3) and [Fe(SR)4]2−: Key Steps in the Assembly of Cuboidal Fe—S-based Clusters

The reaction of [S₂Mo(μ-S)₂Fe(SPh)₂]²⁻ with [Fe(SPh)₄]²⁻ produces the dicuboidal cluster [{MoFe₃S₄(SPh)₃}₂(μ-(SPh)₃]³⁻ in a three stage process studied by ¹H-n.m.r. spectroscopy and stopped-flow spectrophotometry. The initial stage involves the formation of the linear trinuclear [(PhS)₂Fe(μ-S)₂Mo(μ-S)₂Fe(SPh)₂]²⁻ and the kinetics indicate an equilibrium reaction (k₁^Mo = 2.5±0.3x 10² dm³ mol⁻¹ s⁻¹, k₋₁^Mo = 0.8±0.1 s⁻¹). The second stage involves the reduction of [(PhS)₂Fe(μ-S)₂Mo(μ-S)₂Fe(SPh)₂]²⁻ by [Fe(SPh)₄]²⁻ to form [(PhS)₂Fe(μ-S)₂Mo(μ-S)₂Fe(SPh)₂]³⁻ which subsequently rearranges to form the voided cuboidal [MoFe₂S₄(SPh)₃]²⁻. The kinetics of the second stage exhibits a simple first order dependence on the concentrations of both [(PhS)₂Fe(μ-S)₂Mo(μ-S)₂Fe(SPh)₂]³⁻ and [Fe(SPh)₄]²⁻ (k₂^Mo = 25 ± 2 dm³ mol⁻¹ s⁻¹). The Kinetics of the third stage have not been studied but must involve incorporation of the final Fe and formation of the dicuboidal [{MoFe₃S₄(SPh)₃}₂(μ-SPh)₃]³⁻. The kinetics of the reaction between [(EtS)₂Fe(μ-S)₂V(μ-S)₂Fe(SEt)₂]³⁻ and [Fe(SEt)₄]²⁻ to form [{VFe₃S₄(SEt)₃}₂(μ-SEt)₃]³⁻ have also been studied. An important difference between this reaction and the formation of the analogous [{MoFe₃S₄(SPh)₃}₂(μ-SPh)₃]³⁻ is that the formation of [{MoFe₃S₄(SPh)₃}₂(μ-SPh)₃]³⁻ from [(PhS)₂Fe(μ-S)₂Mo(μ-S)₂Fe(SPh)₂]²⁻ involves a change in the redox state of the cluster, whilst the formation of [{VFe₃S₄(SEt)₃}₂(μ-SEt)₃]³⁻ from [(EtS)₂Fe(μ-S)₂V(μ-S)₂Fe(SEt)₂]³⁻ requires no change in redox state. The reaction between [(EtS)₂Fe(μ-S)₂V(μ-S)₂Fe(SEt)₂]³⁻ and [Fe(SEt)₄]²⁻ involves two stages. The kinetics of the faster phase is associated with a rate law analogous to that observed for the reaction between [(PhS)₂Fe(μ-S)₂Mo(μ-S)₂ Fe(SPh)₂]²⁻ with [Fe(SPh)₄]²⁻: a first order dependence on the concentrations of [(EtS)₂Fe(μ-S)₂V(μ-S)₂Fe(SEt)₂]³⁻ and [Fe(SEt)₄]²⁻ (k₂^V = 1.1±0.1 x 10³ dm³ mol⁻¹ s⁻¹. This observation indicates that whilst there is no change in redox state between [(EtS)₂Fe(μ-S)₂V(μ-S)₂Fe(SEt)₂]³⁻ and [{VFe₃S₄(SEt)₃}₂(μ-SEt)₃]³⁻, reduction is necessary to catalyse the conversion of the linear [(EtS)₂Fe(μ-S)₂V(μ-S)₂Fe(SEt)₂]³⁻ into the voided cuboidal [VFe₂S₄(SEt)₃]³⁻. The results of the studies reported in this paper together with those on other putative reactions involved in the assembly of cuboidal clusters, have been combined to present a scheme of the mechanism of cuboidal cluster assembly. Electronic supplementary material Electronic supplementary material is available for this article at and accessible for authorised users.     

Solvent extraction behavior of hexachlorotechnetate(IV) ion in the HCl-TBP system

The solvent extraction behavior of [TcCl₆]²⁻ in the HCl-TBP system was studied together with that of its aqua complexes. It was deduced from the dependence of the distribution coefficient of [TcCl₆]²⁻ both on the HCl and TBP concentrations that a possible form extracted into TBP is H₂[TcCl₆](TBP)₄. The distribution coefficients of aqua complexes of [TcCl₆]²⁻ were found to be in the order of [TcCl₄(H₂O)₂]<[TcCl₆]²⁻<[TcCl₅(H₂O)]⁻.     

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.     

Estimation of the specific content and nature of sorption sites of iron(<Emphasis Type="SmallCaps">III</Emphasis>) and zirconium(<Emphasis Type="SmallCaps">IV</Emphasis>) oxyhydroxides

The sorption of anions H₂PO₄ ⁻, HPO₄ ²⁻, PO₄ ³⁻, [Fe(CN)₆]³⁻, and [Fe(CN)₆]⁴⁻ from aqueous solutions on the surface of Fe^III and Zr^IV oxyhydroxide hydrogels freshly precipitated at pH 4–13 was studied. The region of sorbate concentrations was from 0.00025 to 0.06 mol L⁻¹. The plots of the anion uptakes vs. their equilibrium concentrations are represented by isotherms of the first type, which are well described by the Langmuir equation if the quantity of the amount adsorbed is expressed as mol-site g⁻¹. The maximum uptakes and constants of the Langmuir equation were calculated. The phosphate anions occupy the same number of sorption sites on the sorbents precipitated at different pH. The average specific content of sorption sites for the ferro- and zirconogels in the metastability period is independent of the pH of their precipitation, being 3.1·10⁻³ and 3.2·10⁻³ mol-site g⁻¹, respectively. The [Fe(CN)₆]³⁻ and [Fe(CN)₆]⁴⁻ anions are sorbed only on the positively charged sites of the hydrogels and occupy not more than 2·10 mol-site g⁻¹ in the studied interval of pH of precipitation. __________ Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1736—1741, August, 2005.     

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.     

A study of the aerobic reaction between dopamine and Fe(III) in the presence of S<Subscript>2</Subscript>O<Subscript>3</Subscript><Superscript>2−</Superscript>

The reaction between Fe(III) and dopamine in aqueous solution in the presence of Na₂S₂O₃ was followed through UV–Vis spectroscopy, pH and oxy-reduction potential (Eh) measurements. The formation and quick disappearing of the complex [Fe(III)HL¹⁻]²⁺, HL¹⁻ = monoprotonated dopamine was observed with or without S₂O₃ ²⁻ at pH 3. An unexpected reaction occurs in presence of thiosulfate forming the stable anion complex [Fe(III)(L²⁻)₂]¹⁻, L²⁻ = dopacatecholate (λ = 580 nm) and the auto-increasing of the pH, from 3 to 7. It was proposed that H⁺ and molecular oxygen are consumed by free radical thiosulfate formed during the reaction.     

Synthesis of the active sites of molybdoenzymes: MoO2(VI) and MoO(IV)-dithiolene complexes mimicking enzymatic reactions of sulphite oxidase with saturation kinetics

[Mo^VIO₂(S₂C₂(CN)₂)₂]₂₋ (┘1) and [Mo^IVO(S₂C₂(CN)₂)₂]²⁻ (2) mimick oxidoreductase enzymatic activities of sulphite oxidase with biological electron donor, SO ₃ ²⁻ , andin vitro electron acceptor, [Fe(CN)₆]³⁻, demonstrating proton coupled electron transfer reaction in water and inhibition of the oxidation of (2) in the presence of KCN. The sulphite exidizing system is characterized by substrate saturation kinetics indicating the biological significance of the reactions     

Oxonium derivatives of <Emphasis Type="Italic">closo</Emphasis>-decaborate in reactions with sulfur-containing nucleophiles

The reaction of the [B₁₀H₉O₂C₄H₈]⁻, [B₁₀H₉OC₄H₈]⁻, and [B₁₀H₉OC₅H₁₀]⁻ anions with negatively charged S-nucleophiles, such as SH⁻, SCN⁻, and S₂O₃ ²⁻, resulted in the ring opening of the cyclic substituent and the formation of derivatives with the terminal thiol, thiocyanate, and thiosulfate groups. The structures of the products were confirmed by the IR, mass, and ¹H, ¹¹B, and ¹³C NMR spectra.     

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.