Kinetics and mechanism of complex formation between beryllium(II) and the 3-nitrosalicylatopentaamminecobalt(III) ion

Summary The kinetics of formation and dissociation of the binuclear complex of Be²⁺ with 3-nitrosalicylatopentaamminecobalt(III) have been investigated in the 20–40 and 25–40 °C ranges (I = 0.3 mol dm ^–3), respectively. At 25 °C the rate and activation parameters for the formation of the binuclear species are: k _f = 26.9 × 10² dm³mol^–1s^–1, H = 104 ± 7kJ mol^–1 S = 91 ± 22JK^–1mor^–1.The rate constant, activation enthalpy and activation entropy for the acid-catalysed dissociation of the binuclear species are: 1.25 ± 0.08dm³mol ^–1 at 25 °C, 53 ± 3kJ mol^–1 and - 67 ± 9 J K ^–1 mol^–1, respectively. The formation of the binuclear species is chelation controlled while the dechelation is acid catalysed.     

Extraction of gallium(III) and aluminium(III) with O,O-dialkyldithiophosphoric acids

The extraction of Ga³⁺ and Al³⁺ with the liquid cation-exchangers di-n-butyldithiophosphoric acid (DBTPA) and di-(2-ethylhexyl)dithiophosphoric acid (DETPA) in kerosene, in the presence and absence of alcohols and tri-n-butyl phosphate (TBP) has been studied. Both Ga³⁺ and Al³⁺ can be extracted in the form of a neutral complex, MA₃, but the distribution coefficient of Ga³⁺ is the higher by about two orders of magnitude, which can be the basis of the solvent extraction separation of gallium and aluminium. The differences can be explained by the interaction between the sulphur donor atoms of the extractants and the d¹⁰ electronic shell of Ga³⁺ as well as by the lower steric hindrance of ligands co-ordinated to Ga³⁺.     

Kinetics and mechanism of pentacyanohydroxoferrate(III) formation from mono(aminocarboxylato)iron(III) complexes

Summary The kinetics and mechanism of the system: [FeL(OH)]^2–n + 5 CN^– [Fe(CN)₅(OH)]^3– + L^n–, where L=DTPA or HEDTA, have been investigated at pH= 10.5±0.2, I=0.25 M and t=25±0.1 C.As in the reaction of [FeEDTA(OH)]^2–, the formation of [Fe(CN)₅(OH)]^3– through the formation of mixed ligand complex intermediates of the type [FeL(OH)(CN)_x]^2–n–x, is proposed. The reactions were found to consist of three observable stages. The first involves the formation of [Fe(CN)₅(OH)]^3–, the second is the conversion of [Fe(CN)₅(OH)]^3– into [Fe(CN)₆]^3– and the third is the reduction of [Fe(CN)₆]^3– to [Fe(CN)₆]^4– by oxidation of L^n– The first reaction exhibits a variable order dependence on the concentration of cyanide, ranging from one at high cyanide concentration to three at low concentration. The transition between [FeL(OH)]^2–n and [Fe(CN)₅(OH)]^3– is kinetically controlled by the presence of four cyanide ions around the central iron atom in the rate determining step. The second reaction shows first order dependence on the concentration of [Fe(CN)₅(OH)]^3– as well as on cyanide, while the third reaction follows overall second order kinetics; first order each in [Fe(CN)₆]^3– and L^n–, released in the reaction. The reaction rate is highly dependent on hydroxide ion concentration.The reverse reaction between [Fe(CN)₅(OH)]^3– and L^n– showed an inverse first order dependence on cyanide concentration along with first order dependence each on [Fe(CN)_5– (OH)]^3– and L^n–. A five step mechanism is proposed for the first stage of the above two systems.     

Equilibrium study of the systems of aluminium(III), gallium(III) and indium(III) with mercaptoacetate, 3-mercaptopropionate and 2-mercaptobenzoate

Equilibrium studies were carried out by pH-potentiometry on the systems of aluminium(III), gallium(III) and indium(III) with mercaptoacetate (MerAc^2?), 3-mercaptopropionate (MerPr^2?) and 2-mercaptobenzoate (MerBe^2?). It was found that the complex-forming properties of the Al³⁺ ion towards these mercaptocarboxylic acid ligands differ from those of Ga³⁺ and Al³⁺. Under the conditions of the study, Al³⁺ forms only hydroxo complexes, while Ga³⁺ and In³⁺ form relatively stable complexes involving the simultaneous coordination of the carboxylate and the deprotonated mercapto group. In all cases the equilibrium systems can be described without the assumption of polynuclear complexes. The complexes Ga(MerAc)₂ and Ga(MerBe)₂ show marked stability; this was interpreted in terms of back-coordination and of interaction between the d¹⁰ electrons of the Ga³⁺ ion and the empty d orbitals of the S donor atom. Complexes of composition ML_i are not formed in the Ga³⁺-MerPr^2? system; this points to the importan roles of the number of atoms in the chelate ring and the higher stability of the Ga(III)-hydroxo complexes.     

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

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

Kinetics and mechanism of the formation of nitroprusside from aquapentacyanoferrate(III) and NO: complex formation controlled by outer-sphere electron transfer

The kinetics and mechanism of the reaction between nitric oxide and aquapentacyanoferrate(III) were studied in detail. Pentacyanonitrosylferrate (nitroprusside, NP) was produced quantitatively in a pseudo-first-order process. The complex-formation rate constant was found to be 0.252 +/- 0.004 M(-1) s(-1) at 25.5 degrees C, pH 3.0 (HClO(4)), and I = 0.1 M (NaClO(4)), for which the activation parameters are DeltaH++ = 52 +/- 1 kJ mol(-1), DeltaS++ = -82 +/- 4 J K(-1) mol(-1), and DeltaV++ = -13.9 + 0.5 cm(3) mol(-1). These data disagree with earlier studies on complex-formation reactions of aquapentacyanoferrate(III), for which a dissociative interchange (I(d)) mechanism was suggested. The aquapentacyanoferrate(II) ion was detected as a reactive intermediate in the reaction of aquapentacyanoferrate(III) with NO, by using pyrazine and thiocyanate as scavengers for this intermediate. In addition, the reactions of other [Fe(III)(CN)(5)L](n-) complexes (L = NCS(-), py, NO(2)(-), and CN(-)) with NO were studied. These experiments also pointed to the formation of Fe(II) species as intermediates. It is proposed that aquapentacyanoferrate(III) is reduced by NO to the corresponding Fe(II) complex through a rate-determining outer-sphere electron-transfer reaction controlling the overall processes. The Fe(II) complex rapidly reacts with nitrite producing [Fe(II)(CN)(5)NO(2)](4)(-), followed by the fast and irreversible conversion to NP.     

Kinetics and mechanism of a trans-tetraazamacrocyclic chromium(III) complex oxidation by hexacyanoferrate(III) in strongly alkaline media

Oxidation of the trans-[Cr(cyca)(OH)₂]⁺ complex, where cyca = meso-5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane, by [Fe(CN)₆ ]^3- ion in strongly alkaline media, leading to [Cr^V O(cyca_ox )]³⁺ ion, has been studied using electronic and e.p.r. spectroscopy. The kinetics of the Cr^III → Cr^IV transformation have been studied using a large excess of the reductant and OH^- ion over the oxidant. The reaction is a second order process: first order in [Cr^III] and [Fe^III] at constant [OH^-]. The second order rate constant is higher than linearly dependent on the OH^- concentration. The mechanism of the reaction has been discussed. A relatively inert intermediate chromium(V) species was detected based on characteristic bands in the visible region and the e.p.r. signal at g_iso = 1.987 for the systems where an excess of oxidant was used. The hyperfine structure of the main e.p.r. signal is consistent with the d¹ -electron interactions with four equivalent nitrogen nuclei and [Cr^V = O(cyca_ox)]³⁺ formula, where cyca_ox = oxidized cyca, can be postulated for the intermediate Cr^V complex.     

Heterotri- and -tetrametallic alkoxides of chromium(III) containing aluminium(III), gallium(III) and niobium(V)

CrCl₃ · 3THF reacts with two equivalents of potassium alkoxometallates K{M(OPr ⁱ ) _x } [M = Al(A), Ga(B), x = 4; M = Nb(C), x = 6] to give heterobimetallic chloride isopropoxides [Cr{M(OPr ⁱ ) _x }₂Cl(THF)] [M = Al(A – 1), Ga(B – 1), and Nb(C – 1)], in which the replacement of the chloride with an appropriate alkoxometallate (tetraisopropoxoaluminate, tetraisopropoxogallate, or hexaisopropoxoniobate) results in the formation of novel heterotrimetallic derivatives. The 'single pot synthesis of an heterotetrametallic isopropoxide [Cr{Nb(OPr ⁱ )₆}{Al(OPr ⁱ )₄}{Ga(OPr ⁱ )₄}] (7) has been carried out by the sequential addition of (A), (B), and (C) to a benzene suspension of CrCl₃ · 3THF. Alcoholysis of [Cr{Al(OPr ⁱ )₄}₂{Nb(OPr ⁱ )₆}] (1) and [Cr{Al(OPr ⁱ )₄}₂{Ga(OPr ⁱ )₄}] (5) with t-BuOH has also been studied and the derivatives characterized by elemental analyses, molecular weight determinations, spectroscopic [Electronic, i.r., ²⁷Al-n.m.r.] and magnetic susceptibility studies.     

Kinetics and mechanism of the oxidation of some carboxylates by a nickel (III) oxime-imine complex

The kinetics of the oxidation of formate, oxalate, and malonate by |Ni^III(L¹)|²⁺ (where HL¹ = 15-amino-3-methyl-4,7,10,13-tetraazapentadec-3-en-2-one oxime) were carried out over the regions pH 3.0–5.75, 2.80–5.50, and 2.50–7.58, respectively, at constant ionic strength and temperature 40°C. All the reactions are overall second-order with first-order on both the oxidant and reductant. A general rate law is given as - d/dt|Ni^III(L¹)²⁺| = k_obs|Ni^III(L¹)²⁺| = (k_d + nk_s |R|)|Ni^III(L¹)²⁺|, where k_d is the auto-decomposition rate constant of the complex, k_s is the electron transfer rate constant, n is the stoichiometric factor, and R is either formate, oxalate, or malonate. The reactivity of all the reacting species of the reductants in solution were evaluated choosing suitable pH regions. The reactivity orders are: k_HCOOH > k; k > k > k, and k > k < k for the oxidation of formate, oxalate, and malonate, respectively, and these trends were explained considering the effect of hydrogen bonded adduct formation and thermodynamic potential. © 1997 John Wiley & Sons, Inc. Int J Chem Kinet 29: 225–230, 1997.     

Kinetics and equilibria of the interactions of hydroxamic acids with gallium(III) and indium(III)

The thermodynamics and kinetics of the binding of Ga(III) and In(III) to two hydroxamic acids, C6H5-C(O)N(OH)H (BHA) and C6H5-C(O)N(OH)C6H5 (PBHA), have been investigated in acidic media. Spectrophotometric titrations in the UV region reveal that, with excess metal, only the chelate ML forms, whereas the concentration of the protonated species, MHL, is negligible. The thermodynamic parameters indicate that the driving force for formation of ML from MOH2+ and HL is mainly enthalpic, with entropic contributions favoring InL2+ and disfavoring GaL2+ formation. The kinetic (stopped-flow) experiments are interpreted on the basis of two parallel reaction paths both involving reaction of the undissociated ligand (HL): (a) M + HL <==> MHL <==> ML + H where MHL is in a steady state and (b) MOH + HL <==> ML + H2O. Whereas gallium binding to BHA and PBHA proceeds mainly through path b, indium binding to PBHA proceeds through both a and b paths. The rates of both the a and b steps are ligand dependent. Two alternative mechanisms are proposed. The first is based on the electronic characteristics of the ligands and is of the Ia type. The second, of the Id type, assumes that a considerable fraction of the ligand is unreactive owing to intramolecular hydrogen bonding (possibly including a water molecule) which blocks the reaction site. The reasons for preferring the former mechanism are discussed.     

Kinetics and mechanism of photolysis of the iron(III) complex with tartaric acid

The formation of MV^•+ radical cations was observed upon the laser flash photolysis of the iron(III) tartrate complex [Fe^IIITart]⁺ (1) in the presence of methyl viologen (MV²⁺). The rate constants of the reactions involving MV^•+ were measured. The intramolecular electron trans-fer to form Fe^II and escape of the organic radical to the solvent bulk upon the photolysis of 1 were proposed. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 866–869, May, 2007.     

Kinetics and mechanism of oxidation of chromium(III)-l-arginine complex by periodate

Oxidation of the chromium(III)-l-arginine complex [CrIII(L)2(H2O)2]+ by periodate has been investigated. In aqueous solutions, [CrIII(L)2(H2O)2]+ is oxidized by IO−4 according to the rate law: d[CrVI]/dt=k2K5[CrIII]T [IVII]T/1 +([H+]/K1)+K5[IVII]T where k2 is the rate constant for the electron transfer process, K1 the equilibrium constant for the dissociation of [CrIII(L)2- (H2O)2]+ to [CrIII(L)2(H2O)(OH)]+H+, and K5 the pre-equilibrium formation constant. Values of k2= 4.02×10−3s−1, K1=5.60×10−4m and K5=171m−1 were obtained at 30°C and I=0.2m. Thermodynamic activation parameters were calculated. It is proposed that electron transfer proceeds through an inner-sphere mechanism via coordination of IO−4 to chromium(III). This revised version was published online in June 2006 with corrections to the Cover Date.     

Kinetics and mechanism of oxidation of chromium(III)-tetraoxalylurea complex by periodate

A novel chromium(III) complex of tetraoxalylurea was prepared. In aqueous solutions, [Cr^III(H₂L)(H₂O)]⁺ (H₂L = diprotonated tetraoxalylurea) is oxidized by IO ₄ ^– according to the rate law

Interaction of Triapine and related thiosemicarbazones with iron(III)/(II) and gallium(III): a comparative solution equilibrium study

Stoichiometry and stability of Ga(III), Fe(III), Fe(II) complexes of Triapine and five related α-N heterocyclic thiosemicarbazones with potential antitumor activity have been determined by pH-potentiometry, UV-vis spectrophotometry, (1)H NMR spectroscopy, and spectrofluorimetry in aqueous solution (with 30% DMSO), together with the characterization of the proton dissociation processes. Additionally, the redox properties of the iron complexes were studied by cyclic voltammetry at various pH values. Formation of high stability bis-ligand complexes was found in all cases, which are predominant at physiological pH with Fe(III)/Fe(II), whilst only at the acidic pH range with Ga(III). The results show that among the thiosemicarbazones with various substituents the N-terminal dimethylation does not exert a measurable effect on the redox potential, but has the highest impact on the stability of the complexes as well as the cytotoxicity, especially in the absence of a pyridine-NH(2) group in the molecule. In addition the fluorescence properties of the ligands in aqueous solution and their changes caused by Ga(III) were studied.