Extraction-spectrophotometric determination of trace iron (II) in sea water with 2-[2-(3,5-dibromopyridyl)azo]-5-diethylaminobenzoic acid

An extraction-spectrophotometric method is described for the determination of traces of iron(II) with 2-[2-(3,5-dibromopyridyl)azo]-5-diethyl-aminobenzoic acid. The reagent forms a stable and blue 12 iron/reagent complex that can be extracted into chloroform. The apparent molar absorptivity of the iron(II) complex is 1.09 × 10⁵ 1 mol^–1 cm^–1 at 624 nm in chloroform. The reagent is relatively selective; interferences from cobalt, copper, nickel and vanadium can be removed by using dimethylglyoxime and EDTA. The method is applied to the determination of iron (II) in sea water and aluminium alloys with good precision and accuracy.     

Determination of Lead Using a New Chromogenic Reagent 2-(2-Sulfo-4-acetylphenylazo)-7-(2,4,6-trichlorophenylazo)-1,8-dihydroxynaphthalene-3,6-disulfonic Acid

A novel chromogenic reagent, 2-(2-sulfo-4-acetylphenylazo)-7-(2,4,6-trichlorophenylazo)-1,8-dihydroxynaphthalene-3,6-disulfonic acid 1, was prepared by diazo coupling of 4-acetylaniline-2-sulfonic acid and 2,4,6-trichloroaniline to chromotropic acid through –N=N– groups. Based on this reagent, a simple, sensitive and selective spectrophotometric method was developed for the determination of lead. In 0.20M phosphoric acid medium, lead reacts with 1 to form a 1:2 blue complex with an absorption maximum of 654nm. Beers law is obeyed in the range of 0–0.6mgL^–1 of lead. The apparent molar absorptivity is 1.25×10⁵Lmol^–1cm^–1. The detection limit and quantification limit were found to be 0.63µgL^–1 and 2.1µgL^–1, respectively. The relative standard deviation for eleven replicate measurements was of 2.6%. The interference of foreign ions was also investigated. All the other foreign ions studied did not interfere with lead determination except for Ca(II) and Ba(II). The interference caused by Ca(II) and Ba(II) can be eliminated by prior extraction of lead with potassium iodide-methylisobutylketone (KI-MIBK). The proposed method was applied to the determination of lead in certified samples with satisfactory results.     

Investigation of 2-[(<Emphasis Type="Italic">E</Emphasis>)-2-(4-diethylaminophenyl)-1-ethenyl]-1,3,3-trimethyl-3H-indolium as a new highly sensitive reagent for the spectrophotometric determination of nitrophenols

A new, simple, rapid, and sensitive spectrophotometric method has been developed for the determination of nitrophenols [picric acid (PA); dinitrophenols (DNP)] in wastewater samples. The method is based on the reaction of nitrophenols with 2-[(E)-2-(4-diethylaminophenyl)-1-ethenyl]-1,3,3-trimethyl-3 H-indolium chloride reagent to form the colored ion associates, which are extracted by organic solvents. The molar absorptivity of the ion associates of PA with the investigated reagent ranges from 8.3×10⁴ to 11.3×10⁴ L mol^–1 cm^–1, depending on the extractant. Because only PA is extracted in an acidic medium with the investigated reagent, but both PA and DNP are extracted in an alkaline medium, it is possible to determine both substances in a mixture. Appropriate reaction conditions have been established. The absorbance of the colored extracts obeys Beers law in the range of 0.04–4.58 mg L^–1 PA, 1.0–18.4 mg L^–1 2,4-DNP and 1.2–14.7 mg L^–1 2,6-DNP, respectively. The limit of detections, calculated from a blank test (n=10; P=0.95), are 0.05 mg L^–1 PA, 0.9 mg L^–1 (2,4-DNP), and 1.1 mg L^–1 (2,6-DNP), respectively.     

A new polymeric chromogenic reagent for dual-wavelength spectrophotometric determination of iron(III)

By condensing chitosan with 7-(4-formyl-phenylazo)-8-quinolinol-5-sulfonic acid (FPAQS), a new polymeric chromogenic reagent C-FPAQS has been synthesized and its properties investigated. In acidic media (pH 2.7), C-FPAQS reacts with iron(III) to yield an orange complex with a molar absorptivity of 2.8 × 10⁴ lmol^–1 cm^–1 at 420 nm, and in the meantime a negative peak at 524 nm. The apparent molar absorptivity (420–524 nm) obtained by dual-wavelength measurements is 7.9 × 10⁴lmol^–1cm^–1 which is about two times higher than that by single-wavelength measurements at 420 nm Beer's law is obeyed in the range of 0–0.8 g ml^–1 for iron(III). The developed method has been satisfactorily used to determine iron at the 0.03 to 3% (ww) level in a nylon-6 and in a soil sample. Compared to the corresponding low-molecular weight FPAQS and other chromogenic reagents, C-FPAQS has not only good sensitivity but also largely increased acid solubility and improved selectivity for iron, which may be explained by the incorporation of FPAQS into an acid-soluble polymer.     

Flotation and spectrophotometric determination of silver with 4-(p-nitrophenylazo)-2-amino-3-pyridinol

A sensitive flotation-spectrophotometric method, based on the complex formed between Ag(I) and 4-(p-nitrophenylazo)-2-amino-3-pyridinol is described. The complex precipitates when the aqueous solution is shaken with benzene, and the solid formed is then dissolved in dimethylformamide. The molar absorptivity of the resulting solution is 10.7×10⁴l·mole^–1·cm^–1 at 605 nm. Beer's law is obeyed between 0.08 and 1 ppm of silver. The molar ratio of Agreagent in the separated complex is 12. The effect of foreign ions has been determined and the proposed method can be applied to determination of silver in an exhausted photographic developing solution.     

Synthesis and properties of disulfonated (2-benzimidazolyl)(phenyl)methanone 5-nitro-2-pyridylhydrazone (S2BINPH) and spectrophotometric determination of trace amounts of nickel with S2BINPH

Disulfonated (2-benzimidazolyl)(phenyl)methanone 5-nitro-2-pyridylhydrazone (S₂BINPH) has been synthesized and its reactivity with metal ions investigated. A sensitive and selective spectrophotometric method for the determination of nickel with this reagent has been developed. S₂BINPH reacts with nickel(II) to form a stable 12 (metal ligand) complex with an absorption maximum at 501 nm. The complex formation is quantitative in the pH range 7.2–8.5. Beer's law is obeyed over the range 60–700 ng ml^–1 of nickel and the apparent molar absorptivity of the complex is 8.86 × 10⁴ mol^–1 1 cm^–1 at 501 nm. The proposed method was applied to the determination of nickel in a standard iron- and -steel sample with satisfactory results. Furthermore, proton dissociation constants of S₂BINPH and the overall formation constant of its nickel complex were also determined spectrophotometrically.     

Extraction-spectrophotometric determination of iron with 2-(2′-benzothiazolylazo)-4,6-dimethylphenol

An extraction-spectrophotometric method for the determination of trace amounts of iron based on its extraction into chloroform with 2-(2-benzothiazolylazo)-4,6-dimethylphenol (BTADMP) from a pH 6.5 medium has been developed. The extracted 12 FeBTADMP complex species allow the determination of 4–30gmg of iron (=3.92×10⁴1·mol^–1·cm^–1 at 790 nm). The method is highly selective and has been applied to the determination of iron in polymineral-polyvitamin pharmaceutical products.     

Preconcentration and Determination of Trace Nickel Using 1-(2-Pyridylazo)-2-Naphtol (PAN) Immobilized on Surfactant-Coated Alumina

A microcolumn of alumina modified with sodium dodecyl sulfate (SDS) and 1-(2-pyridylazo)-2-naphthol (PAN) was prepared for the preconcentration of trace nickel from water samples for a flame atomic absorption spectrometry (FAAS) determination. Under optimized conditions (pH = 4.0; flow rate, 5 mL min^–1) nickel (II) was retained on the column. The nickel collected on the column was eluted with 5 mL of 0.5 M nitric acid. Recovery was greater than 96.7%. A concentration factor of 300 can be achieved by passing 1500 mL of sample through the microcolumn. The relative standard deviation (ten replicate analyses) at the 40 ng mL^–1 level for nickel was 2.4%, and the corresponding limit of detection (based on 3) was 0.06 ng mL^–1. The method was applied to the determination of Ni in waste and mineral waters.     

Complexing properties of thiazolylazophenols and spectrophotometric determination of iron(II) with 2-(2-benzothiazolylazo)-4-methoxyphenol

Summary Thiazolylazophenols containing a methyl or a phenyl group in the thiazole ring were synthesized and their potential for the spectrophotometric determination of iron were studied. These dyes react with iron(II) to form brownish complexes, which show a characteristic absorption in near-infrared region. Among them, 2-(2-benzothiazolylazo)-4-methoxyphenol is most suitable and the iron(II) complex has the absorption maximum at 815 nm in chloroform. The optimum pH for iron extraction lies between 5.5–9.5 and Beer's law holds up to 6 ppm of iron, with a molar absorptivity of 1.47×10⁴ 1 mol^–1 cm^–1. The composition and the extraction constant of the complex was found to be FeBTAMP=12 and log K_ex=–0.76±0.12, respectively. Many kinds of ions can be tolerated even in the presence of large amounts. The method has been applied to the determination of iron in mixtures containing 3d type metal ions, natural waters and native sulfurs with satisfactory results.

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Komplexbildende Eigenschaften von Thiazolylazophenol und spektro-photometrische Bestimmung von Eisen(II) mit 2-(2-Benzothiazolylazo)-4-methoxyphenol

Zusammenfassung Thiazolylazophenol mit einer Methyl- bzw. Phenylgruppe am Thiazolring wurde synthetisiert und seine Eignung für die spektrophotometrische Bestimmung von Eisen(II) studiert. Diese Farbstoffe bilden mit Fe(II) bräunliche Komplexe, die charakteristische Absorption im nahen IR zeigen. Unter ihnen ist 2-(2-Benzothiazolylazo)-4-methoxyphenol am besten geeignet. Der Fe(II)-Komplex hat ein Absorptionsmaximum bei 815 nm in Chloroform. Das optimale pH für die Eisenextraktion liegt zwischen 5,5 und 9,5; das Beersche Gesetz ist bis zu 6 ppm Fe erfüllt, der molare Extinktionskoeffizient beträgt 1,47×10⁴ l·mol^–1·cm^–1. Fe(II)BTMP=12. Die Extraktionskonstante ist log K_ex=–0,76±0,12. Zahlreiche Ionen stören auch in größerer Menge nicht. Die Methode wurde zur Eisenbestimmung in Gemischen mit 3d-Typ-Metallen, in natürlichen Wässern und in inländischem Schwefel angewandt.

     

Extraction spectrophotometric investigation of mixed ligand complex of molybdenum(V) with thiocyanate and 4-acetyl-2-(acetylamino)-5-dimethyl-Δ2-1,3,4-thiadiazole

A sensitive spectrophotometric method is developed for the determination of small amounts of molybdenum based on the extraction of molybdenum-thiocyanate-4-acetyl-2-(acetylamino)-5-dimethyl- ²-1,3,4-thiadiazole complex into chloroform from hydrochloric acid medium which is orange red in colour. The complex has an absorption maximum at 470 nm with a molar absorptivity of 2.01×10⁴l·mole^–1·cm^–1. Beer's law is valid over the concentration range 0.06–2.5 ppm of molybdenum with an optimum concentration range of 0.15–2.2 ppm. The ternary complex is stable for over one week at room temperature. Equilibrium shift method indicates 142 composition for molybdenum-thio-cyanate-4-acetyl-2-(acetylamino)-5-dimethyl- ²-1,3,4-thiadiazole complex. The effects of acidity, reagent concentrations, time, temperature and diverse ions upon the absorbance of the complex are critically assessed. This method has been used successfully for the determination of molybdenum in molybdenum steels.     

Complex formation between nickel(II) and 2-(2-aminoethyl) benzimidazole: A kinetic and equilibrium study

Summary The reversible complex formation between 2-(2-aminoethyl) benzimidazole (AEB) and nickel(II) was studied by stopped flow spectrophotometry at I = 0.30 mol dm^–3. Both the neutral and monoprotonated form of AEB reacted to give the NiAEB²⁺ chelate. At 25 °C, the rates and activation parameters for the reactions Ni_II + AEB NiAEB²⁺ and Ni^II + AEBH⁺ NiAEB²⁺ + H⁺ are k _f ^L(dm^–3 mol^–1 s^–1) = (2.17 ± 0.24) × 10³, H (kJ mol^–1) = 40.0 ± 0.8, S (JK^–1 mol^–1) = – 47 ± 3 and k _inff ^pHL (dm³ mol^–1 s^–1) = 33 ± 10, H (kJ mol^–1) = 42.0 ±2.7, S (JK^–1 mol^–1) = – 72 ± 9. The dissociation of NiAEB²⁺ was acid catalysed and k _obs for this process increased linearly with [H⁺] in the 0.01–0.15 mol dm^–3 (10–30 °C) range with k _H(dm³ mol^–1s^–1) (25 °C) = 329 ± 6, H (kJ mol^–1) = 40 ± 2 and S (JK^–1 mol^–1) = – 61 ± 8. The results also indicated that the formation of NiAEB²⁺ involves a chelation-controlled, rate-limiting process. Analysis of the S ° data for the acid ionisation of AEBH _inf2 ^p2+ and the formation of NiAEB²⁺ showed that the bulky AEBH⁺ ion has a solvent structure breaking effect as compared to AEB [s _aqS ° (AEBH⁺) – s _aq ° (AEB) = 69 JK^–1 mol^–1], while AEBH _inf2 ^p2+ is a solvent ordering ion relative to NiAEB²⁺ [s _aq° (NiAEB²⁺) – ovS _aq ° (AEBH _inf2 ^p2+ ) = 11 JK^–1 mol^–1].Author to whom all correspondence should be directed.     

Cyclodextrin Complexation of the Stilbene 4-(2-(4-Tert-butylphenyl)ethen-1-yl)- benzoate and the Self-assembly of Molecular Devices

E-4-(2-(4- tert - butylphenyl) ethen-1- yl)benzoate, E-1^–, photoisomerizes to the Z-1^– isomer and vice versa in the free state and in the binary complexes 2·E-1^–, 2·Z-1^–, 3·E-1^– and 3·Z-1^– where 2 is the urea-linked cyclodextrin N-(6 ^A -deoxy--cyclodextrin-6 ^A - yl)-N-(6 ^A -deoxy--cyclodextrin-6 ^A - yl)urea and 3 is N,N- bis(6 ^A -deoxy--cyclodextrin-6 ^A - yl)urea. In 2·E-1^–and 3·E-1^– the stilbene occupies both cyclodextrin (CD) components of 2 and 3, whereas in 2·Z-1^– and 3·Z-1^– it only occupies one CD component while the other CD component is unoccupied. 4- tert - Butylphenolate, 4^–, and its carboxylate, 5^–, and sulfonate, 6^–, analogues form the ternary complex 2·Z-1^–·4^– and its analogues and also 3·Z-1^–·4^– and its analogues. These photoisomerize to 2·E-1^–and 3·E-1^– and either free 4^–, 57^– or 6^– and thereby function as molecular devices.     

Extractive spectrophotometric determination of niobium(V) with 3-hydroxy-2-(2-thienyl)-4H-chromen-4-one

A coloured complex of niobium (V) with 3-hydroxy-2-(2-thienyl)-4H-chromen-4-one (HTC) is produced in perchloric acid medium (4 mol/L); it is quite stable and extractable into dichloromethane. Beer's law is obeyed in the range 0.0–1.9 g Nb(V) mL^–1. The molar absorptivity at _max=420 nm is 5.0357×10⁴ L mol^–1 cm^–1. Using Job's method and the mole ratio method, the ratio of metal to ligand (Nb:HTC) in the extracted species has been found to be 1:2. The effect of anions and foreign metals is described.     

Spectrophotometric determination of microamounts of thorium with disodium salt of 2-(2-hydroxy-3,6-disulfo-1-naphthylazo)-benzenearsonic acid (Thorin) as a chromogenic reagent

A sensitive spectrophotometric method has been developed for the determination of microamounts of thorium using 0.05% thorin in a 3M perchloric acid solution as a chromogenic reagent and measuring the absorbance at 544 nm. The complex of thorium thus formed, is stable for more than two months with a constant absorbance of ±0.55%. Beer's law is obeyed from 0 to 25 g g^–1 of thorium in a solution with a molar absorptivity (544 nm) = 1.69×10⁴ M^–1 cm^–1 at 26±1 °C. Among the anions tested, only phosphate, acetate and cyanide at >200-fold excess of thorium interfere in the determination, whereas cations like Zn(II), Al(III), Na(I), Mg(II), and Ca(II) do not effect the absorbance. Thorium can be determined in the presence of oxalate, nitrate, tartrate, sulfate, thiosulfate, citrate, and ascorbate. The accuracy of the method has been checked by measuring the known concentration of thorium in the range of 100 g-5 mg g^–1 and found to be in the range of 7.7–0.9%. The method has been applied successfully to determine thorium at g g^–1 level in local ore samples with a precision of ±0.3%. The sensitivity of the method on Sandell's scale is 0.082±0.002 g g^–1 cm^–1.     

Extraction and spectrophotometric determination of titanium(iv) with 3-hydroxy-2-methyl-1-(4-tolyl)-4-pyridone

Titanium(IV) in sulphuric, perchloric and hydrochloric acid media reacts with 3-hydroxy-2-methyl-1-(4-tolyl)-4-pyridone (HY) to give a complex which is extractable into chloroform. The composition of the extractable complex depends on the acidity of the aqueous phase and on which mineral acid is used. The mixed titanium-perchlorate-HY complex which is formed in the presence of excess of perchlorate is the most suitable for the spectrophotometric determination of titanium. The molar absorptivity of the complex is 1.6×10⁴1·mole^–1·cm^–1 at 355 nm. The optimum titanium concentration range is 0.5–6,g/ml. The method has been successfully applied to the determination of titanium in samples of bauxite and alumina refractory.     

Über Reaktionen von Arylbiguaniden mit Benzoin beimpH der Biguanidbasen

Summary Arylbiguanides2 a–e react with benzoin (1) at thepH of the base to two different products.1 undergoes in presence of the base2 a–e oxidation to benzil and benzoic acid, which reacts fast with the arylbiguanides2 a–e to yield N-[4-(arylamino)-6-phenyl-1,3,5-triazine-2-yl]benzamides3 a–d. After lowering thepH of the reaction mixture, the bases2 b–e react with benzil to yield 2-[1-aryl-5-oxo-4,4-diphenyl-2-imidazoline-2-yl]guanidine4 b–e. The mechanism of the formation is discussed. The structure of4b was established from a single crystal x-ray structure analysis. The analysis was carried out at 100K: C₂₃H₂₁N₅O,M _r=383.5, monoclinic, C 2/c,a=15.842(6),b=8.419(3),c=30.223(10) Å, =98.44(3)°,V=3 987.3(9) ų,Z=8,d _x=1.277 g/cm³, =0.81 cm^–1,R=5.89%R _w=4.97% (1 537 observations, 233 parameters).

     

Crystal structure of 1-(2-pyridiniomethyl)-2,4-<Emphasis Type="Italic">bis</Emphasis>(phenylsulfonyl)benzene bromide

The crystal structure of 1-(2-pyridiniomethyl)-2,4-bis(phenylsulfonyl)benzene bromide, (C₂₄H₂₀NO₄S₂)⁺. Br^– (I) has been investigated by X-ray diffraction (XRD)analysis. The triclinic structure of I (space group P1, a = 7.863 , b = 8.350 , c = 9.043 , = 94.00°, = 97.81°, = 104.62°, Z = 1) was solved by direct methods and refined by full-matrix least-squares analysis in an anisotropic approximation to R = 0.048 for all 4570 reflections collected (CAD-4 automatic diffractometer, CuK ). The geometrical parameters of the organic cation were determined with a sufficient degree of accuracy. The crystal structure of I involves a very strong interionic hydrogen bond N⁺-HBr^–.Original Russian Text Copyright © 2004 by A. N. ChekhlovTranslated from Zhurnal Strukturnoi Khimii, Vol. 45, No. 4, pp. 759–763, July–August, 2004.This revised version was published online in April 2005 with a corrected cover date.     

Thermodynamics of complexation of lanthanides and some of transition metal ions by 5,5-dimethyl-cyclohexane-2-(2-hydroxyphenyl)-hydrazono-1,3-dione (DCPHD) and its derivatives

Summary Equilibrium betweenDCPHD,DC-4-Cl-PHD, andDC-4-Me-PHD and protons, transition, and lanthanide ions have been investigated at 30 °C by means of potentiometric titration in 75% (v/v) methanol-water mixture containing 0.10M KNO₃ as a constant ionic medium. Thermodynamic parameters (G, H and S) referring to the formation of species HL ^–,L ^––,ML ^+n–2 andML ₂ ^+n–4 (L ^–– denotes the ligand anion) have been determined in solutions. The solvent effects on the thermodynamic parameters of the complex formation are discussed in terms of differences in the donor ability of methanol and water solvents. The plots of thermodynamic parameters versus ionic potential (Z ²/r) of the lanthanide elements is not linear as expected from ionic theory. The obtained curve can be resolved in an initial group (the lighter lanthanides), an intermediate group (Sm-Dy), and a final group (the heavier ones, Tb-Lu). This behavior was explained in terms of differences in the dehydration of lighter lanthanide(III) from that of heavier ones.

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Thermodynamik der Komplexierung von Lanthaniden und einigen Übergangsmetall-Ionen mit 5,5-Dimethylcyclohexyl-2-(2-hydroxyphenyl)-hydrazono-1,3-dion (DCPHD) und seinen Derivaten

Zusammenfassung Die Gleichgewichte zwischenDCPHD,DC-4-Cl-PHD undDC-4-Me-PHD mit Protonen, Übergangsmetall- und Lanthaniden-Ionen wurden bei 30 °C mittels potentiometrischer Titration in 75% (v/v) Methanol-Wasser mit einem Gehalt an 0.10M KNO₃ als konstantem ionischem Medium untersucht. Die thermodynamischen Parameter G, H und S zur Bildung der Spezies HL ^–,L ^––,ML ^+n–2 undML ₂ ^+n–4 (L ^–– steht für das Ligandenanion) wurden in Lösung bestimmt. Die Lösungsmitteleffekte auf diese Komplexbildungsparameter werden auf Basis der Differenz im Donorvermögen von Methanol und Wasser als Solventien diskutiert. Die Diagramme der thermodynamischen Parameter gegen die ionischen Potentiale (Z ²/r) der Lanthaniden sind, wie nach der Ionentheorie zu erwarten, nicht linear. Die erhaltene Kurve läßt eine Anfangsgruppe (die leichteren Lanthaniden), eine mittlere Gruppe (Sm-Dy) und eine Endgruppe (die schwereren Lanthaniden. Tb-Lu) erkennen. Dieses Verhalten kann aus dem Unterschied im Dehydratationsverhalten erklärt werden.

     

3-Hydroxy-2-(2"-Thienyl)-4H-Chromon-4-one as a Spectrophotometric Reagent for the Trace Determination of Zirconium in an Aqueous Phase

A rapid, simple, selective, and sensitive method for the trace determination of zirconium has been developed based on the reaction of 3-hydroxy-2-(2"-thienyl)-4H-chromon-4-one in an hydrochloric acid medium to form a yellow–colored complex which is rendered water soluble by the micellar action of Triton X-100 and measured at 415 nm. Most of the metal ions do not interfere with the determination. Beer's law is obeyed in the concentration range 0–2.0 g/mL and the molar absorptivity of the complex is 2.73 × 10⁴L mol^–1cm^–1; Sandell's sensitivity is found to be 0.0034 g cm^–2. The method has been applied for the determination of zirconium in various samples, and satisfactory results have been obtained.     

Synthesis,metal-exchange kinetics and X-ray structure of the nickel(II) complex of the diazacyclam ligand 1,3,6,10,12,15-hexa-azatricyclo [13.3.1.16,10]eicosane, [NiL] (ClO4)2

The preparation of the nickel(II) complex of the diazacyclam ligand 1,3,6,10,12,15-hexaazatricyclo [13.3.1.16,10]eicosane (2) by the reaction of the nickel(II) complex of N-(2-aminoethyl)-1,3-diaminopropane with formaldehyde in MeOH solution is described. The crystal structure of [NiL](ClO4)2 has been determined. The nickel atom is four coordinate and planar with Ni-N bond lengths of 1.969(4) and 1.928(3)Å in a centrosymmetric structure. The basic diazacyclam ring system has a trans III configuration with the two additional six-membered rings fused in a chair conformation.The kinetics of the metal exchange:for the nickel complexes (1) and (2) have been studied in detail. Under the experimental conditions employed, with copper(II) in at least a tenfold excess, the reaction is independent of the copper(II) concentration. The copper(II) effectively scavenges the free ligand as the nickel(II) complex dissociates. For the nickel complex (1) k = 2 × 10–4 s–1 at 60°C and H = 126 ± 5 kJmol–1 and S298 = 61 ± 15 JK–1mol–1. For the complex (2), k = 1.8 × 10–4 s–1 at 60°C and H = 99 ± 6 kJmol–1 and S298 = –21 ± 10 JK–1 mol–1.