A proton magnetic resonance study of ligand exchange on hexakis(1,1,3,3-tetramethylurea) thulium(III)

A ¹H NMR study of the rate of exchange of 1,1,3,3-tetramethylurea (tmu) on [Tm(tmu)₆]³⁺ is found to be independent of [tmu] over a five-fold variation in CD₃CN solution, and to be characterized by k(298.2 K) = 145 ± 1 s⁻¹, ΔH^# = 29.3±0.3 kJ mol⁻¹, and ΔS^# = 105±1 J K⁻¹ mol⁻¹. These data are discussed in conjunction with data from related lanthanide and pseudo-lanthanide systems and the mechanistic implications are considered.     

Kinetics and mechanism of ligand substitution in binuclear nickel(II) and cobalt(II) complexes

The kinetics and mechanism of the removal of M²⁺ from bis-(heptane-2,4,6-trionato)M(II) [M = Ni, Co] by ethylenediminetetraacetic acid (EDTA), nitrilotriacetic acid (NAT), 1,2-cyclohexanediamine-N, N, N′, N′-tetraacetic acid (C_yDTA), and ethylenebis(oxyethylenenitrilo)tetraacetic acid (EGTA) have been investigated using stopped-flow spectrophotometry in methanol-water at 25°C and ionic strength 0.1 mol dm^?3 KNO₃. The reactions were investigated at a number of different pHs. An associative mechanism is proposed to account for the kinetic data. Although all the ligands have similar functional groups, their reactivity towards the parent complex is quite different. The pH dependence of the rate constants has been used to determine the relative reactivities of the various ligand species present. In the case of nitrilotriacetic acid, a nonlinear dependence on ligand concentration is observed, thus confirming the mechanism proposed. © 1995 John Wiley & Sons, Inc.     

Spectrophotometric estimation of cobalt(II) with nitrilotriacetic acid

A convenient and efficient method for the estimation of cobalt(II) ions in the presence of other metal ions is described. Interference of metal ions such as iron(II), iron(III), nickel(II), manganese(II), and copper(II) have been investigated. Only iron(III) ions seriously affect this determination. Copper(II) and nickel(II) ions do not interfere if present in a molar-ratio less than 1:2 in the cobalt(II) ion solution. Cobalt(II)-nickel(II) and cobalt(II)-copper(II) binary mixtures can be efficiently analyzed at selective wavelengths.     

Spectrophotometric study of coproporphyrin-I complexes of copper(II) and cobalt(II)

The reagent 3,8,13,18-tetramethyl-21H,23H-porphine-2,7,12,17-tetrapropionic acid or coproporphyrin-I (CPI) was used for the spectrophotometric determination of copper(II) and cobalt(II) in the presence of pyridine and imidazole catalysts. Optimum conditions were investigated and the methods were applied to the determination of parts per billion levels of copper(II) and cobalt(II). The Sandell sensitivities of the recommended procedures were 0.568 μm cm⁻² and 0.464 μg cm⁻² (for A = 0.001) for copper and cobalt, respectively. The relative standard deviations were 2.0% for copper and 1.0% for cobalt. The kinetics of the reaction of CPI with copper(II) and cobalt(II) in the presence of the catalysts and the influence of the temperature were studied, and their kinetic constants determined.The influence of light on the photodecomposition of CPI was also studied.     

Copper(II), cobalt(II), and nickel(II) complexes with a bulky anthracene-based carboxylic ligand: syntheses, crystal structures, and magnetic properties

To systematically explore the influence of the bulky aromatic ring skeleton with a large conjugated pi-system on the structures and properties of their complexes, six CuII, CoII, and NiII complexes with the anthracene-based carboxylic ligand anthracene-9-carboxylic acid (HL1), were synthesized and characterized, sometimes incorporating different auxiliary ligands: [Cu2(L1)4(CH3OH)2](CH3OH) (1), [Cu4(L1)6(L2)4](NO3)2(H2O)2 (2), {[Cu2(L1)4(L3)](CH3OH)0.25}infinity (3), [Co2(L1)4(L4)2(micro-H2O)](CH3OH) (4), {[Co(L1)2(L5)(CH3OH)2]}infinity (5), and {[Ni(L1)2(L5)(CH3OH)2]}infinity (6) (L2 = 2,2'-bipyridine, L3 = 1,4-diazabicyclo[2.2.2]octane, L4 = 1,10-phenanthroline, and L5 = 4,4'-bipyridine). 1 has a dinuclear structure that is further assembled to form a one-dimensional (1D) chain and then a two-dimensional (2D) network by the C-H...O H-bonding and pi...pi stacking interactions jointly. 2 takes a tetranuclear structure due to the existence of the chelating L2 ligand. 3 possesses a 1D chain structure by incorporating the related auxiliary ligand L3, which is further interlinked via interchain pi...pi stacking, resulting in a three-dimensional (3D) network. 4 also has a dinuclear structure and then forms a higher-dimensional supramolecular network through intermolecular pi...pi stacking and/or C-H...pi interactions. 5 and 6 are isostructural complexes, except they involve different metal ions, showing 1D chain structures, which are also assembled into 2D networks from the different crystallographic directions by interchain pi...pi stacking and C-H...pi interactions, respectively. The results reveal that the steric bulk of the anthracene ring in HL1 plays an important role in the formation of 1-6. The magnetic properties of the complexes were investigated, and the very long intermetallic distances result in weak magnetic coupling, with the exception of 1 and 3, which adopt the typical paddle-wheel structure of copper acetate and are thus strongly coupled.     

Extraction-polarographic determination of cobalt(II) and nickel(II) as 2,2′-bipyridine complexes in acetonitrile

The tris(2,2′-bipyridine)cobalt(II) complex gives a reversible d.c. wave with E$\text{1}\text{2}$ = ?1.02 V vs. SCE and a sharp differential pulse peak at E_p = ?1.03 V in a salted-out acetonitrile phase. A simple selective method is described for the determination of cobalt(II); down to 0.25 μg of cobalt(II) can be determined in presence of large amounts of Ni, Zn, Cd, Pb, and Cu; iron(III) can be masked with sodium fluoride. The method is applicable to the determination of >0.0l% cobalt in nickel salts and >5 × 10^?5% cobalt in iron salts. Nickel(II) can also be extracted from aqueous solution and determined by differential pulse polarography, even in presence of a 20-fold amount of cobalt(II) by masking with EDTA; >0.01% of nickel in cobalt salts can be determined reproducibly.     

Electro-oxidation and electro-reduction of some iron (II), cobalt(II) and nickel(II) polypyridyl complexes in acetonitrile

Cyclic voltammetry and controlled potential coulometry studies of 2,2′-bipyridyl, 4,4′-dimethyl-2,2′-bipyridyl and 2,2′, 2″-terpyridyl complexes of Fe(II), Co(II) and Ni(II) in acetonitrile are described. E_1/2 values for M(III)/M(II) and M(II)/M)I) couples are compared and crystal field effects discussed. A series of M(I) and M(III) polypyridyl complexes have been prepared by controlled potential electrolysis; these include a number of 3+ and 1+ oxidation state complexes of nickel which have not previously been isolated — [Ni(bipy)₃]-(ClO₄)₃, [Ni(dimbipy)₃](ClO₄)₃, [Ni(terpy)₂](ClO₄)₃ and [Ni(bipy)₂]ClO₄.     

Bimetallic reactivity. One-site addition two-metal oxidation reactions using a di-Co(II) complex of a binucleating ligand with 5- and 6-coordinate sites

The preparation of an unsymmetrical binucleating ligand bearing a bridging oxadiazole ring flanked on one side by three ligands and on the other by four ligands is described. When bound to two metals, the ligand forms complexes where the metals are in 5- and 6-coordinate sites after the incorporation of an exogenous bridging ligand. A di-Co(2+) complex of this ligand has been prepared containing a hydroxide bridge. The complex is readily oxidized to the di-Co(3+) state by outer sphere electron transfer with ferrocenium ions. Addition of Br(2) or NO(2)(+) to the di-Co(2+) complex leads to the rapid formation of the di-Co(3+) bromo or nitro complexes, respectively. The ligand characteristics which allow for double oxidation with ferrocenium ions and for the one-site addition two-metal oxidations with Br(2) and NO(2)(+) are discussed in terms of mechanical coupling between the two metal sites.     

Spectrophotometric determination of cobalt(II) based on the ion-pair of tetrathiocyanatocobaltate(II) with neotetrazolium chloride

The ion-pair formed between tetrathiocyanatocobaltate(II) and [3,3′-(4,4′- biphenylene)bis(2,5-diphenyl)]-2H-tetrazolium chloride (neotetrazolium chloride) can be extracted into 4-methylpentan-2-one and used for the spectrophotometric determination of cobalt (2.5–9.8 μg ml^?1) at pH 3.5–5.0. The Sandell sensitivity of the method is 0.02 μg Co cm^?2. With suitabte masking, the method is quite selective and is applicable to nickel wire alloys.     

Spectrophotometric determination of cobalt by extraction of tetrathiocyanatocobaltate(II) with propylene carbonate

Cobalt is extracted as ammonium tetrathiocyanatocobaltate(II) propyelene carbonate. The effects of pH and diverse ions and masking are reported. The system is applied to the determination of cobalt (0.01–0.20%) in mild steels without prior separation of iron.     

Spontaneous reduction of a low-spin Fe(III) complex of a neutral pentadentate N(5) Schiff base ligand to the corresponding Fe(II) species in acetonitrile

The iron complexes of a designed pentadentate Schiff base ligand N,N-bis(2-pyridylmethyl)amine-N-ethyl-2-pyridine-2-aldimine (SBPy(3)) have been synthesized. The low-spin mononuclear Fe(III) complex [(SBPy(3))Fe(DMF)](ClO(4))(3) (2), though stable in the solid state, is spontaneously reduced to the corresponding Fe(II) species [(SBPy(3))Fe(MeCN)](2+) in MeCN. Fe(II) complex [(SBPy(3))Fe(MeCN)](BF(4))(2) (3) has been isolated independently and characterized by crystallography. Electrochemical studies indicate that SBPy(3), like other pentadentate polypyridine ligands, stabilizes the Fe(II) center to a great extent (E(1/2) = 1.01 V vs SCE in MeCN). This fact is responsible for the ready reduction of 2. It is evident that such reactivity has brought complications in the syntheses of iron complexes of polypyridine ligands reported in previous accounts. Very low solubility of 2 in MeOH has allowed isolation of analytically pure 2 in the present work. Storage of dilute methanolic solution of 2 results in the formation of the mu-oxo Fe(III) dimer [(SBPy(3))FeOFe(SBPy(3))](ClO(4))(4) (5), the structure of which has also been determined. Fe(II) complex 3 reacts with CN(-) to afford cyanide adduct [(SBPy(3))Fe(CN)](BF(4)) (4) but does not exhibit any reactivity toward NO. The azomethine moiety (CH=N-py) of 2 is rapidly oxidized by H(2)O(2) to a pyridine-2-carboxamido (C(=O)-N-py) unit and affords [(PaPy(3))Fe(MeCN)](ClO(4))(2) (1), a complex previously reported by us.     

Spectrophotometric determination of cobalt(II) and cyanocobalamin with vanillilfluorone and its applications

Spectrophotometric determination of cobalt(II) was accomplished with vanillilfluorone (VF) in the presence of dimethylbenzyltetradecylammonium chloride (Zephiramine, Zep). In the determination of cobalt(II), Beer's law was obeyed in the range of 24-470 ng/ml, with an effective molar absorption coefficient (at 575 nm) and relative standard deviation of 1.35×10(5) l mol(-1) cm(-1) and 0.66% (n=5), respectively. The composition ratio of the colored complex was determined by the mole ratio and continuous variation methods, and it was found to be Co(II) : VF : Zep=1 : 2 : 4. Analysis of cyanocobalamin by the same procedure showed that cyanocobalamin could be determined in the concentration range of 0.5-0.11 μg/ml using the proposed method.     

Spectrophotometric determination of cobalt with 2,2′-dipyridyl ketoxime. II

Summary A rapid spectrophotometric method for the determination of cobalt in the presence of nickel using 2,2-dipyridyl ketoxime in aqueous 1.44 M hydrochloric acid solution is described. The reliability of the method was tested using an NBS standard nickel oxide sample containing small amounts of cobalt, copper, iron, magnesium, manganese, titanium, aluminum and chromium. The effects due to pH, time, nickel concentration and the above mentioned diverse cations are reported. Beer's law is obeyed and the molar extinction coefficient at 388 nm is 2.1×10⁴l mole^–1 cm^–1.

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Zusammenfassung Eine spektrophotometrische Methode zur Bestimmung von Kobalt in Gegenwart von Nickel mit 2,2-Dipyridylketoxim in wäßriger 1,44-m Salzsäure wurde beschrieben. Die Brauchbarkeit des Verfahrens wurde an einer NBS-Standardprobe von Nickeloxid erprobt, die geringe Mengen Co, Cu, Fe, Mg, Mn, Ti, Al und Cr enthielt. Über den Einfluß des pH, der Zeit, der Nickelkonzentration und der angeführten Metalle wurde berichtet. Das Beer'sche Gesetz ist erfüllt. Der molare Extinktionskoeffizient bei 388 nm beträgt 2,1·10⁴ 1·Mol^–1·cm^–1.