Physicochemical model for choosing complexes for chromium-plating solutions based on Cr(III) compounds

A prognostic physicochemical model for choosing complexes for chromium-plating solutions is constructed on the basis of a combination of the thermodynamic principles and data on the kinetics of some electrochemical reactions. It is shown that high-quality chromium coating can be produced from solutions of Cr(III) complexes, whose logarithm of stability constants ranges from 10 to 20, and the logarithm of stability constants of Cr(II) complexes formed in electrolysis ranges from 7 to 10. The influence of the nature of ligands on chromium electrodeposition is studied. Using the model proposed, a chromium-plating solution based on a chromium(III) malonate complex is developed. This solution makes it possible to produce high-quality coatings in a wide interval of current densities.Translated from Koordinatsionnaya Khimiya, Vol. 31, No. 1, 2005, pp. 17–21.Original Russian Text Copyright © 2005 by Vinokurov, Demidov, Bondar.     

Corrosion–Electrochemical Behavior of Chromium Deposits Obtained from Sulfuric Acid Solutions Containing Oxalates

The corrosion–electrochemical behavior of chromium electrodeposits, which are formed in sulfuric acid solutions of Cr(III) containing oxalates, is studied by taking steady-state polarization measurements in a 0.5 M H₂SO₄ solution. No region of the metal's active dissolution is observed for such coatings, and the open-circuit potential is localized in the passivity region, i.e. it is substantially displaced in the positive direction as compared with that for metallurgic chromium or the coatings deposited from standard chromium-plating electrolytes. According to XPS data, the surface layer of the passive metal a few nanometers thick includes oxide compounds of chromium and also carbides formed during the coating electrodeposition. Specific features of the corrosion–electrochemical behavior of the deposits are attributed to the presence of carbide compounds of chromium in them, with the compounds playing the role of a cathodic alloying agent.     

Specific feature of synthesis and technological scheme for manufacture of nonstoichiometric hydroxo chromium(III) formates

Specific features of the hydrothermal interaction between chromium(VI) oxide and formic acid to give water-soluble nonstoichiometric hydroxo chromium(III) formates with compositions from the range Cr(OH)_0.66(HCOO)_2.34-Cr(OH)_1.55(HCOO)_1.45 were studied. These compounds can be used in formulations for chromium-plating electrolytes and tanning agents.     

The nephelauxetic effect and ortho- to para-positronium conversion reactions promoted by Cr(III) complexes

The ortho- (o-Ps) to para-positronium (p-Ps) conversion reactions promoted by ten Cr(III) complexes were investigated at several temperatures. The results obtained are discussed together with those of three other Cr(III) complexes measured previously.It was found that all the reactions studied are diffusion controlled and that their rate constants,k _SE, increased as the corresponding ratiosB/B _o between the Racah interelectronic repulsion parameters of Cr(III) complexes and Cr(III) gaseous ion decrease; that is,K _SE increases as 3d electron cloud expansion promoted by ligands, increases. Therefore the o-Ps to p-Ps conversion reactions can be considered a complementary probe for the spectroscopic method in investigating the ligand capabilities to cause 3d cloud expansion. The effect is called nephelauxetic which means expanding (electron) cloud.Finally an empirical average environmental rule, similar to that hypothesized for B/B_o ratios, is proposed for estimating the rate constants,K _SE, of complexes formed with mixed ligand sets.This work was supported by Ministero Universitá e Ricerca Scientifica e Tecnologica (M.U.R.S.T.) and by Consiglio Nazionale Ricerche (C.N.R.).     

Composition, Structure, and Corrosion–Electrochemical Properties of Chromium Coatings Deposited from Chromium(III) Electrolytes Containing Formic Acid and Its Derivatives

To study electrocatalytic properties of chromium, its deposits are obtained from Cr(III) electrolytes containing formic acid and its four derivatives (formaldehyde, methyl alcohol, formamide, dimethylformamide) and comprehensively examined. Amorphous structure of the deposits is established with an x-ray diffraction analysis. The valence state of chromium and major extrinsic elements is studied by x-ray photoelectron spectroscopy. The carbon reduction degree and the organization of carbon particles in deep layers of the deposits (chainlike or graphite-like structures) is shown to depend on the nature of organic compounds added into the chromium-plating electrolyte. The corrosion–electrochemical behavior of the obtained deposits in 0.5 M H₂SO₄ is compared to that of polycrystalline chromium and deposits plated from sulfate–oxalate Cr(III) electrolytes.     

Regularities of complexation in the series of sorbents on the polystyrene matrix and their complexes with chromium(III) and manganese(II)

The physicochemical properties of polymeric complexing sorbents with the o,o′-dihydroxy-(1-azo-1′)-functional complexing group were studied. Optimal sorption parameters for chromium(III) and manganese(II) (medium acidity, temperature, time), the sorption capacity of the sorbents, and the stability constants (pK _a′)of the complexes were determined. Correlation relations were established between the dissociation constants (pK _a′) of the sorbent functional group and pH₅₀ of the complexation of the considered elements; between and the stability of the formed complexes (logβ); and between pK _a′ and the deprotonation energy (E _depr) of the sorbent hydroxy group located in the para-position to the introduced substituent. The established correlation relations are useful for targeted prediction of the physicochemical parameters of the sorbents, their complexes, and the sorption of manganese(II) and chromium(III) aimed at recovery and pre-concentration of these ions from objects with various chemical composition.     

On the Possibility of Replacing Standard Chromium-Plating Electrolytes with Sulfate-Oxalate Solutions of Cr(III)

A method for preparing sulfate-oxalate chromium-plating solutions by reducing Cr(VI) from industrial chromium-plating electrolytes to Cr(III) with a Na₂SO₃ solution is proposed.     

Sharp-line electronic spectroscopy and metal-ligand geometry in chromium(III) complexes with triazacyclononane ligands

Summary An angular overlap model analysis of the sharp-line electronic spectrum of bis(triazacyclononane)chromium(III) chloride indicates that the nitrogen coordination sphere is only slightly distorted from an octahedral arrangement. The best fit to the spectrum was obtained for a geometry in which the nitrogens within a ring are displaced 1.1° away from the cartesian axes towards the molecular C₃ axis and the two rings are twisted by 3.4° away from an antiprismatic orientation. The blue-shift often observed for triazacyclononane complexes compared to complexes with other amine ligands appears not to be due to large geometric distortions in the [9]aneN₃ complexes, and in fact the nitrogen ligand field strengths in [9]aneN₃ and ethylenediamine complexes are not very different when geometric distortions in both complexes are accounted for. A similar analysis of the sharp-line spectrum of triazacylononanetriacetatochromium(III) was undertaken, making use of the known ligand geometry. The ligand field strength of the nitrogens is significantly smaller for this ligand because of inductive effects from the acetato groups. The acetato ligands are strong - and -donors.     

The effect of complexing agents on the DMF–chromium(VI) reaction. A kinetic study

The kinetics of the reduction of chromium(VI) by dimethylformamide (DMF) in the presence of ethylenediaminetetraacetic acid (EDTA) and 2,2-bipyridyl (bpy) was investigated in aqueous HClO₄ acid solutions spectrophotometrically. The experimental findings, that the reaction has an induction period followed by autoacceleration, is explained. After the reduction, chromium(III) is present as the EDTA- and bpy-complexes, although such complexes form very slowly under the same experimental conditions from chromium(III) and the EDTA and bpy, respectively. Increases in the reaction rate with increasing [EDTA] were observed, while added bpy had negligible effect on the reaction rate. The reaction is first-order each in [Cr^VI] and [H⁺]. The first-order kinetics with respect to EDTA at low concentrations shift to zero-order at higher concentrations. The reaction is considered to proceed through the formation of a very stable chromium(VI)–DMF–EDTA complex. The suggested mechanism refers only to the induction period of the reaction. The net rate of oxidation of DMF, as measured by the consumption of chromium (VI), is given by: –d[Cr^VI]/dt = kK ₁ K _b[H⁺][DMF]_T[EDTA][Cr^VI]_T/(1 + K ₁[EDTA])(1 + K _b[H⁺])     

Komplexe von Cu(II), Zn(II), Ni(II) und Mn(II) mit N-m-Tolyl-p-methylbenzhydroxamsäure

The thermodynamic proton ligand and metal ligand stability constants of N-m-tolyl-p-methylbenzohydroxamic acid with Cu(II), Zn(II), Ni(II), and Mn(II) have been determined at 25° and 35° in several dioxane-water media. The pK _a and logK ₁ (logK ₂ or log ₂) varies linearly with the mole fraction of dioxane at a given temperature but not linearly with the reciprocal of dielectric constants of the medium. Values of G ^o, H ^o, and S ^o are tubulated. The stabilities of the complexes mostly follow the order of electron affinities of the metal ions. An attempt has been made to calculate the ligand field stabilization energy of the complexes.

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Reactions of 2-(1-carboxyl-2-hydroxyphenyl)thiazolidine with chromium(III) salts leading to newer synthesis of chromium(III) complexes

Reactions of 2-(L-carboxyl-2-hydroxyphenyl)thiazolidine with different chromium(III) salts [CrCl₃?·?6H₂O, K₃[Cr(SCN)₆], NH₄[Cr(NH₃)₂(SCN)₄]?·?H₂O, [Cr(urea)₆]Cl₃?·?3H₂O and [Cr(CH₃COO)₂H₂O]₂] under varied reaction conditions afforded many new mixed-ligand chromium(III) complexes. The ligand is a tridentate dibasic NSO donor except for complexes 1 and 4 where two moles of the ligand are present for each molecule of complex, one functioning as a dibasic tridentate (NSO) and the other as a monobasic bidentate (NS) (phenolic OH and carboxylic COOH groups remaining uncoordinated). The complexes have been characterized by elemental analyses, magnetic susceptibilities, molar conductances, molecular weights and spectroscopic (IR, Uv-vis) data. The ligand field parameters and NSH Hamiltonian parameters suggest tetragonal geometries of the complexes.     

1,3-pyrimidinyl(2,4,6-pyrimidine trione) and its metal chelates

Summary A new tetradentate ligand 1,3-pyrimidinyl(2,4,6-pyrimidine trione) and its chromium(III), manganese(II), iron(III), cobalt(II), nickel(II), copper(II), zinc(II) and cadmium(II) complexes have been prepared and characterized by elemental analysis, magnetic susceptibility measurements, and electronic and i.r. spectra. Conductivity measurements show the complexes to be non-electrolytes. An octahedral geometry for trivalent ions [iron(III) and chromium(III)] has been proposed, whereas all divalent metal ions, except for copper(II), appear to be tetrahedral. The 10 Dq values suggest that the ligand is as strong as ethylenediamine. The values indicate a considerable orbital overlap in the metal-ligand bond.     

Electromigration of carrier-free radionuclides

Using a special type of on-line electromigration measurements of -emitting radio-nuclides in homogeneous aqueous electrolytes free of supporting material, complex formation of carrier-free²⁴¹Am-Am(III) has been studied in perchlorate electrolytes, T=298.1 (1) K. Stoichiometric stability constants for oxalate ligand of log K₁=5.01 (13), 5.11 (13) and 5.38 (18) as well as log K₂=3.15 (15), 3.19 (14) and 3.58 (26) were obtained at the overall ionic strength of =0.10, 0.05 and 0.01, respectively. The corresponding thermodynamic stability constants are log K ₁ ⁰ =5.90 (15) and log K ₂ ⁰ =3.73 (18). The sulfate ligand values of log K₁=2.5 (2) were obtained both in acidic and neutral solutions, =0.10.Present address: Central Institute of Nuclear Research, Rossendorf, GDR, 8051 Dresden, PSF 19, Department of Radioactive Isotopes.     

Mixed ligand complexes of palladium(II) and platinum(II) with methionine and 2,4-disubstituted pyrimidines

Summary Mixed ligand complexes ofcis-[M(MetH)Cl₂] (M=Pd²⁺ and Pt²⁺; MetH=methionine) with 2,4-disubstituted pyrimidines were prepared and characterised. Thecis-[Pd(MetH)Cl₂] complex reacted with cytosine (2-hydroxy-4-aminopyrimidine), isocytosine (2-amino-4-hydroxypyrimidine) and thiocytosine (2-thio-4-amino-pyrimidine) to form ternary complexes.cis-[Pt(MetH)Cl₂] however reacted with cytosine, uracil (2,4-pyrimidine dione or 2,4-dihydroxypyrimidine) to yield the corresponding mixed ligand complexes. The primary ligand, methionine, binds to the metal ion through sulphur and amino nitrogenvia a six membered chelate ring. The secondary ligands (substituted pyrimidines) bind to the Pd²⁺ or Pt²⁺ metal ion through the ring nitrogen (N₃), as monodentate ligand. Thiocytosine however acts as a bidentate ligand, coordinating to the metal ion through-SH and ring nitrogen (N₃). All complexes are 11 electrolytes, except the thiocytosine complex, which is a 12 electrolyte.     

Coordination Properties of Sterically Stressed Zincporphyrins

Spectrophotometric titration and computer simulation were used to study how the nature of porphyrin and extra ligand affect the formation of extra complexes of zincporphyrins in o-xylene. The compounds under study were zincporphyrins (ZnP) with different substituents and phenyl radicals in meso-positions (zinc-5,15-(p-butyloxyphenyl)-2,8,12,18-tetramethyl-3,7,13,17-tetraethylporphyrin (ZnP¹), zinc-5,15-(p-butyloxyphenyl)-2,8,12,18-tetramethyl-3,7,13,17-tetrabutylporphyrin (ZnP²), zinctetraphenylporphine (ZnP³), and zinc complexes with overlapped porphyrin (ZnP⁴). N-Methylimidazole, imidazole, pyridine, 3,5-dimethylpyrazole, and dimethylformamide were used as extra ligands (L). The strength of Zn–L bonding was found to decrease in extra complexes (L)ZnP in the series of ZnP as follows: ZnP⁴> ZnP¹> ZnP²> ZnP³. It was established that the stability constant (logK _st) for sterically nonstressed complexes (L)ZnP⁴linearly increases with growth in the extra ligand basicity (log ) and is proportional to the shift of the main absorption bands () in the electronic spectra of extra complexes of zinctetraphenylporphine. For spatially distorted (L)ZnP¹, (L)ZnP², and (L)ZnP³, the values of logK _stand log , as well as logK _stand , change symbatically. The geometric structure and energy characteristics of pentacoordinated zincporphyrins were calculated by quantum-chemical methods. Correlations were established between the calculated values of the energy of the interaction of the central metal atom with the extra ligand molecule and the stability of the extra complexes of zincporphyrins.     

Linear free energy relationships between stability of complex and basicity of ligands: V. The stability of complex formed from 1,10-phenanthroline,N-(p-substituted phenyl) glycines with copper(II), nickel(II), cobalt(II) and zinc(II)

The formation constants (log K ) of ternary mixed ligand complex formed from 1,10-phenanthroline (A, phen) and N-(p-substituted phenyl) glycines,R NHCH₂COOH (B, p RPhG, CH₃, H, CL), with copper (II), nickel (II), cobalt (II) and zinc (II) were determined by pH method at 25°C in 30% (V/V) ethanol solution in presence of 0.1 M NaClO₄. It was found that linear free energy relationships exist between the stability of M(II)-phen-p-RPhG ternary complex and the base strengths of the ligands p-RPhG, conforming to the equation log K = α pK_B +C. The factors influencing the stability of the ternary complex have been discussed. The enhancement of stability of ternary complex was explained in terms of formation of π back bonding between metal ion and the ligand phen. In contrast to the binary system, the ternary ni (II) and zn (II) complexes showed higher stability which could be accounted for by the formation of π back bonding and by intramolecular ligand-ligand interaction, possibly aromatic-ring stacking.     

Aqueous Solutions of Cr(III) Sulfate: Modeling of Equilibrium Composition and Physicochemical Properties

Chemical equilibria in aqueous solutions of chromium sulfate are studied and the mathematical model of chromium complexation is developed in a wide range of concentrations (0.001–1.2 mol/l). The results of calculations satisfactorily agree with the experimental data, which confirms the adequacy of the suggested mathematical model of the equilibrium composition of an aqueous chromium sulfate solution. The nature of green modification of Cr₂(SO₄)₃ is established. This modification occurs in its diluted solutions (c < 0.1 mol/l) due to predominant polynuclear chromium hydroxo complexes, whereas in the concentrated solutions (c > 0.1 mol/l), it forms due to prevailing chromium sulfate complexes. The diagram was plotted that shows the regions with different prevailing forms of the complexes in the equilibrium system H₂O–Cr₂(SO₄)₃ at different concentrations and pH and explains the experimental data on the basis of some physicochemical properties (electric conductivity, viscosity, light absorption) of chromium sulfate solutions within a wide range of concentrations.     

Metal chelates of heterocyclic nitrogen-containing ketones,Part XXII. Spectroscopic studies on rhodium(III) complexes of phenyl-2-picolylketone-2-pyridyl hydrazone

Summary Mono, bis and tris complexes of rhodium(III) with phenyl-2-picolylketone-2-pyridyl hydrazone (PPKPyH) have been characterized. In every case, the imino-proton of PPKPyH shows marked acidity associated with the coordination to rhodium(III). Electronic spectra show that all complexes are octahedral. The B-values suggest a strong covalency in the metal-ligand -bond and the Dq-values indicate a medium-strong ligand field. The magnetic susceptibility indicates that PPKPyH forms low-spin complexes with rhodium(III).¹H n.m.r. spectra show that the tris(ligand) complexes arecis isomers. I.r. spectra show that the ligand is neutral or monobasic tridentate or bidentate. Far i.r. studies show that [Rh(PPKPyH)X₃] · 2 H₂O (X = Cl, Br or I) aremer isomers. The effect of pH variation on the rection products is also discussed.     

Equilibrium studies on Schiff base complexes of copper(II) and nickel(II): Ligand substitution and formation of mixed ligand complexes

Summary Vis spectrophotometry has been used to study various ligand substitution equilibria (1) and (2) involving four-coordinate copper(II) and nickel(II) his chelate complexes in methanol, propan-2-ol and toluene. MA₂ + HB MAB+HA, K₁ (1) MAB + HB MB₂ + HA, K₂ (2) The Schiff base ligands, HA and HB, which are monobasic and bidentate, represent salicylaldimine type N,O-ligands ( HSA=NR) (1) or pyrrole-2-aldimine type N,N-ligands ( HPA=NR) (2) with different branching at the - or (3-carbon of the organic group R. For both types of ligand the relative thermodynamic stability of their copper and nickel complexes is governed mainly by the steric demands of R, which determine the degree of tetrahedral distortion. The order of stability as given by = K₁ K₂ is: t-Bu < neo-Pent < i-Pr < i-Bu < Et < n-Pr. The K₁/K₂ ratio is strongly solvent dependent in the sense that the mixed ligand species MAB is stabilised in toluene relative to methanol. Such a solvent effect is not observed for . The MAB complexes could not be isolated. The vis spectrum of the mixed ligand species Ni(SA=NiPr, SA=NEt) was calculated by computer fitting of the experimental data.     

Chelate Compounds of a Selected Non-Transition and Transition Metal Ions with Some Salicylic Acid Derivatives

A series of binary complexes of Zn(II), Hg(II), Pb(II), La(III), Ce(III), Th(IV) and UO₂(II) with 3,5-dinitrosalicylic acid and 5-sulphosalicylic acid have been isolated and characterized. The solution equilibria of these complexes have been studied pH-metrically. The formation constants (log K) at 25±1°C and ionic strength of 0.2M NaClC₄ have been calculated. Stability of the complexes formed in relation to ligand molecular structure and nature of the metal ion has been examined and discussed.