Nickel complexes stability constants

Amine-extraction equilibria can also be modeled by chemical-reaction equilibrium constants. Figure 8.3-3 indicates that cations such as iron(IIl), zinc, cobelt(ll) and coppeifU) exhibit high distribution coefficients with chloride solutions, wherese nickel. iron(II), and manganese are not extracted to any great extent. The besis for the differences in distribution coefficients lies mainly in the tendency for the former group of cations to fonn chloride complexes. Stability constants for these complexes are available in the literature,11 and they can be used to develop quantitative phase-equilibrium models. 

Due to the anionic nature of rhamnolipids, they are able to remove metals from soil and ions such as cadmium, copper, lanthanum, lead and zinc due to their complexation ability [57-59], More information is required to establish the nature of the biosurfactant-metal complexes. Stability constants were established by an ion exchange resin technique [60], Cations of lowest to highest affinity for rhamnolipid were K+ < Mg + < Mn + < Ni " " < Co " < Ca2+ < Hg2+ < Fe + < Zn2+ < Cd2+ < Pb2+ < Cu2+ < M +. These affinities were approximately the same or higher than those with the organic acids, acetic, citric, fulvic and oxalic acids. This indicated the potential of the rhamnolipid for metal remediation. Molar ratios of the rhamnolipid to metal for selected metals were 2.31 for copper, 2.37 for lead, 1.91 for cadmium, 1.58 for zinc and 0.93 for nickel. Common soil cations, magnesium and potassium, had low molar ratios, 0.84 and 0.57, respectively. 

The extent of hydrolysis of (MY)(n 4)+ depends upon the characteristics of the metal ion, and is largely controlled by the solubility product of the metallic hydroxide and, of course, the stability constant of the complex. Thus iron(III) is precipitated as hydroxide (Ksal = 1 x 10 36) in basic solution, but nickel(II), for which the relevant solubility product is 6.5 x 10 l8, remains complexed. Clearly the use of excess EDTA will tend to reduce the effect of hydrolysis in basic solutions. It follows that for each metal ion there exists an optimum pH which will give rise to a maximum value for the apparent stability constant. 

The Chelate Effect and Polydentate Ligands 147 Table 8-1. Stability constants for some nickel(ii) complexes of ammonia and 1,2-diaminoethane. 

Table 8-4. Stability constant data for copper(ii) and nickel(ii) ammine complexes.

Mapsi et al. [16] reported the use of a potentiometric method for the determination of the stability constants of miconazole complexes with iron(II), iron(III), cobalt(II), nickel(II), copper(II), and zinc(II) ions. The interaction of miconazole with the ions was determined potentiometrically in methanol-water (90 10) at an ionic force of 0.16 and at 20 °C. The coordination number of iron, cobalt, and nickel was 6 copper and zinc show a coordination number of 4. The values of the respected log jSn of these complexes were calculated by an improved Scatchard (1949) method and they are in agreement with the Irving-Williams (1953) series of Fe2+ < Co2+ < Ni2 < Cu2+ < Zn2+. 

In a similar investigation of the tautomeric tridentate ligand 2 -hydroxyphenylazo-2-naphthol (5.65 in Scheme 5.17), the first and second acidic dissociation constants (pKa) related to the two hydroxy groups in the parent structure (X = H) were found to be 11.0 and 13.75 respectively. On introduction of an electron-withdrawing substituent (X) the first dissociation constant decreased from 11.0 to 10.55 (X = Cl) or 7.67 (X = N02). The stability constants (log K1 1) of the derived 1 1 complexes were dependent on the metal ion introduced [46], being particularly high for nickel(n) at 19.6 and copper(II) at 23.3. 

We can now make sensible guesses as to the order of rate constant for water replacement from coordination complexes of the metals tabulated. (With the formation of fused rings these relationships may no longer apply. Consider, for example, the slow reactions of metal ions with porphyrine derivatives (20) or with tetrasulfonated phthalocyanine, where the rate determining step in the incorporation of metal ion is the dissociation of the pyrrole N-H bond (164).) The reason for many earlier (mostly qualitative) observations on the behavior of complex ions can now be understood. The relative reaction rates of cations with the anion of thenoyltrifluoroacetone (113) and metal-aqua water exchange data from NMR studies (69) are much as expected. The rapid exchange of CN " with Hg(CN)4 2 or Zn(CN)4-2 or the very slow Hg(CN)+, Hg+2 isotopic exchange can be understood, when the dissociative rate constants are estimated. Reactions of the type M+a + L b = ML+(a "b) can be justifiably assumed rapid in the proposed mechanisms for the redox reactions of iron(III) with iodide (47) or thiosulfate (93) ions or when copper(II) reacts with cyanide ions (9). Finally relations between kinetic and thermodynamic parameters are shown by a variety of complex ions since the dissociation rate constant dominates the thermodynamic stability constant of the complex (127). A recently observed linear relation between the rate constant for dissociation of nickel complexes with a variety of pyridine bases and the acidity constant of the base arises from the constancy of the formation rate constant for these complexes (87). 

Dale Margerum Ralph Wilkins has mentioned the interesting effect of terpyridine on the subsequent substitution reaction of the nickel complex. I would like to discuss this point—namely the effect of coordination of other ligands on the rate of substitution of the remaining coordinated water. However, before proceeding we should first focus attention on the main point of this paper-which is that a tremendous amount of kinetic data for the rate of formation of all kinds of metal complexes can be correlated with the rate of water substitution of the simple aquo metal ion. This also means that dissociation rate constants of metal complexes can be predicted from the stability constants of the complexes and the rate constant of water exchange. The data from the paper are so convincing that we can proceed to other points of discussion. 

Eichhom and his co-workers have thoroughly studied the kinetics of the formation and hydrolysis of polydentate Schiff bases in the presence of various cations (9, 10, 25). The reactions are complicated by a factor not found in the absence of metal ions, i.e, the formation of metal chelate complexes stabilizes the Schiff bases thermodynamically but this factor is determined by, and varies with, the central metal ion involved. In the case of bis(2-thiophenyl)-ethylenediamine, both copper (II) and nickel(II) catalyze the hydrolytic decomposition via complex formation. The nickel (I I) is the more effective catalyst from the viewpoint of the actual rate constants. However, it requires an activation energy cf 12.5 kcal., while the corresponding reaction in the copper(II) case requires only 11.3 kcal. The values for the entropies of activation were found to be —30.0 e.u. for the nickel(II) system and — 34.7 e.u. for the copper(II) system. Studies of the rate of formation of the Schiff bases and their metal complexes (25) showed that prior coordination of one of the reactants slowed down the rate of formation of the Schiff base when the other reactant was added. Although copper (more than nickel) favored the production of the Schiff bases from the viewpoint of the thermodynamics of the overall reaction, the formation reactions were slower with copper than with nickel. The rate of hydrolysis of Schiff bases with or/Zw-aminophenols is so fast that the corresponding metal complexes cannot be isolated from solutions containing water (4). 

Table 39 Stepwise Stability Constants of some Six-coordinate Nickel(II) Amino Complexes...

The strong chelating ability of phen and bipy, due to the extended n system, makes the tris chelates [NiL3]2 > easy to obtain from the direct combination of a nickel salt with the appropriate ligand in aqueous solution.839,840 The stability constants of the complexes in aqueous solution (20 °C, 0.1 M NaNOs)841 are reported in Table 39. 

Complex formation of nickel(II) with a variety of hydroxy acids has been investigated. It has been reported that in aqueous solution in the pH range 6-7.2 at 25 °C, a 1 1 neutral complex is formed between nickel(II) and the salicylato monoanion, HOCsH Oj (Keq —1.4 x 10 6), or its 5-substituted analogue, with the further release of a proton.1789,1790 The effects of substituents on the stability constants of nickel complexes with various 4- and 5-substituted salicylic acids were also studied.1791,1792... 

A series of papers describe solution studies on nickel(II) complexes of aminoalkyltetrazoles (62, 63, 122). Stepwise stability constants for... 

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