Nickel stability constant

Table I. Acidity and Nickel Stability Constants for Complexing Ligands "...

The stability constant for the nickel chelate of pyrido[2,3-d]-pyrimidine-4(3 r)-one has been measured. ... 

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. 

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... 

Table 8.1. Experimentally determined and calculated stability constants of high-spin nickel(II) amines with five- and six-membered chelates131.

X-Ray Data (Copper), Physical Data (Copper), Formation and Stability Constants (Cobalt, Nickel, and Copper)... 

Formation and Stability Constants (Cobalt, Nickel, and Copper)... 

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