A new computer program, squad, has been developed enabling the evaluation of the best set of stability constants from absorbance measurements. The method can be used to determine p>fa's, metal Ion hydrolysis constants and study multicomponent equilibria. A rigorous testing procedure has been used to investigate the limitations of the method, squad has been applied to the nickel ethylenediamine system to yield log 01O1 = 7.36, log 01O2 = 13.74, log 0103 = 18.06.

The acidity constants of citric acid and the stability constants of the citrate complexes of copper(II), iron(II), and iron(III) have been measured at 25 °C in 0.1 M KNO3 background. The measurements were based on mass balances on hydrogen ions, supplemented in the case of Cu(II) and Fe(III) complexes by measurements of free metal concentration with a solid-state copper-ion-sensitive electrode and a redox electrode respectively. The data required the assumption of a binuclear copper complex, but all complexes characterized were of 1:1 stoichiometry.

Dans le but de relier les constantes de stabilitt mttal-complexe B certaine propriCtC des ions rn6talliques, on a dCveloppC un indice numkrique de corrClation Q posskdant les caractCristiques suivantes: ( i ) Q est un parametre double engageant la charge ionique et I'electronCgativitC. I1 peut avoir des valeurs diffkrentes pour un mCtal particulier selon le type de ligand avec lequel les complexes se foment. [ Efforts to relate metal-complex stability constants to some property of the metal ions involved have led to the development of a correlating numerical index Q with the following characteristics: (i) Q is a two parameter quantity involving ionic charge and electronegativity, which may assume different values for a particular metal according to the type of ligand with which complexes are formed; (ii) linear relationships are obtained for plots of log KML 1's. Q for various metals and more than thirty ligands. The effect of the thermodynamic and structural properties of ligands and metal complexes on the slope and intercept in this correlation is discussed. Two new indices to class b behavior are introduced, one for metal ions and one for ligands. The index Q is shown to be related to other published indices of ionic or covalent, class a or class b, behavior of metals and ligands; the alternative ways of assigning Q may be regarded a s reflecting the varying degrees of ionic or covalent character in the complexes of a given metal. IntroductionThe classification of donors and acceptors in inorganic reactions has provided a useful framework for the rationalization of metalcomplex formation in solution. The original division of acceptors (metal ions) by Ahrland, Chatt, and Davies (1) into class a and class b was based on complex stability in solution as measured by equilibrium constants. These ideas were then extended to the ligand donor atoms: class a donors (e.g., N, 0, F) are preferred by class a acceptors, and class b donors (e.g., P, S, C1) by class b acceptors. More recently Ahrland (2-5) has made the generalization that class a reactions in aqueous medium tend to be entropy stabilized, while class b interactions tend to be enthalpy controlled (exothermic). This thermodynamic criterion is useful, but cannot be used in a clear-cut diagnostic test for any individual reaction (see below).

Results obtained with three commercial pH meters, by measuring the e.m.f. of cells consisting of glass electrodes coupled with either calomel or silver chloride reference electrodes, are compared with the hydrogen-ion concentrations in solutions with three different background electrolytes at five or six different levels of salt concentration. The hydrogen-ion functions measured in this way can be empirically converted into concentrations for use in calculating equilibrium constants. When thermodynamic equilibrium constants cannot be calculated from such measurements it is preferable to report concentration quotients. Concordance is demonstrated among values of the second-stage ionisation constant for sulphosalicylic acid, measured in different ionic media and with different reference electrodes, when all values are converted into concentration quotients.RESULTS from measurements with commercial pH meters have been widely incorporated into calculations of equilibrium "constants" of two principal types. The first type are the dissociation constants of weak acids and bases, with which hydrolysis constants of various ionic species such as metal cations in aqueous solution can appropriately be grouped. The second type comprises formation constants of metal complexes, which may be obtained from the study of reactions that take #ace by proton displacement, with methods clearly enunciated by Bjerruml and widely applied since.The ionisation of a weak acid, HA, although perhaps better regarded as a proton-transfer reaction with the solvent, can be considered seciently well for our purposes as a simple dissociation, HA + H + A. (Charges will be omitted, if the meaning is not obscured, in the interest of simpler representation and of greater generalisation). The ionisation constant is defined by the equation Activities are represented by ai (concentrations are given by square brackets) and activity coefficients by yi. A concentration quotient Qa is defined by the equation 337 -, 09. cit., p. 409.

Acidity constants and stability constants of certain metal complexes (nickel(II), zinc(II), and manganese(I1)) with ethylenediamine and glycine were determined a t 25 OC and in 0.1 M ionic strength in mixed aqueous solutions of methanol ( M e O H ) , dioxane (DOX) , acetonitrile (MeCN) , and dimethylformamide ( D M F ) . In general, t h e stability constants of the metal complexes and the first protonation constant of glycine increase with increasing composition of the co-solvent in the orderThe protonation constants of ethylenediamine pass through a minimum at some particular solvent composition, while the second protonation constant of glycine exhibits behavior characteristic of the particular solvent employed.These results are discussed in terms of medium and solvent effects and properties o f the solvent that might affect these, but find more satisfactory explanation in terms o f solvent displacement during the association reactions. The observations can be accounted for in terms of a complete equilibrium constant incorporating the concentration (activity) of each solvent substance. . En gtntrale, les constantes de stabilitt des complexes de mttaux et la constante de la premiere protonation de la glycine sont croissantes quand la composition de leur solvant augmente selon I'ordreLes constantes de protonation de l'tthyltnediamine passe par u n minimum B u n e certaine composition particulitre du solvant, alors que la constante de la seconde protonation de la glycine manifeste un comportement caracttristique du solvant particulier utilisi.Ces rtsultats sont discutts en termes du.milieu, des effets du solvant et des propritt6s du solvant qui pourraient influencer ces derniers; mais ils sont expliquts, de fafon plus satisfaisantes, en termes de dtplacement de solvant durant les rtactions d'association. Les observations peuvent Btre interprtties en termes de constante d'tquilibre complet incluant la concentration (activitt) de chaque substance de solvant.[ Introduction The possibility of utilizing mixed solvents to improve analytical methods involving complex formation have been noted elsewhere (1). Furthermore, studies of equilibrium and kinetics in mixed solvents may be of assistance in interpreting the role of solvent in inorganic reactions. Previous studies of the solvent effect on complex equilibria have been concerned mainly with simple organic acids and metal chelates in selected solvents such as dioxane and alcohols of low molecular weight. Of the few metal ions and ligands studied, the metal complexes increased in stability as the concentration of non-aqueous solvent (co-solvent) increased. This trend also held good for protonation constants of ligands with a carboxylate group, but those with amine groups behaved differently. So far no systematic studies in different solvents with a wide range of solvent compositions have been made.As part of a larger study of free-energy correlations in coordination chemistry we have undertaken the measurement of complex equilibria for three metal ions (NiZt, zn2', an...