Stepwise stability constants

The equilibrium constant for the formation of a metal complex is known as its stability constant. (Some authors, however, present the datum as its reciprocal, the instability constant of the complex, by analogy with the dissociation of a weak acid.) There are two kinds of stability constants stepwise (Ad, Ad, Ad,..., Kn) and overall (/ ). We will assume that there are six aqua ligands to be replaced by some other unidentate ligand Xx, in an aqueous solution of Mm+ ... 

For complexes that contain more than one ligand or central metal ion, there are two ways of writing stability constants. Stepwise formation constants are equilibrium constants for the reactions in which the central metal ion consecutively adds one ligand. An overall formation constant is the equilibrium constant for the reaction in which the central metal ion combines with all of the ligands necessary to form a specific complex. We can illustrate these two types of constants using the formation constants of the chloromercury(II) complexes as an example. For stepwise formation we have... 

The following entry defines the commonly used stability constants (stepwise, overall, conditional, association, dissociation, and pK) and relates the values to a rigorous thermodynamic definition of equilibrium constants. In addition, the article briefly outlines experimental techniques (potentiometric titration, spectroscopic methods involving ultraviolet/visible, infrared, Raman, fluorescence. and nuclear magnetic resonance spectroscopy), together with the numerical methods and computer programs that can be used to derive stability constants from such experimental data. 

A significant achievement of Anatoly K. Babko was in the area of systematic physicochemical research of complex compounds in solution based on their photometric properties. Anatoly K. Babko showed the stability constants of complexes to be highly important, and demonstrated the relevance of the stepwise character of the dissociation of complex compounds. 

The equilibrium constants Kl,K2,...,Kn are referred to as stepwise stability constants. 

The equilibrium constants pi,p2,...,p are called the overall stability constants and are related to the stepwise stability constants by the general expression... 

The stability of a complex is conveniently expressed in terms of the thermodynamic stepwise stability constant K as defined in Eq. (8.1). 

Willems et al. [37] used a polarographic method to study the miconazole complexes of some trace elements. Manganese, iron, cobalt, and zinc element formed miconazole complexes with different stability constants. Polarography was used for detecting stability constants. The evolution of the respective formation constants followed the natural (Irving-Williams) order. The stepwise constant of the complexes formed increased from manganese to cobalt and decreased for zinc. The results are discussed with respect to the possible mechanism of action of miconazole. 

For these reactions, we can write the equilibrium constants (known as stepwise formation or stability constants) as... 

Thus, the overall stability constants for the various steps, [3-v can be expressed in terms of the product of several stepwise stability constants, Kt (e.g., [32 = Kiff2 P4 = K1K2K3K4). 

The equation relating the stepwise and overall stability constants is indicated by equation 1.7 ... 

The stepwise formation of fluoro complexes in aqueous solution has been extensively studied in recent years by standard techniques (192). The introduction of the LaF3 electrode has enabled fluoride activities to be measured directly, even with the "insoluble fluorides such as CaF2 or PbF2 (34, 81). Values obtained depend on the salts added to maintain constant ionic strength, and care must be taken in making comparisons since not all results are extrapolated to zero strength (15). Key references to work in this field can be gleaned from standard compilations of stability constants. 

Stepwise stability constants decrease regularly in the normal manner 178), log > log K2 > log K, as may be exemplified by the values for the Ni -maltolate system, viz. 5.5, 4.3, and 2.7, respectively 179). Stability constants (log Kf) for a selection of 3-hydroxy-4-pyranonate and 3-hydroxy-4-pyridinonate complexes of some first-row transition metal 2+ cations are listed in Table IV 10,128,180—184). The values for the 3d transition metal cations conform to the long-established Irving-Williams order... 

Stepwise Stability Constants for the Formation of 3-Hydroxy-4-pyranonate Complexes... 

Many factors play a role in establishing the value of the stability constants of a particular metal-ligand system. J. Bjerrum [4] considered such factors and their effect on the successive stability constants. The ratio of two stepwise constants is defined as ... 

The stepwise stability constants for zirconium and hafnium thiocyanato-complexes have been determined by solvent-extraction techniques. The values ) i = 12.1 2.2, = 215 11, P4. = 205 + 20 for zirconium, and Pi =... 

The stepwise stability constants for V complexes with glutamic, aspartic, W -ethylenediaminedisuccinic, and W -ethylenediamine-bis-(a-glutaric acid) have been reported.The 1 1 oxovanadium(iv) complex of 5-nitrosalicylic acid has been characterized = 710 nm), and the mixed species formed between vanadium(iv) with protocatechuic acid (H3L ) or gallic acid (H L ) and 4-aminoantipyrine (aant) have been identified spectroscopically as the 1 1 1 and 1 2 2 HL (HL ) aant complexes. The species present in solutions of and tartaric acid below pH 2 have been identified as [VO-... 

Platinum(iv).—Group VII Donors. Simple syntheses of hexafluoro-complexes of some noble metals have been reported, including K2[PtF ]. The method involved the melt reaction between KHFj and K2[PtXg] (X = Cl, Br, or I), which has the advantage of not requiring corrosive reagents such as fluorine or BF3- The stepwise stability constants K have been measured for the bromoaquo-complexes [PtBr , (H20)-7 ] "" (n = 4, 5, or 6) formed in... 

The corresponding stepwise binding constants (also called formation constant Kf, association constant Kay or stability constant Ks) are given by the equations... 

TABLE III. Stepwise Stability Constants for the Interaction of Some Carrier Molecules with Different Ions (Water or Ethanol3 in kg/mole)... 

Figure 7 Stability constants (log Ks) of various podand alkali metal complexes as a function of ionic radius (a) stepwise loss of terminal groups (b) ligands with stiffened chains (c) variation of terminal group (reproduced with...

Fig. 11.50 Stepwise stability constants. K,. K+ and Ky for ettiylenediamne complexes of several first-row transition metals in aqueous solution.

The stepwise constants written for the equilibria above are called formation or stability constants and may be written ... 

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

Stepwise stability constants for complexation between Zn2+ and Cd2+ and the acids X(CH2CH2C02H)2 (X = 0, S, Se or Te), Se(CH2C02H)2, X(CH(Me)C02H)2 (X = S or Se) and H02CCH2SCH2CH2C02H have been measured.882 The structures of benzeneselenic acid complexes of zinc, cadmium and mercury have been investigated by IR spectroscopy the bonding of the areneseleninato ligand depends on the water content of the compound. The hydrated complexes are always of the 0,0-type the anhydrous complexes are mainly 0,0... 

Stability constants for higher oligomers have, until now, been reported only for the aqua chromium(III) system. In addition to Bjerrum s classical study of this system (14), stability data for this very complicated system have been reported by several independent groups (26,27, 28, 200). The Qxy values given in Table XIV show some deviation between the values reported by different authors, and it should further be noted that ( 46 applies to a mixture of at least two tetranuclear species (see Section IV). The stability constants Kn for the stepwise polymerization process, Eq. (32), have been determined as K2 1 x 105 M 1, K3 x6 x 106 M l, and K x 2 x 105 M The change in free... 

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

The consecutive or stepwise formation (stability) constants kn are expressed as... 

Narula and Gupta76 have measured the thermodynamic stepwise stability constants for the formation of complexes of dialkyltin(TV) ions, R2Sn2+ (R = Me or Bu ), with hydroxamic acids, RCONHOH (R = Ph, Me2CH, Me) potentiometrically in 75% dioxane-water media. This has been followed by determination of formation constant of Af-arylhydroxamic acids-dialkyltin(IV) ions.77... 

---