Conditional metal-ligand formation constants

Conditional Metal—Ligand Formation Constants Recognizing EDTA s acid-base properties is important. The formation constant for CdY in equation 9.11 assumes that EDTA is present as Y . If we restrict the pH to levels greater than 12, then equation 9.11 provides an adequate description of the formation of CdY . for pH levels less than 12, however, K overestimates the stability of the CdY complex. 

Aqueous solution stability constants for the aryl substituted complexes have been determined. The ML3 complexes are highly stable at 25 °C the overall stability constant (/Ss) for the parent complex (17) R = H is 10 . At ligand to metal ratios > 1, the hgands prevent gallium hydrolysis even at millimolar concentrations and under slightly basic conditions the effective formation constant for ML3... 

The equilibrium formation constant for a metal-ligand complex for a specific set of solution conditions, such as pH. 

Under these conditions, the formation rate constant, k, can be estimated from the product of the outer sphere stability constant, Kos, and the water loss rate constant, h2o, (equation (28) Table 2). The outer sphere stability constant can be estimated from the free energy of electrostatic interaction between M(H20)q+ and L and the ionic strength of the medium [5,164,172,173]. Consequently, Kos does not depend on the chemical nature of the ligand. A similar mechanism will also apply to a coordination complex with polydentate ligands, if the rate-limiting step is the formation of the first metal-ligand bond [5]. Values for the dissociation rate constants, k, are usually estimated from the thermodynamic equilibrium constant, using calculated values of kf ... 

Vanadyl compounds hydrolyze much more easily than any other metal chelates of the first transition series, and in some conditions the vanadyl 1 1 chelates prefer combination with hydroxyl to a second ligand molecule.385 Unless otherwise stated formation constants fixyz presented in this review correspond to the reaction shown in equation (15). 

In Section 12-3 we had no auxiliary complexing ligand and we implicitly assumed that aM + = 1. In fact, metal ions react with water to form M(OH) species. Combinations of pH and metal ion in Section 12-3 were selected so that hydrolysis to M(OH) is negligible. We can find such conditions for most M2+ ions, but not for M3+ or M4+. Even in acidic solution, Fe3+ hydrolyzes to Fe(OH)2+ and Fe(OH)(,17 (Appendix I gives formation constants for hydroxide complexes.) The graph shows that aFe., is close to 1 between pH 1 and 2 (log c[Pg.240]

The greater the effective formation constant, the sharper is the EDTA titration curve. Addition of auxiliary complexing agents, which compete with EDTA for the metal ion and thereby limit the sharpness of the titration curve, is often necessary to keep the metal in solution. Calculations for a solution containing EDTA and an auxiliary complexing agent utilize the conditional formation constant K" = aM aY4- Kt, where aM is the fraction of free metal ion not complexed by the auxiliary ligand. 

Stability constant determinations were based on the methods of Shuman and Woodward (43,44) using ASV complexometric titration. The conditional formation constant for the formulation of a metal-ligand complex with an assumed stoichiometry of 1 1 can be determined from Equation 16... 

VO " bonds most effectively to electronegative atoms, e.g. F, Cl, O and N, and bonds to S and P are also known. Fluoro and oxygen complexes are specially stable. Complexes with oxygen donors follow the expanded Irving-Williams series VO >Cu >Ni >Co > > Mn. The presence of one or two nitrogen donors places VO " lower in the series. Vanadyl compounds hydrolyze much more easily than any other metal chelates of the first transition series, and in some conditions the vanadyl 1 1 chelates prefer combination with hydroxyl to a second ligand molecule. Unless otherwise stated formation constants presented in this review correspond to the reaction shown in equation (15). 

When we considered metal complex formation equilibria, it was found very convenient to introduce the parameter, the conditional formation constant P. The use of p simplified the proper handling of side reactions, including proton transfer reactions of the ligand, complexation of the metal with other ligands present in the solution, and even those cases where the primary metal complex was itself involved. With redox equilibria there is also such a parameter, called the formal potential, E", enabling us to write the Nemst equation as... 

Binding of Cu + ions by catechol-terminated SAMs was studied by thin-layer UV-vis spectrophotometry combined with a long-optical-pass cell [43]. Since the pK values are 9.23 and 13.0 for the first and second catechol deprotonation, respectively, experiments were carried out at pH = 7.3 to ensure that the ligand is in its neutral form. Under these conditions, assuming formation of a 1 1 ligand-metal ion complex both in solution and on the surface, formation constants... 

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