Metal-ligand complexation

The equilibrium constant for a reaction in which a metal and a ligand bind to form a metal—ligand complex K ). 

The product of this reaction is called a metal-ligand complex. In writing the equation for this reaction, we have shown ammonia as NH3 to emphasize the pair of electrons it donates to Cd +. In subsequent reactions we will omit this notation. 

The formation of a metal-ligand complex is described by a formation constant, K. The complexation reaction between Cd + and NH3, for example, has the following equilibrium constant... 

The formation constant for a metal—ligand complex in which only one ligand is added to the metal ion or to a metal—ligand complex Ki). 

Stepwise and cumulative formation constants for selected metal-ligand complexes are given in Appendix 3C. 

Ladder diagram for metal-ligand complexes of ethylenediaminetetraacetic add (EOTA) with Ca + and Mg +. 

The most important types of reactions are precipitation reactions, acid-base reactions, metal-ligand complexation reactions, and redox reactions. In a precipitation reaction two or more soluble species combine to produce an insoluble product called a precipitate. The equilibrium properties of a precipitation reaction are described by a solubility product. 

Liquid-liquid extractions using ammonium pyrrolidine dithiocarbamate (APDC) as a metal chelating agent are commonly encountered in the analysis of metal ions in aqueous samples. The sample and APDC are mixed together, and the resulting metal-ligand complexes are extracted into methyl isobutyl ketone before analysis. 

The relevant equilibria for extracting a neutral metal-ligand complex from an aqueous solution into an organic phase are shown in the following diagram. 

Four possible mechanisms for solid-state extraction (a) adsorption onto a solid substrate (b) absorption into a thin polymer or chemical film coated on a solid substrate (c) metal-ligand complexation in which the ligand is covalently bound to the solid substrate and (d) antibody-antigen binding in which the receptor is covalently bound to the solid substrate. 

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

EDTA Must Compete with Other Ligands To maintain a constant pH, we must add a buffering agent. If one of the buffer s components forms a metal-ligand complex with Cd +, then EDTA must compete with the ligand for Cd +. For example, an NH4+/NH3 buffer includes the ligand NH3, which forms several stable Cd +-NH3 complexes. EDTA forms a stronger complex with Cd + and will displace NH3. The presence of NH3, however, decreases the stability of the Cd +-EDTA complex. 

This is simplified for titrations involving EDTA where the stoichiometry is always 1 1 regardless of how many electron pairs are involved in the formation of the metal-ligand complex. 

Spectrophotometric titrations are particularly useful for the analysis of mixtures if a suitable difference in absorbance exists between the analytes and products, or titrant. Eor example, the analysis of a two-component mixture can be accomplished if there is a difference between the absorbance of the two metal-ligand complexes (Eigure 9.33). 

A more important source of UV/Vis absorption for inorganic metal-ligand complexes is charge transfer, in which absorbing a photon produces an excited state species that can be described in terms of the transfer of an electron from the metal, M, to the ligand, L. 

Molecular absorption, particularly in the UV/Vis range, has been used for a variety of different characterization studies, including determining the stoichiometry of metal-ligand complexes and determining equilibrium constants. Both of these examples are examined in this section. 

If there is no wavelength where only the metal-ligand complex absorbs, then the measured absorbances must be corrected for the absorbance that would be exhibited if the metal and ligand did not react to form ML, . 

In essence, the corrected absorbance gives the change in absorbance due to the formation of the metal-ligand complex. An example of the application of the method of continuous variations is shown in Example 10.7. 

Mole-ratio plots used to determine the stoichiometry of a metal-ligand complexation reaction. 

Both the method of continuous variations and the mole-ratio method rely on an extrapolation of absorbance data collected under conditions in which a linear relationship exists between absorbance and the relative amounts of metal and ligand. When a metal-ligand complex is very weak, a plot of absorbance versus Ay or n-J m may be curved, making it impossible to determine the stoichiometry by extrapolation. In this case the slope ratio may be used. 

In the slope-ratio method two sets of solutions are prepared. The first set consists of a constant amount of metal and a variable amount of ligand, chosen such that the total concentration of metal, Cm, is much greater than the total concentration of ligand. Cl- Under these conditions we may assume that essentially all the ligand is complexed. The concentration of a metal-ligand complex of the general... 

Structure of (a) alizarin garnet R, and (b) its metal-ligand complex with AP+. 

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