Chapter 17 - Chapter 17 Complexation Reactions and...

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Unformatted text preview: Chapter 17 Complexation Reactions and Titrations Complex-formation reactions are widely used in analytical chemistry. One of the first uses of these reagents was for titrating cations. In addition, many complexes are colored or absorb ultraviolet radiation; the formation of these complexes is often the basis for spectrophotometric determinations. Some complexes are sparingly soluble and can be used in gravimetric analysis. Complexes are also widely used for extracting cations from one solvent to another and for dissolving insoluble precipitates. The most useful complex forming reagents are organic compounds that contain several electron donor groups that form multiple covalent bonds with metal ions. FORMING COMPLEXES Most metal ions react with electron-pair donors to form coordination compounds or complexes. The donor species, or ligand is an ion or a molecule that forms a covalent bond with a cation or a neutral metal atom by donating a pair of electrons that are then shared by the two. The number of covalent bonds that a cation tends to form with electron donors is its coordination number. Typical values for coordination numbers are two, four, and six. The species formed as a result of coordination can be electrically positive, neutral, or negative. A ligand that has a single donor group, such as ammonia, is called unidentate (single-toothed), whereas one such as glycine, which has two groups available for covalent bonding, is called bidenate . Tridentate , tetradentate , pentadentate , and hexadentate chelating agents are also known. Another important type of complex, a macrocycle , is formed between a metal ion and a cyclic organic compound. The selectivity of a ligand for one metal ion over another relates to the stability of the complexes formed. The higher the formation constant of a metal-ligand complex, the better the selectivity of the ligand for the metal relative to Producing Soluble Compelxes Complexation reactions involve a metal ion M reacting with a ligand L to form a complex ML. M + L ML Complexation reactions occur in a stepwise fashion, and the reaction above is often followed by additional reactions: ML + L ML 2 ML 2 + L ML 3 ML n-1 + L ML n Unidentate ligands invariably add in a series of steps. With multidentate ligands, the maximum coordination number of the cation may be satisfied with only one or a few added ligands. continued The equilibrium constants for complex formation reactions are generally written as formation constants. M + 2L ML 2 M + 3L ML 3 M + nL ML n The overall formation constants are products of the stepwise formation constants for the individual steps leading to the product. [ ] [ ][ ] 2 2 1 2 = = ML M L K K [ ] [ ][ ] 3 3 1 2 3 = = ML M L K K K [ ] [ ][ ] n n n ML M L K K K = = 1 2 .......
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Chapter 17 - Chapter 17 Complexation Reactions and...

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