Nomenclature and Definition of Terms

Coordination compounds or complexes consist of one or more central atoms or central ions, usually metals, with a number of ions or molecules, called ligands, surrounding them and attached to them. The complex can be nonionic, cationic, or anionic, depending upon the charges of the central ions and the ligands. Usually, the central ions and ligands can exist individually as well as combined in complexes. The total possible number of attachments to a central atom or central ion or the total possible 

The superscript o on the magnesium and calcium complexes indicates the complex is uncharged. 

The coordination number of a metal ion may vary from one ion to another. For example, Ni(II) has a coordination number of 4 in its complex with carbonyl (CO), Ni(CO)4 with 1,10-phenanthroline in the complex Ni(phenanthroline 3 the coordination number is 6. Iron(III) has coordination numbers of 6 for water, FefHaOe , and cyanide, Fe(CN)6 , but 4 for chloride, FeCb . 

The chelating agent EDTA can attach at six sites, since each of the acetate groups and the two nitrogen atoms have free electron pairs necessary for coordinate bond formation. For example, the EDTA complex with Ca that forms during the titrimetric determination of water hardness 

Each mole of the chelating agent 1,10-phenanthroline, which is used as an indicator for Fe in the titrimetric finish of the COD test, can satisfy two coordination sites with the electron pairs on the two nitrogen atoms in each molecule. With Fe a red-colored complex is formed with three molecules of 1,10-phenanthroline satisfying the coordination number of 6 for the Fe ion  

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