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Coordination Number

The number of ligands which are directly bonded to he central metal atom is known as the coordination number of that complexes. The coordination number of silver ion in the complex [ Ag(NH3)2 ]+ is two and that of the copper ion in [ Cu(H2O)4 ]2+ is four. Some common coordination numbers exhibited by metal atoms are 2, 3, 4 and 6. The coordination numbers 4 and 6 are commonly exhibited by light transition elements. The heavy transition elements generally prefer to attain coordination numbers greater than six, and may exhibit coordination numbers up to 7, 8 or 9. A metal atom may exhibit more than one coordination number in different complexes. For example, in complexes FeCI4- and FeF63- , iron atom shows coordination number for and six, respectively. Normally speaking, a metal atom in a particular oxidation state always exhibits a preferential coordination number. For example, Cr(III) always show coordination number six in their complexes. It has been commonly observed that the coordination number of a metal ion is related to its oxidation state. In practice, the coordination number is twice the oxidation state.

Metal ion__Oxidation state__Coordination number
Ag  __________  I______________ 2
Cu__________   II______________ 4
Cr__________   III______________ 6

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