Terminology of coordination compounds

A species like [Co(NH3)e] is called a coordination entity . The atom in the middle is called the central atom , the attached groups are called ligands (Latin, ligare, bind), and the atoms by which they are attached, coordinating or ligating atoms. The ligands are said to be coordinated or ligated to the central atom. They are also said to be in the coordination sphere of the central atom. 

Following Werner, the number of coordinating atoms around the central atom is called the coordination number of the central atom. This term is also used by ciystallographers (Chap. 6). However, the latter use it to describe a particular structure, whereas Werner used it more as a valency. A better term for this purpose would be coordinate valency . 

A ligand containing more than one coordinating atom is called a multidentate (literally many-toothed ) ligand, the number of coordinating atoms being indicated by the terms unidentate , bidentate , etc. An example of such a ligand is ethylenediamine, usually abbreviated en  

This can coordinate through its two nitrogen atoms in the same way as two molecules of CH3NH2 or NH3. Thus, just as the addition of ammonia to a solution containing Cu leads to the formation of the deep blue [Cu(NH3)4] ion, so the addition of ethylenediamine leads to the formation of the dark blue-violet [Cu(en)2]  

A ligand is said to function as a bridging ligand when it is attached to more than one coordinating centre. An example is the hydroxide ion in 

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