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the number of atoms, ions, or molecules that a central atom or ion holds as its nearest neighbours in a complex or coordination compound or in a crystal. Thus the metal atom has coordination number 8 in the coordination complexes [Mo(CN)8]4- and [Sr(H2O)8]2+; 7 in the complex [ZrF7]3-; 4 in the complexes [Zn(CN)4]2-, [Cu(CN)4]3-, and [Ni(CN)4]4-; 2 in the complexes [Ag(NH3)2]+, [AuCl2]-, and [HgCl2]. Coordination numbers from 2 to 9 have been observed in complexes; higher coordination numbers occur, although rarely. A given ion or atom does not necessarily have one characteristic coordination number; e.g., Al3+ has coordination number 4 in [AlCl4]- but 6 in [AlF6]3-.