Editing Coordination complex (section)

==Nomenclature and terminology==
Coordination complexes are so pervasive that their structures and reactions are described in many ways, sometimes confusingly.  The atom within a ligand that is bonded to the central metal atom or ion is called the '''donor atom'''.  In a typical complex, a metal ion is bonded to several donor atoms, which can be the same or different. A [[Ligand#Polydentate and polyhapto ligand motifs and nomenclature|polydentate]] (multiple bonded) ligand is a molecule or ion that bonds to the central atom through several of the ligand's atoms; ligands with 2, 3, 4 or even 6 bonds to the central atom are common. These complexes are called [[chelate complex]]es; the formation of such complexes is called chelation, complexation, and coordination.

The central atom or ion, together with all ligands, comprise the [[coordination sphere]].<ref>{{cite news |url= http://www.chemistry-dictionary.com/definition/coordination+sphere.php |title= Definition of coordination sphere |newspaper= chemistry-dictionary.com }}</ref><ref>{{cite web |url= http://www.chem.purdue.edu/gchelp/cchem/whatis.html |title= What Is A Coordination Compound? |publisher= Purdue University Department of Chemistry }}</ref>  The central atoms or ion and the donor atoms comprise the first coordination sphere.

'''Coordination''' refers to the "coordinate covalent bonds" ([[dipolar bond]]s) between the ligands and the central atom. Originally, a complex implied a reversible association of [[molecule]]s, [[atom]]s, or [[ion]]s through such weak [[chemical bond]]s. As applied to coordination chemistry, this meaning has evolved. Some metal complexes are formed virtually irreversibly and many are bound together by bonds that are quite strong.<ref>{{Cite book|isbn=978-0-471-19957-1 |page=1355|last=Cotton|first=Frank Albert|author2=Geoffrey Wilkinson |author3=Carlos A. Murillo |title=Advanced Inorganic Chemistry|year=1999|publisher=John Wiley & Sons }}</ref><ref>{{Cite book|isbn=978-0-13-841891-5 |page=642|last=Miessler|first=Gary L.|author2=Donald Arthur Tarr|title=Inorganic Chemistry|year=1999|publisher=Prentice Hall }}</ref>

The number of donor atoms attached to the central atom or ion is called the [[coordination number]]. The most common coordination numbers are 2, 4, and especially 6. A hydrated ion is one kind of a complex ion (or simply a complex), a species formed between a central metal ion and one or more surrounding ligands, molecules or ions that contain at least one lone pair of electrons.

If all the ligands are [[Denticity|monodentate]], then the number of donor atoms equals the number of ligands. For example, the cobalt(II) hexahydrate ion or the hexaaquacobalt(II) ion&nbsp;[Co(H<sub>2</sub>O)<sub>6</sub>]<sup>2+</sup> is a hydrated-complex ion that consists of six water molecules attached to a metal ion&nbsp;Co. The oxidation state and the coordination number reflect the number of bonds formed between the metal ion and the ligands in the complex ion. However, the coordination number of&nbsp;Pt([[Ethylenediamine|en]]){{su|b=2|p=2+}} is 4 (rather than 2) since it has two bidentate ligands, which contain four donor atoms in total.

Any donor atom will give a pair of electrons. There are some donor atoms or groups which can offer more than one pair of electrons. Such are called bidentate (offers two pairs of electrons) or polydentate (offers more than two pairs of electrons). In some cases an atom or a group offers a pair of electrons to two similar or different central metal atoms or acceptors—by division of the electron pair—into a [[three-center two-electron bond]]. These are called bridging ligands.