text: Metal complexes, also known as coordination compounds, include virtually all metal compounds.  The study of "coordination chemistry" is the study of "inorganic chemistry" of all alkali and alkaline earth metals, transition metals, lanthanides, actinides, and metalloids. Thus, coordination chemistry is the chemistry of the majority of the periodic table.  Metals and metal ions exist, in the condensed phases at least, only surrounded by ligands.
  The areas of coordination chemistry can be classified according to the nature of the ligands, in broad terms:
  * Classical (or "Werner Complexes"): Ligands in classical coordination chemistry bind to metals, almost exclusively, via their lone pairs of electrons residing on the main-group atoms of the ligand. Typical ligands are HO, NH, Cl, CN, en.  Some of the simplest members of such complexes are described in metal aquo complexes, metal ammine complexes,
  : Examples: [Co(EDTA)], [[Cobalt(III) hexammine chloride|[Co(NH)]]], [[Potassium ferrioxalate|[Fe(CO)]]]
  * Organometallic chemistry:  Ligands are organic (alkenes, alkynes, alkyls) as well as "organic-like" ligands such as phosphines, hydride, and CO.
  : Example: (CH)Fe(CO)CH
  * Bioinorganic chemistry: Ligands are those provided by nature, especially including the side chains of amino acids, and many cofactors such as porphyrins.
  : Example: hemoglobin contains heme, a porphyrin complex of iron
  : Example: chlorophyll contains a porphyrin complex of magnesium
  : Many natural ligands are "classical"  especially including water.
  * Cluster chemistry: Ligands include all of the above as well as other metal ions or atoms as well.
  : Example Ru(CO)
  * In some cases there are combinations of different fields:
  :Example: [[Iron–sulfur protein|[FeS(Scysteinyl)]]], in which a cluster is embedded in a biologically active species.
  Mineralogy, materials science, and solid state chemistry – as they apply to metal ions – are subsets of coordination chemistry in the sense that the metals are surrounded by ligands.  In many cases these ligands are oxides or sulfides, but the metals are coordinated nonetheless, and the principles and guidelines discussed below apply. In hydrates, at least some of the ligands are water molecules. It is true that the focus of mineralogy, materials science, and solid state chemistry differs from the usual focus of coordination or inorganic chemistry. The former are concerned primarily with polymeric structures, properties arising from a collective effects of many highly interconnected metals. In contrast, coordination chemistry focuses on reactivity and properties of complexes containing individual metal atoms or small ensembles of metal atoms.