coordination complex
/ koh-or-dih-NAY-shun KOM-pleks /
Picture a single bead at the centre of a small cluster, with several little arms reaching in to hold it from around. A coordination complex is that cluster: a central metal ion surrounded and bound by a set of attached partners.
Formally, it is a structure in which a central metal ion is joined to one or more ligands by bonds where the ligand supplies both shared electrons. The whole assembly behaves as one chemical unit, often with its own distinctive colour, solubility, and stability that differ sharply from the bare metal.
Coordination complexes matter because their formation is the entire basis of complexometric titration, and they underlie colours in dyes, the action of catalysts, and the way the body carries iron in blood. The practical point for analysis: how strongly the complex forms (its formation constant) decides whether it can be used to measure a metal cleanly.
When colourless EDTA is added to a solution of metal ions, the resulting metal-EDTA coordination complex stays fully dissolved and behaves as a single, very stable species in solution.
Metal plus ligands joined into one dissolved unit is a coordination complex.
'Complex' and 'coordination complex' are used interchangeably. A chelate is the special case where one ligand grips at multiple points; coordination complexes also include ones held by several separate single-point ligands.