ligand
/ LIG-and or LY-gand /
Imagine a metal ion as a central magnet and a crowd of small molecules each offering it a lone pair of electrons like an outstretched hand. A ligand is any one of those molecules or ions that reaches out and attaches itself to the metal.
Formally, a ligand is an ion or molecule that donates a pair of electrons to a metal ion, forming the bond that builds a coordination complex. Common ligands include water, ammonia, chloride, and large multi-armed molecules like EDTA; the metal acts as the electron-pair acceptor.
Ligands matter because they are the binding partners in every complex, and their nature decides how strong and selective that binding is. The key honest point: ligands range from those that attach at a single point to those that wrap around at many points, and that difference — denticity — largely sets how stable the resulting complex will be.
In the deep-blue solution formed when ammonia is added to copper salts, four ammonia molecules act as ligands, each donating an electron pair to the central copper ion.
Ammonia molecules act as ligands binding a central copper ion.
A ligand that binds at one point is monodentate; one that binds at several points is multidentate, and a multidentate ligand forming a ring gives a chelate. EDTA is a six-point (hexadentate) ligand.