Denticity

Atom with
monodentate ligands

Incoordination chemistry,denticity(fromLatindentis'tooth') refers to the number ofdonorgroups in a givenligandthat bind to the central metal atom in acoordination complex.In many cases, only one atom in the ligand binds to the metal, so the denticity equals one, and the ligand is said to beunidentateormonodentate.Ligands with more than one bonded atom are calledmultidentateorpolydentate.The denticity of a ligand is described with the Greek letterκ('kappa').For example, κ⁶-EDTAdescribes an EDTA ligand that coordinates through 6 non-contiguous atoms.

Denticity is different fromhapticitybecause hapticity refers exclusively to ligands where the coordinating atoms are contiguous. In these cases theη('eta') notation is used.Bridging ligandsuse theμ('mu') notation.

Classes

Polydentate ligands arechelating agentsand classified by their denticity. Some atoms cannot form the maximum possible number of bonds a ligand could make. In that case one or morebinding sitesof the ligand are unused. Such sites can be used to form a bond with anotherchemical species.

• Bidentate (also called didentate) ligands bind with two atoms, an example beingethylenediamine.

Structure of the pharmaceuticalOxaliplatin,which features two different bidentate ligands.

• Tridentate ligandsbind with three atoms, an example beingterpyridine.Tridentate ligands usually bind via two kinds of connectivity, called "mer" and "fac." "fac" stands for facial, the donor atoms are arranged on a triangle around one face of the octahedron. "mer" stands for meridian, where the donor atoms are stretched out around one half of the octahedron. Cyclic tridentate ligands such asTACNand9-ane-S3bind in a facial manner.
• Quadridentateortetradentate ligandsbind with four donor atoms, an example beingtriethylenetetramine(abbreviated trien). For different central metal geometries there can be different numbers of isomers depending on the ligand's topology and the geometry of the metal center. For octahedral metals, the linear tetradentate trien can bind via three geometries. Tripodal tetradentate ligands, e.g.tris(2-aminoethyl)amine,are more constrained, and on octahedra leave two cis sites (adjacent to each other). Many naturally occurringmacrocyclicligands are tetradentative, an example being theporphyrininheme.On an octahedral metal these leave two vacant sites opposite each other.
• Quin(qui)dentateorpentadentate ligandsbind with five atoms, an example being ethylenediaminetriacetic acid.
• Sexidentateorhexadentate ligandsbind with six atoms, an example beingEDTA(although it can bind in a tetradentate manner).
• Ligands of denticity greater than 6 are well known. The ligands1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetate(DOTA) anddiethylene triamine pentaacetate(DTPA) are octadentate. They are particularly useful for binding lanthanide ions, which typically have coordination numbers greater than 6.

Relationship between "linear" bi-, tri- and tetradentate ligands (red) bound to an octahedral metal center. The structures marked with * are chiral owing to the backbone of the tetradentate ligand.

Stability constants

In general, the stability of a metal complex correlates with the denticity of the ligands, which can be attributed to thechelate effect.Polydentate ligands such as hexa- or octadentate ligands tend to bind metal ions more strongly than ligands of lower denticity, primarily due to entropic factors.Stability constantsare a quantitative measure to assess the thermodynamic stability of coordination complexes.

See also

• Chelate

External links

• EDTA chelation lecture notes.2.4MB PDF - Slide 3 on denticity