Hexadentate ligand

A hexadentate ligand (or sexidentate ligand) is a ligand that combines with a central metal atom with six bonds. Such ligands are desirable because they have high affinities for metal ions as expressed in their formation constants.

Examples

• Aminopolycarboxylates comprise a major class of hexadentate ligands. A commercially important member is EDTA, a tetraanion.

• Siderophores are naturally occurring ligands secreted by organisms to bind iron, required by the organism's metabolism. Those of the catecholate and hydroxamate types are often hexadentate.

• Crown ethers are often hexadentate, presenting six ether donors. For example 18-crown-6 tightly binds K⁺ and NH⁴⁺ ions. Related thiacrowns and aza-crown ethers are also known. Several antibiotics are hexadentate ligands with comparable polyether bonding motifs. Several polyether antibiotics function as hexadentate ligands.

• Clathrochelates are hexadentate ligands constructed around a metal ion.

Nomenclature

In coordination chemistry, the denticity of hexadentate ligands is often denoted with the prefix κ⁶.

Topology

The arrangement of the donor atoms in hexadentate ligands, its topology, strongly affects the stability of its complexes. Some topologies are simple, such as the linear or ring shapes. The ligand can also be branched, either at a donor atom, or at a non-donor atom. Example shapes are the tripod, and amplector, with a bifurcation at each end. Rigid molecules can be used to force unusual coordination such as trigonal prism. F. Lions identified 36 different hexadentate topologies.

References