Tetradentate

Tetradentate ligands are ligands that bind with four donor atoms to a central atom to form a coordination complex. This number of donor atoms that bind is called denticity and is a way to classify ligands. Tetradentate ligands are common in nature in the form of chlorophyll which has a core ligand called chlorin, and heme with a core ligand called porphyrin. They add much of the colour seen in plants and humans. Phthalocyanine is an artificial macrocyclic tetradentate ligand that is used to make blue and green pigments.

Tetradentate ligands can be classified by the topology of the connections between the donor atoms. Common forms are linear (also called sequential), ring or tripodal. A tetrapodal ligand that is also tetradentate has four legs with donor atoms, and a bridgehead that is not a donor. On binding with a central atom there are several arrangements possible called geometric isomers.

A linear tetradentate ligand has the four donor atoms in a line, each subsequent donor is connected by one of three bridges.

A linear tetradetate ligand bound to a metal in tetrahedral coordination can only connect in one way. If the ligand is unsymmetrical then there are two chiral arrangements.

A linear tetradetate ligand bound to a metal in square planar coordination in one way. Anticlockwise or clockwise arrangements are equivalent.

A linear tetradentate ligand has its donor atoms arranged along or in a chain, so that each adjacent donor atom has to be adjacent on the central atom. This leads to three arrangements on a central atom in octahedral coordination. The chain can be in a circle surrounding the equator of the central atom. This is called trans because the remaining unoccupied positions on the octahedron are trans (opposite) to each other. The triangles formed by the coordinating atoms and the central atom are all coplanar. However the two remaining octahedral positions opposite each other may not be equivalent.

The ligand can have one bend so that one is at the pole and three on the equator of the central atom. This is called beta cis-β (beta). The remaining octahedrap positionas are cis (adjacent) to each other. The triangles of coordinating atoms and the central atom have two coplanar, and one perpendicular. This arrangement is chiral, so there are two possible mirror images. The arrangement where the chain goes down and clockwise is termed lambda Λ, and where is goes down and anticlockwise is called delta Δ. If the chain is not symmetrical, then different isomers can be produced by which end of the ligand has the bend. If three donor atoms are the same at one end of the chain, the mer- and fac- prefixes used for tridentate ligands can be used giving β-mer- if the three are arranged on a meridian or β-fac if the three are arranged on a face of the octahedron.

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