Tetradentate ligand

This number of donor atoms that bind is called denticity and is a method of classifying ligands.

Phthalocyanine is an artificial macrocyclic tetradentate ligand that is used to make blue and green pigments.

A linear tetradentate ligand can also bind to a metal in square planar coordination in one way, where anticlockwise or clockwise arrangements are equivalent.

[1][2][3] When the two internal donor atoms are pyramidal (such as the secondary amines in trien or EDDA), two diastereomers for the trans arrangement are determined by the relative stereochemistry of these centers.

The arrangement where the chain goes down and clockwise is termed lambda, Λ, and where it goes down and anticlockwise is called delta, Δ.

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.

The arrangement where the chain goes down and clockwise and down is termed lambda, Λ, and where is goes down and anticlockwise and down is called delta (Δ).

Tripodal tetradentate ligands can have three identical chains attached to an atom (such as nitrogen, phosphorus, or arsenic) in tertiary arrangement.

Molecules containing phosphorus, or arsenic donor atoms remain stiff at the P or As and can hold their shape, unlike nitrogen compounds which rapidly racemize.

The longer leg (with three bridging atoms) connects to the apex of the pyramid, and symmetry is lost.

[6] For the trigonal bipyramid, the tripod shaped ligand has its most symmetrical position with the bridging donor at one of the apexes, and the feet of the tripod are arranged around the base, leaving a vacant position at the opposite apex, resulting in C3v symmetry.

One further characteristic is the size of the rings formed by the central metal with two donor atoms and the intervening chain of the ligand.

A tetradentate monoanionic (TMDA) ligand has one donor atom with a negative charge.