Tetradentate ligands can be classified by the topology of the connections between 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. Upon binding with a central atom, there are several arrangements possible (known as geometric isomers).
Linear ligands A linear tetradentate ligand has the four donor atoms in a line and each subsequent donor is connected by one of three bridges. Such a ligand bound to a metal in
tetrahedral coordination can only connect in one way, though if the ligand is unsymmetrical then there are two
chiral arrangements. A linear tetradentate ligand can also bind to a metal in
square planar coordination in one way, where anticlockwise or clockwise arrangements are equivalent.
Linear ligands in octahedral coordination 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 arrangement leads to three stereochemical outcomes, and the four donor groups can be co-equatorial. This geometry is called
trans because the remaining unoccupied positions on the octahedron are mutually trans (opposite). 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. Typically these donors are mutually trans, resulting in a chiral complex of
C2-symmetric complexes. This arrangement is illustrated by complexes of the
Trost ligand. The ligand can bend so that one donor atom is at the pole and the remaining three are on the equator of the central atom. This is called
cis-β (beta). The remaining octahedral positions are
cis (adjacent) to each other. The triangles of coordinating atoms and the central atom have two coplanar atoms, and one perpendicular atom. 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 it goes down and anticlockwise is called delta, Δ. Atoms with five coordinate positions are usually
trigonal bipyramidal or
square pyramid geometry. A symmetric tripodal tetradentate ligand can form two isomers on a square pyramid, depending on whether the bridging donor is on the apex or the base of the pyramid. The extra vacant position on the square pyramid is on the base. Square pyramidal coordination tends to occur where a six-member ring is formed with the bridgehead, bridge, feet donor atom and central atom. The longer leg (with three bridging atoms) connects to the apex of the pyramid, and symmetry is lost. In four coordination a tripodal ligand would fill all the positions available, the geometry is
trigonal pyramid. The shape is distorted from the
tetrahedron due to the non-symmetry of the tripod. ==Classification==