Planar Isomers

Metal complexes that differ only in which ligands are adjacent to one another (cis) or directly across from one another (trans) in the coordination sphere of the metal are called geometrical isomers. They are most important for square planar and octahedral complexes.

Because all vertices of a square are equivalent, it does not matter which vertex is occupied by the ligand B in a square planar MA₃B complex; hence only a single geometrical isomer is possible in this case (and in the analogous MAB₃ case). All four structures shown here are chemically identical because they can be superimposed simply by rotating the complex in space:

For an MA₂B₂ complex, there are two possible isomers: either the A ligands can be adjacent to one another (cis), in which case the B ligands must also be cis, or the A ligands can be across from one another (trans), in which case the B ligands must also be trans. Even though it is possible to draw the cis isomer in four different ways and the trans isomer in two different ways, all members of each set are chemically equivalent:

Because there is no way to convert the cis structure to the trans by rotating or flipping the molecule in space, they are fundamentally different arrangements of atoms in space. Probably the best-known examples of cis and trans isomers of an MA₂B₂ square planar complex are cis-Pt(NH₃)₂Cl₂, also known as cisplatin, and trans-Pt(NH₃)₂Cl₂, which is actually toxic rather than therapeutic.

Square planar complexes that contain symmetrical bidentate ligands, such as [Pt(en)₂]²⁺, have only one possible structure, in which curved lines linking the two N atoms indicate the ethylenediamine ligands: