Square planar complexes are also formed and can display cis–trans isomerism.

The idea

Square planar complexes have four ligands in one plane at 90° to each other, and platinum(II) is the classic example. When two of the four ligands differ, as in Pt(NH₃)₂Cl₂, two distinct compounds exist: the cis isomer with the chlorides on adjacent corners, 90° apart, and the trans isomer with them on opposite corners, 180° apart. They are stereoisomers, having the same atoms joined in the same order but arranged differently in space, and they cannot interconvert without breaking bonds. Octahedral complexes do the same: [Cr(NH₃)₄Cl₂]⁺ has cis and trans forms. A mark scheme for drawing them gives one mark for the pair of structures and a separate mark for showing the charge on the complex ion.

What earns the marks

• Two correctly drawn isomers, with ligands on adjacent corners for cis and opposite corners for trans.
• Showing the overall charge on the complex ion in the drawing.
• Naming the isomerism as cis-trans or geometric.
• The bond angles, 90° between cis ligands and 180° between trans ligands.

Common mistakes and how to put them right

• Drawing a square planar complex as a tetrahedron, which cannot show cis-trans isomerism at all.

• Omitting the charge on the complex ion in a drawing, which loses a separate mark.

• Drawing the same isomer twice with the square simply rotated.

• Calling the two forms optical isomers or structural isomers rather than stereoisomers.

• Using E-Z nomenclature, which applies to alkenes, not square planar complexes.

Exam tips

• Draw the square with ligands at the corners, not on the edges, so adjacent and opposite are unambiguous.
• Write the charge outside the square bracket every time you draw a complex ion.
• Rotate your second drawing mentally to check that it really is a different isomer.