20.6: Stereoisomerism

Isomerism in Complexes

Isomers are different chemical species that have the same chemical formula.

Transition metal complexes often exist as geometric isomers, in which the same atoms are connected through the same types of bonds but with differences in their orientation in space. Coordination complexes with two different ligands in the cis and trans positions from a ligand of interest form isomers. For example, the octahedral [Co(NH₃)₄Cl₂]⁺ ion has two isomers (Figure 1) In the cis configuration, the two chloride ligands are adjacent to each other. The other isomer, the trans configuration, has the two chloride ligands directly across from one another.

Figure 1. The cis and trans isomers of [Co(H₂O)₄Cl₂]⁺ contain the same ligands attached to the same metal ion, but the spatial arrangement causes these two compounds to have very different properties.

Different geometric isomers of a substance are different chemical compounds. They exhibit different properties, even though they have the same formula. For example, the two isomers of [Co(NH₃)₄Cl₂]NO₃ differ in color; the cis form is violet, and the trans form is green. Furthermore, these isomers have different dipole moments, solubilities, and reactivities. As an example of how the arrangement in space can influence the molecular properties, consider the polarity of the two [Co(NH₃)₄Cl₂]NO₃ isomers. Remember that the polarity of a molecule or ion is determined by the bond dipoles (which are due to the difference in electronegativity of the bonding atoms) and their arrangement in space. In one isomer, cis chloride ligands cause more electron density on one side of the molecule than on the other, making it polar. For the trans isomer, each ligand is directly across from an identical ligand, so the bond dipoles cancel out, and the molecule is nonpolar.

Another important type of isomers are optical isomers, or enantiomers, in which two objects are exact mirror images of each other but cannot be lined up so that all parts match. This means that optical isomers are non-superposable mirror images. A classic example of this is a pair of hands, in which the right and left hand are mirror images of one another but cannot be superposed. Optical isomers are very important in organic and biochemistry because living systems often incorporate one specific optical isomer and not the other. Unlike geometric isomers, pairs of optical isomers have nearly identical properties (boiling point, polarity, solubility, etc.). Optical isomers differ only in the way they affect polarized light and how they react with other optical isomers. For coordination complexes, many coordination compounds such as [M(en)₃]ⁿ⁺ [in which Mⁿ⁺ is a central metal ion such as iron(III) or cobalt(II)] form enantiomers, as shown in Figure 2. These two isomers will react differently with other optical isomers. For example, DNA helices are optical isomers, and the form that occurs in nature (right-handed DNA) will bind to only one isomer of [M(en)₃]ⁿ⁺ and not the other.

Figure 2. The complex [M(en)₃]ⁿ⁺ (Mⁿ⁺ = a metal ion, en = ethylenediamine) has a non-superposable mirror image.

The [Co(en)₂Cl₂]⁺ ion exhibits geometric isomerism (cis/trans), and its cis isomer exists as a pair of optical isomers (Figure 3).

Figure 3. Three isomeric forms of [Co(en)₂Cl₂]⁺ exist. The trans isomer, formed when the chlorines are positioned at a 180° angle, has very different properties from the cis isomers. The mirror images of the cis isomer form a pair of optical isomers, which have identical behavior except when reacting with other enantiomers.

This text is adapted from Openstax, Chemistry 2e, Chapter 19.2 Coordination Chemistry of Transition Metals.

Stereoisomers of coordination compounds are molecules that have the same chemical formula and the same connectivity of the ligands to the metal atom but differ in the arrangement of the ligand molecules around the central atom.

Based on the molecular structure, stereoisomers of coordination compounds are classified as geometric isomers or optical isomers.

Geometric isomers are stereoisomers that have completely different molecular geometries and exist with distinct physical and chemical properties.

cis–trans isomers are an example of geometric isomers. They occur in square-planar complexes with two sets of two identical ligands, or MA₂B₂, and in octahedral complexes with a set of four identical ligands and a pair of another ligand, or MA₄B₂. 

For example, in the cis isomer of the square-planar complex diamminedichloroplatinum(II), known as cisplatin, the chloride ligands are adjacent to each other and on the same side of the molecule. In the trans isomer, transplatin, the chloride ligands are on the opposite sides of the molecule. 

Octahedral complexes with two sets of three identical ligands, or MA₃B₃, can exhibit another type of geometric isomerism, known as fac–mer isomerism. 

For example, in the fac isomer of triamminetrichlorocobalt(III), the three amine ligands and three chloride ligands are on opposite sides of the molecule and thus make up opposite faces of the octahedron. In the mer isomer, three of the same ligands form an arc around the central metal atom.

The other class of stereoisomers is defined as optical isomers. Like our hands, optical isomers are mirror images of each other and non-superposable. These isomers, also called enantiomers, are described as chiral and have very few differences in their physical and chemical properties.

Optical isomers are distinguished by their interaction with plane-polarized light. Polarized light has electric field vectors oscillating in a single plane, which is rotated by a certain amount when the polarized light passes through a solution of an enantiomer. 

For example, one enantiomer of tris(ethylenediamine)cobalt(III) rotates the plane to the right and is called dextrorotatory, or the d optical isomer. The other enantiomer rotates the plane to the left and is called levorotatory, or the l optical isomer.