Chemistry · Ch 5 — Coordination Compounds

Optical Isomerism

5.4.2

Optical Isomerism

Optical isomers are mirror images of one another that cannot be superimposed on one another, no matter how they are rotated in space. Such a pair of isomers is called a pair of enantiomers, and a molecule or ion that cannot be superimposed on its own mirror image is said to be chiral.

The two non-superimposable mirror-image forms of a chiral coordination entity are distinguished as:

the dextro (d) form, and
the laevo (l) form,

named for the direction in which each rotates the plane of plane-polarised light when examined in a polarimeter — the d form rotates the plane to the right, the l form to the left.

Where it occurs

Optical isomerism is common in octahedral complexes involving didentate ligands. It is illustrated for the tris-chelate ion $[Co(en)_3]^{3+}$ , whose dextro and laevo forms are non-superimposable mirror images of each other (Fig. 5.6).

Figure 5.6Optical isomers (d and l) of [Co(en)₃]³⁺

Fig. 5.6 — Optical isomers (d and l) of [Co(en)₃]³⁺

Drawn by us to help you understand the concept clearly, and verified to make sure it's accurate. For exams, practice from your textbook's own diagram.

The figure presents two octahedral cobalt(III) complexes, each with the formula $[\text{Co(en)}_3]^{3+}$ , placed on opposite sides of a vertical dashed line representing a mirror plane. Each complex shows the central $\text{Co}^{3+}$ ion surrounded by three bidentate ethylenediamine (en) ligands, $\text{H}_2\text{N–CH}_2–\text{CH}_2–\text{NH}_2$ , which form five-membered chelate rings. The three rings are arranged around the metal so that they resemble a three-bladed propeller.

On the left side of the mirror line, the complex is labelled dextro (d); on the right side it is labelled laevo (l) — exactly the two labels NCERT prints beneath the panels. The d and l designations refer to the direction in which each isomer rotates the plane of polarised light in a polarimeter (d to the right, l to the left). The two structures are non-superimposable mirror images: one cannot be rotated in space to exactly match the other, just as a left hand cannot be turned into a right hand. This property is chirality, and the pair of isomers are called optical isomers or enantiomers.

The physical idea taught is that even though both complexes have identical connectivity (same ligands, same coordination number, same geometry), the spatial arrangement of the chelate rings creates a handedness. Because the three en ligands are bidentate and form a helical twist, the molecule lacks any plane of symmetry — it is chiral. This leads to optical activity: the Δ form rotates plane-polarised light to the right, and the Λ form rotates it to the left by an equal amount.

The key formula developed with this figure is the relationship between the specific rotation $[\alpha]$ and the observed rotation $\alpha$ :

[\alpha] = \frac{\alpha}{l \cdot c}

where: …

It also occurs in complexes carrying a mixture of unidentate and didentate ligands. In a coordination entity of the type $[PtCl_2(en)_2]^{2+}$ , only the cis-isomer is optically active — the corresponding trans-isomer possesses a plane of symmetry and is superimposable on its mirror image, so it shows no optical activity. The $d$ and $l$ forms of cis- $[PtCl_2(en)_2]^{2+}$ are shown in Fig. 5.7.

Figure 5.7Optical isomers (d and l) of cis-[PtCl₂(en)₂]²⁺

Fig. 5.7 — Optical isomers (d and l) of cis-[PtCl₂(en)₂]²⁺

Drawn by us to help you understand the concept clearly, and verified to make sure it's accurate. For exams, practice from your textbook's own diagram.

The figure depicts two octahedral complexes of platinum(IV), with the formula $\text{cis-}[\text{PtCl}_2(\text{en})_2]^{2+}$ . The central Pt atom is at the centre of an octahedron. Two chloride ligands ( $\text{Cl}^-$ ) occupy two adjacent positions (the cis arrangement), and two bidentate ethylenediamine (en) ligands each occupy the remaining two coordination sites, forming five-membered chelate rings.

The key visual feature is a vertical mirror plane drawn between the two structures. The complex on the left is the d (dextrorotatory) enantiomer, and the complex on the right is the l (laevorotatory) enantiomer. They are non-superimposable mirror images of each other. The trans isomer of this complex (with the two Cl ligands opposite each other) would have a plane of symmetry and therefore would not be chiral — the figure explicitly shows only the cis form to highlight the origin of optical activity.

The physical idea taught is that optical isomerism arises when a coordination compound lacks a plane of symmetry (or any improper rotation axis). In this cis geometry, the two chelate rings create a helical arrangement of the ligand atoms around the metal centre, making the molecule chiral. The two enantiomers rotate plane-polarised light in opposite directions. …

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