Ch 4 · Stereoisomerism in transition element complexes · AS & A Level Chemistry 9701

Topic 4 of 5 in Chemistry of transition elements — AS & A Level Chemistry 9701 — 58 lessons.

Introduction

Complexes with a fixed formula can still differ in how their ligands are arranged in space. This lesson covers the two forms of stereoisomerism shown by transition metal complexes — geometrical and optical — and shows how to decide whether a given complex is polar, all building directly on the shapes met in the previous lesson.

Key ideas — Geometrical isomerism in square planar complexes

Geometrical (cis–trans) isomerism arises when two different spatial arrangements of ligands are possible for the same formula and coordination number, because rotation about the dative bonds is restricted. In a square planar complex such as [Pt(NH_3)_2Cl_2], the two chloride ligands can be adjacent to each other, 90° apart (the cis isomer), or opposite each other, 180° apart (the trans isomer). The cis isomer, cisplatin, is used as an anticancer drug; the trans isomer is biologically inactive.

🧑‍🏫 Teacher's explanation

Imagine you are sitting at a square dining table with three friends. If you and your best friend sit next to each other (on adjacent sides), that is like a 'cis' arrangement. If you sit directly across from your best friend (on opposite sides), that is like a 'trans' arrangement. Even though the same four people are at the table, the spatial relationship between you and your friend has changed.

📖 Definitions

🔍 Worked example — Easy

Draw the two geometrical isomers of the complex Pt(NH_3)_2 extCl_2 and identify which one is the 'cis' isomer.

🔍 Worked example — Standard

A square planar complex has the formula MA_2 extB_2. If the bond angle between the two B ligands is 180^°, what is the name of this isomer?

🔍 Worked example — Exam-style

Determine the overall polarity of the cis-isomer of Pt(NH_3)_2 extCl_2. Assume the Pt-Cl and Pt-N bond dipoles are significant and the geometry is perfectly square planar.

⚠️ Common errors

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🎯 Exam question

(a) Name the isomer where the two Cl^- ligands are adjacent to each other. [1] (b) Explain why the trans-isomer of Pt(NH_3)_2 extCl_2 is non-polar. [2] [3 marks]

Key ideas — Geometrical isomerism in octahedral complexes

Octahedral complexes with two different monodentate ligands in a 4:2 ratio also show cis–trans isomerism: in [Co(NH_3)_4Cl_2]^+, the two chloride ligands can be adjacent (cis, 90° apart) or opposite (trans, 180° apart). The same applies with bidentate ligands: in [Co(en)_2Cl_2]^+, the two chloride ligands can again be cis or trans to each other, with the two bidentate "en" ligands occupying the remaining four coordination positions.

🧑‍🏫 Teacher's explanation

Imagine an octahedral complex as a central atom sitting inside a 3D cage made of six arms. In an octahedral geometry, these arms point to the corners of a cube. If you have two identical 'special' ligands, you can place them right next to each other (like two people sitting in adjacent seats in a movie theater) or directly across from each other (like two people sitting on opposite sides of a small table).

📖 Definitions

🔍 Worked example — Easy

Draw the two geometrical isomers of [Co(NH_3)_4Cl_2]^+ and state the bond angle between the two Cl ligands in the cis-isomer.

🔍 Worked example — Standard

A complex has the formula [Co(en)_2Cl_2]^+. How many geometrical isomers are possible?

🔍 Worked example — Exam-style

Determine if the complex [Pt(NH_3)_2Cl_2] is polar or non-polar for the trans-isomer.

⚠️ Common errors

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🎯 Exam question

(a) Explain why the trans-isomer is not optically active. [2] (b) Draw the structure of the cis-isomer. [2] [4 marks]

Key ideas — Optical isomerism in octahedral complexes with bidentate ligands

Optical isomerism occurs when a complex and its mirror image are non-superimposable, making the complex chiral; the two mirror-image forms are called enantiomers and rotate plane-polarised light in opposite directions. Octahedral complexes containing bidentate (or polydentate) ligands frequently lack any plane of symmetry and so are chiral. mathrm[Co(en)_3]^3+, with three bidentate ligands, has a propeller-like arrangement with no plane of symmetry, and exists as two non-superimposable enantiomers.

🧑‍🏫 Teacher's explanation

Imagine you are wearing a pair of gloves. Your left hand and your right hand are mirror images of each other, but no matter how you twist or turn them, you can never slide a left-handed glove onto your right hand and have it fit perfectly. They are 'non-superimposable'. In chemistry, some molecules are like these gloves; they have a 'handedness' called chirality.

📖 Definitions

🔍 Worked example — Easy

A complex ion has the formula [Co(en)_3]^3+. If the molar mass of the en ligand (ethane-1,2-diamine) is 60.1 g/mol, calculate the molar mass of the complex ion.

🔍 Worked example — Standard

Explain why the complex [Co(en)_3]^3+ exhibits optical isomerism.

🔍 Worked example — Exam-style

A sample of [Co(en)_3]^3+ contains only one enantiomer. If it rotates plane-polarised light by +5.0^°, what is the rotation of its enantiomer?

⚠️ Common errors

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🎯 Exam question

(a) Define the term 'enantiomer'. [1] (b) Explain why the complex [Co(en)_3]^3+ exhibits optical isomerism, whereas the complex [Co(NH_3)_6]^3+ does not. [2] [3 marks]

Key ideas — Complexes showing both types of isomerism

[Co(en)_2Cl_2]^+ shows both geometrical and optical isomerism. The cis isomer has no plane of symmetry, so it is chiral and exists as two enantiomers, showing optical isomerism as well as being one of the two geometrical isomers. The trans isomer, however, does have a plane of symmetry, so it is achiral and shows no optical isomerism, even though it is a distinct geometrical isomer of the cis form. mathrm[Co(en)_3]^3+, by contrast, shows optical but not geometrical isomerism, since three identical bidentate ligands allow no cis–trans distinction.

🧑‍🏫 Teacher's explanation

Imagine you are holding a pair of gloves. If you hold them side-by-side, they are like 'cis' isomers—they are similar in arrangement. If you hold one in each hand, they are like 'trans' isomers—they are mirrored or opposite. In transition metal chemistry, ligands (the molecules attached to the metal) can be 'monodentate' (one attachment point) or 'bidentate' (two attachment points, like a pair of pincers).

📖 Definitions

🔍 Worked example — Easy

Identify if the complex ce[Co(NH3)4Cl2]^+ exhibits geometrical isomerism and determine if the cis-isomer is chiral.

🔍 Worked example — Standard

A complex has the formula ce[Co(en)_2Cl_2]^+. Explain why the trans-isomer does not show optical isomerism.

🔍 Worked example — Exam-style

Determine the number of stereoisomers for ce[Co(en)_3]^3+ and classify them.

⚠️ Common errors

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🎯 Exam question

(a) Draw the structures of the cis and trans isomers. (b) Explain why the cis-isomer is optically active while the trans-isomer is not. [3] [3 marks]

Key ideas — Deducing the polarity of a complex

The overall polarity of a complex is found from the vector sum of its individual ligand–metal bond dipoles, taking its symmetry into account. In trans-[Pt(NH_3)_2Cl_2], the two identical pairs of ligands lie directly opposite each other, so their bond dipoles cancel and the complex is non-polar. In cis-[Pt(NH_3)_2Cl_2], the two chlorides are on one side and the two ammine ligands on the other, so the bond dipoles do not cancel and the complex has a net dipole moment; it is polar.

🧑‍🏫 Teacher's explanation

Imagine you are standing in the center of a room. If two people are pulling you toward the North wall and two other people are pulling you toward the South wall with equal strength, you won't move. Your net movement is zero because the forces cancel out. This is exactly how polarity works in a symmetrical complex.

Ch 4 · Stereoisomerism in transition element complexes · AS & A Level Chemistry 9701

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