Chemistry (9701) | Stereoisomerism in transition element complexes

🧠 Chapter 28.4: Stereoisomerism in Transition Element Complexes

Hello! Welcome to one of the most visually interesting and important parts of transition element chemistry:

This topic links the 3D shapes of complex ions (which you learned about earlier in Topic 28.2) with the concept of isomerism (which you probably studied in Organic Chemistry, Topic 13.4). Don't worry if complex ions still feel a bit abstract; we'll break down how changing the 3D arrangement of ligands creates completely different compounds.

Why is this important? The exact spatial arrangement of ligands can drastically change a complex's chemical properties, its colour, and even its biological activity. For example, one stereoisomer of a platinum complex is a life-saving chemotherapy drug, while the other is inactive!

Section 1: Quick Review of Complex Ion Shapes

To understand stereoisomerism, we first need to recall the common geometries for transition metal complexes:

• Coordination Number (CN) = 4: Two main shapes:
  Tetrahedral (no isomerism for simple complexes, like methane).
  Square planar (very common for $\text{Pt(II)}$ and $\text{Ni(II)}$).
• Coordination Number (CN) = 6: Always Octahedral (like two square pyramids joined at the base). This shape is central to complex ion isomerism.

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Section 2: Geometrical (Cis/Trans) Isomerism

Geometrical isomerism (often called cis-trans isomerism) occurs when ligands occupy different spatial positions around the central metal ion. It happens when the coordination number is 4 (square planar) or 6 (octahedral).

1. Geometrical Isomerism in Square Planar Complexes (CN=4)

This is typically seen in complexes of the form \(MA_2B_2\), where M is the metal ion, and A and B are two different monodentate ligands.

Key Example: Diamminedichloroplatinum(II), $\text{[Pt}(\text{NH}_3)_2\text{Cl}_2]$

In this square planar complex, the $\text{NH}_3$ ligands (A) and the $\text{Cl}^-$ ligands (B) can be arranged in two ways:

• Cis-isomer (90°): The two identical ligands (e.g., the two $\text{Cl}$ atoms) are next to each other, occupying positions at a 90° angle.
• Trans-isomer (180°): The two identical ligands are opposite each other, occupying positions at a 180° angle.

Did you know? The cis-isomer of $\text{[Pt}(\text{NH}_3)_2\text{Cl}_2]$ is the famous chemotherapy drug Cisplatin, effective against various cancers. The trans-isomer, however, is biologically inactive. This highlights how crucial stereochemistry is!

2. Geometrical Isomerism in Octahedral Complexes (CN=6)

Geometrical isomerism is also common in octahedral complexes of the form \(MA_4B_2\), or complexes involving bidentate ligands like $M(AA)_2B_2$.

Example 1: $\text{[Co}(\text{NH}_3)_4(\text{H}_2\text{O})_2]^{2+}$ (Tetramminediaquacobalt(II) ion)

Here, A is $\text{NH}_3$ and B is $\text{H}_2\text{O}$.

• Cis-isomer: The two $\text{H}_2\text{O}$ ligands (the B ligands) are adjacent (90°).
• Trans-isomer: The two $\text{H}_2\text{O}$ ligands (the B ligands) are opposite (180°).

Example 2 (Involving Bidentate Ligands): $\text{[Ni}(\text{H}_2\text{NCH}_2\text{CH}_2\text{NH}_2)_2(\text{H}_2\text{O})_2]^{2+}$

The bidentate ligand here is 1,2-diaminoethane ($\text{en}$). The formula is \(M(AA)_2B_2\). This complex still exhibits cis-trans isomerism based on the arrangement of the two monodentate $\text{H}_2\text{O}$ ligands.

• Cis-isomer: The two water ligands are adjacent (90°).
• Trans-isomer: The two water ligands are opposite (180°).

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Section 3: Optical Isomerism (Chirality)

Optical isomerism, also known as enantiomerism, occurs when a complex ion is chiral.

What is a Chiral Complex?

A complex is chiral if it is non-superimposable on its mirror image.

Think of your hands: your left hand and your right hand are mirror images, but you cannot perfectly superimpose them—if you put your palms together, they match, but if you try to stack them, they don't. A chiral complex is like a "left-handed" and "right-handed" version of the same molecule.

Optical isomers (enantiomers) have identical chemical and physical properties, except for their ability to rotate the plane of plane-polarised light in opposite directions.

When Does Optical Isomerism Occur?

Optical isomerism in transition metal complexes is almost exclusively associated with bidentate or polydentate ligands that wrap around the central ion, preventing the molecule from having a plane of symmetry.

Key Example 1: $\text{[Ni}(\text{H}_2\text{NCH}_2\text{CH}_2\text{NH}_2)_3]^{2+}$ (Tris(ethylenediamine)nickel(II) ion)

The ligand 1,2-diaminoethane ($\text{en}$) is bidentate (it binds at two points). This complex is \(M(AA)_3\) type (where AA is a bidentate ligand). The three 'en' ligands wrap around the central nickel ion like the blades of a propeller, creating a structure that is chiral.

The two enantiomers are:

1. One isomer rotates plane-polarised light to the right (dextrorotatory).
2. Its mirror image rotates plane-polarised light equally to the left (levorotatory).

Key Example 2: $\text{[Ni}(\text{H}_2\text{NCH}_2\text{CH}_2\text{NH}_2)_2(\text{H}_2\text{O})_2]^{2+}$

This complex, which also showed geometrical isomerism (Section 2), can show optical isomerism too!
Specifically, the cis-isomer of this \(M(AA)_2B_2\) type octahedral complex is chiral and exists as a pair of enantiomers. The *trans*-isomer, however, has a plane of symmetry (cutting straight through the two $\text{H}_2\text{O}$ ligands and the $\text{Ni}$ centre) and is therefore achiral (optically inactive).

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Section 4: Overall Polarity of Complexes (Dipole Moments)

The overall polarity of a complex is determined by whether the individual bond dipoles created by the metal-ligand bonds cancel each other out in the overall structure.

When determining polarity, we need to look at the symmetry of the isomers:

1. Square Planar Complex: $\text{[Pt}(\text{NH}_3)_2\text{Cl}_2]$

• Trans-isomer: The identical ligands (e.g., the two $\text{Cl}$ atoms) are arranged exactly opposite each other (180°). The bond dipoles cancel out perfectly.
  Result: The trans isomer is non-polar (or has a very low net dipole moment).
• Cis-isomer: The identical ligands are adjacent (90°). Their bond dipoles point in the same general direction and cannot cancel each other out fully.
  Result: The cis isomer is polar.

2. Octahedral Complexes (Geometrical Isomers)

Example: $\text{[Co}(\text{NH}_3)_4(\text{H}_2\text{O})_2]^{2+}$

The principles remain the same for octahedral complexes:

• Trans-isomer: High symmetry. The two unique $\text{H}_2\text{O}$ ligands are opposite (180°). Their dipoles cancel.
  Result: The trans isomer is non-polar.
• Cis-isomer: Low symmetry. The two $\text{H}_2\text{O}$ ligands are adjacent (90°). Their dipoles do not cancel.
  Result: The cis isomer is polar.

3. Octahedral Complexes (Optical Isomers)

Example: $\text{[Ni}(\text{en})_3]^{2+}$

Since optical isomers are mirror images, they both have the exact same distribution of charge and symmetry elements.

• The complex has a high degree of symmetry (although it is chiral). The charge distribution is uniform.
  Result: Both enantiomers (the optical isomers) are typically non-polar overall, or have zero net dipole moment, due to their symmetric octahedral geometry.

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