28.4 Stereoisomerism in Transition Element Complexes

2026 Syllabus Objectives

By the end of these notes, you should be able to:

  1. Describe the types of stereoisomerism shown by transition element complexes, including those involving bidentate ligands:

    • (a) Geometrical (cis/trans) isomerism — in square planar complexes such as [Pt(NH₃)₂Cl₂] and in octahedral complexes such as [Co(NH₃)₄(H₂O)₂]²⁺ and [Ni(H₂NCH₂CH₂NH₂)₂(H₂O)₂]²⁺
    • (b) Optical isomerism — in complexes such as [Ni(H₂NCH₂CH₂NH₂)₃]²⁺ and [Ni(H₂NCH₂CH₂NH₂)₂(H₂O)₂]²⁺

  2. Deduce the overall polarity of the complexes described above.

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1. What is Stereoisomerism?

Stereoisomers are molecules that have the exact same chemical formula and the same bonds connecting their atoms, but those atoms are arranged differently in space. Think of it like having the same pieces of a jigsaw puzzle but placing them in different positions — the result looks different even though every piece is identical.

Transition element complexes (which are structures where a central metal ion is surrounded by molecules or ions called ligands — particles that donate a pair of electrons to the metal) can show two types of stereoisomerism:

  • Geometrical (cis/trans) isomerism
  • Optical isomerism

2. Quick Reminder: What Are Bidentate Ligands?

Before going further, it helps to understand what a bidentate ligand is.

  • Most ligands are monodentate — they attach to the metal through just one atom (one "bite point"). Example: NH₃ (ammonia), H₂O (water), Cl⁻ (chloride).
  • A bidentate ligand attaches to the metal through two atoms (two "bite points") at the same time. The word "bidentate" comes from the Latin for "two teeth."
  • The key example you need to know is 1,2-diaminoethane, written as H₂NCH₂CH₂NH₂ (sometimes shortened to "en"). It has two nitrogen atoms, and both of them can bond to the metal simultaneously, forming a ring shape.

Bidentate ligands are important in stereoisomerism because their shape restricts how they can sit around the metal, creating different possible arrangements.

3. Geometrical (Cis/Trans) Isomerism

What is it?

Geometrical isomerism happens when ligands can be arranged in two different ways around the central metal ion — either next to each other (called the cis arrangement) or opposite each other (called the trans arrangement).

  • Cis means "on the same side" — the two different ligands are adjacent (next to each other).
  • Trans means "across" — the two different ligands are on opposite sides.

Key idea: Geometrical isomerism in complexes does NOT require a double bond (unlike in organic chemistry). It only requires that at least two different types of ligand are present.

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