---
title: "Complex Ions, Ligands and Their Shapes"
description: "A ligand donates a lone pair into a vacant orbital on a metal ion. How many fit, and how they arrange themselves, sets the shape of the complex and whether two mirror forms are possible."
canonical: https://lightmysky.com/learn/science/complex-ions-ligands-and-their-shapes-mt_xxWYYqgZp_
source: https://lightmysky.com/learn/science/complex-ions-ligands-and-their-shapes-mt_xxWYYqgZp_.md
retrieved: 2026-09-12
---

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# Complex Ions, Ligands and Their Shapes

A ligand donates a lone pair into a vacant orbital on a metal ion. How many fit, and how they arrange themselves, sets the shape of the complex and whether two mirror forms are possible.

Subject: Science · Area: Chemistry · Ages 17 to 18
Page: https://lightmysky.com/learn/science/complex-ions-ligands-and-their-shapes-mt_xxWYYqgZp_

## Ready when they can

- Defines a ligand and a coordination number and picks both out of a given formula
- Predicts octahedral, tetrahedral or square planar geometry from the ligand and its size
- Separates monodentate from bidentate ligands and explains why the larger ones hold on more tightly
- Explains cis-trans isomerism in a named complex and why it matters for a drug such as a platinum complex

## Lesson: Ligands, shapes and isomers

A ligand donates a lone pair into an empty orbital on the metal, and the coordination number counts those ligand to metal bonds. [Cu(H2O)6]2+ holds 6 waters so the number is 6, while [CuCl4]2- holds 4 chlorides so the number is 4.

**Example.** Six small waters fit around copper in an octahedron, but four bulky chlorides only fit as a tetrahedron. Size decides how many ligands fit, and the count sets the shape you get.

Water, ammonia and chloride each bind through one atom, so they are monodentate. Ethane-1,2-diamine binds through two atoms, so it is bidentate, and that double grip makes its complexes far harder to pull apart.

**Tip.** In square planar complexes, matching ligands side by side is cis and opposite corners is trans. Cisplatin, the cis form, fights tumours, while the trans form does not, so the shape really matters.

**Recap.** Lone pairs set the count, size sets the shape, double grips hold tighter, and cis versus trans can save lives.

## Practice

8 questions on this page, each with its working shown.

## Needs first

- [Covalent Bonding: Shared Pairs and Simple Molecules](https://lightmysky.com/learn/science/covalent-bonding-shared-pairs-and-simple-molecules-mt_fn5Dq6qUjP)
- [Transition Metals: Variable Oxidation State, Colour and Catalysis](https://lightmysky.com/learn/science/transition-metals-variable-oxidation-state-colour-and-catalysis-mt_UIVC6d7wak)

## Opens up

- [Aldehydes and Ketones: Nucleophilic Addition and the Tests That Separate Them](https://lightmysky.com/learn/science/aldehydes-and-ketones-nucleophilic-addition-and-the-tests-that-separate-them-mt_bVcJ0UaXk6)
- [Metal Aqua Ions: Acidity, Ligand Exchange and Precipitation](https://lightmysky.com/learn/science/metal-aqua-ions-acidity-ligand-exchange-and-precipitation-mt_jrsKwNq1Oi)
- [Coordination Compounds: Naming, Geometry and Isomerism](https://lightmysky.com/learn/science/coordination-compounds-naming-geometry-and-isomerism-mt_rkHM3jL3Ri)