Ligands, Complex Ions and Their Shapes
A concise revision guide to complex ions: ligands and the co-ordinate bond, co-ordination number, monodentate, bidentate and multidentate ligands, and the octahedral, tetrahedral, square planar and linear shapes, with cis-platin and haemoglobin.
- 3.2.5.1iii
- 3.2.5.1iv
- 3.2.5.1v
- 3.2.5.2i-a
- 3.2.5.2i-b
- 3.2.5.2i-c
- 3.2.5.2v-a
- 3.2.5.2v-b
- 3.2.5.2v-c
- 3.2.5.2vi-a
- 3.2.5.3i
- 3.2.5.3iii
- 3.2.5.3v-a
- 3.2.5.3v-b
What these spec points say
- 3.2.5.1iii know that a ligand is a molecule or ion that forms a co-ordinate bond with a transition metal by donating a pair of electrons
- 3.2.5.1iv know that a complex is a central metal atom or ion surrounded by ligands
- 3.2.5.1v know that co-ordination number is the number of co-ordinate bonds to the central metal atom or ion
- 3.2.5.2i-a know that H2O can act as a monodentate ligand
- 3.2.5.2i-b know that NH3 can act as a monodentate ligand
- 3.2.5.2i-c know that Cl- can act as a monodentate ligand
- 3.2.5.2v-a know that ligands can be bidentate, for example H2NCH2CH2NH2
- 3.2.5.2v-b know that ligands can be bidentate, for example C2O42-
- 3.2.5.2v-c know that ligands can be multidentate, for example EDTA4-
- 3.2.5.2vi-a know that haem is an iron(II) complex with a multidentate ligand
- 3.2.5.3i know that transition metal ions commonly form octahedral complexes with small ligands (eg H2O and NH3)
- 3.2.5.3iii know that transition metal ions commonly form tetrahedral complexes with larger ligands (eg Cl-)
- 3.2.5.3v-a know that cisplatin is the cis isomer
- 3.2.5.3v-b know that Ag+ forms the linear complex [Ag(NH3)2]+ as used in Tollens' reagent
Shapes of Complex Ions
Choose a step to watch each ligand travel in and form a dative bond, and the bonds spread out as far apart as they can.
© Dr. Mohammed Al-Fatah – onlinelearningsystem.net
Before You Start
This page builds on bonding and shapes of molecules from your earlier course.
- A lone pair is a pair of outer electrons not used in bonding. Water has two on its oxygen atom and ammonia has one on its nitrogen atom.
- In a co-ordinate bond, both shared electrons come from the same atom.
- Electron pairs around a central atom repel each other and move as far apart as possible, which fixes the shape.
- The ions used here are those of the d-block: see Electron Configurations and Properties of Transition metals.
Ligands and Complex Ions
A transition metal ion is small and highly charged, and it has vacant orbitals. It attracts species that have lone pairs and accepts those lone pairs into its vacant orbitals.
Definition: A ligand is a molecule or ion with a lone pair of electrons that it donates to a central metal atom or ion, forming a co-ordinate bond.
- A complex is a central metal atom or ion surrounded by ligands. If it carries a charge it is a complex ion.
- Every bond between a ligand and the metal ion is a co-ordinate bond.
- The formula is written in square brackets with the overall charge outside: [Cu(H₂O)₆]²⁺.
- In water, a metal ion written as Cu²⁺(aq) is really the complex ion [Cu(H₂O)₆]²⁺.
The formula of a complex ion taken apart: the central metal ion, the ligands, the square brackets and the overall charge.
Exam focus: State where the electrons come from: the ligand donates a lone pair to the metal ion. A co-ordinate bond is drawn as a line or as an arrow pointing from the ligand to the metal.
Co-ordination number
Definition: The co-ordination number is the number of co-ordinate bonds to the central metal atom or ion.
The co-ordination number counts bonds, not ligands. The two are equal only when every ligand forms one bond.
| Complex | Number of ligands | Bonds per ligand | Co-ordination number |
|---|---|---|---|
| [Cu(H₂O)₆]²⁺ | 6 | 1 | 6 |
| [CuCl₄]²⁻ | 4 | 1 | 4 |
| [Ni(H₂NCH₂CH₂NH₂)₃]²⁺ | 3 | 2 | 6 |
| [Cu(EDTA)]²⁻ | 1 | 6 | 6 |
| [Ag(NH₃)₂]⁺ | 2 | 1 | 2 |
- The most common co-ordination numbers are 6 and 4.
- A co-ordination number of 6 does not always mean six ligands.
Monodentate, Bidentate and Multidentate Ligands
Ligands are classified by the number of lone pairs that one ligand donates to the same metal ion.
| Type | Bonds per ligand | Examples |
|---|---|---|
| monodentate | 1 | H₂O, NH₃, OH⁻, Cl⁻ |
| bidentate | 2 | 1,2-diaminoethane, H₂NCH₂CH₂NH₂ (“en”); ethanedioate ion, C₂O₄²⁻ |
| multidentate | more than 2 | EDTA⁴⁻, which forms six bonds |
- Water has two lone pairs but is monodentate: both are on the same small oxygen atom, so only one can reach the metal ion.
- Ammonia has only one lone pair. A hydroxide ion, like water, has all its lone pairs on one oxygen atom, so it also forms one bond.
- In “en”, each nitrogen atom donates one lone pair. The two nitrogen atoms are far enough apart to bond to the same ion.
- In the ethanedioate ion, one lone pair is donated from each of the two negatively charged oxygen atoms.
- EDTA⁴⁻ donates six lone pairs: two from nitrogen atoms and four from oxygen atoms. One EDTA⁴⁻ ion wraps around one metal ion.
1,2-Diaminoethane has two donor nitrogen atoms. EDTA⁴⁻ has six donor atoms: two nitrogen and four oxygen.
The ethanedioate ion bonds through one lone pair on each of its two negatively charged oxygen atoms.
Ligands sorted by the number of bonds each one forms to a single metal ion, with the bonds counted in three complexes.
Remember: Three bidentate ligands or one EDTA⁴⁻ ion fill all six positions around a metal ion: [Ni(en)₃]²⁺ and [Cu(EDTA)]²⁻ both have a co-ordination number of 6.
Check: Ligands and Complex Ions
Use the terms ligand, complex ion, co-ordinate bond and co-ordination number with complexes not shown above.
Quick Check: Count the Bonds, Not the Ligands
Work out how many bonds each new complex has to its metal ion, where en is the bidentate molecule H₂NCH₂CH₂NH₂, each EDTA⁴⁻ forms six bonds and CN⁻ is monodentate.
Charges and Oxidation states in Complexes
The overall charge on a complex is the charge on the metal ion plus the total charge on the ligands. The same sum, run backwards, gives the oxidation state of the metal.
| Ligand | Charge |
|---|---|
| H₂O, NH₃, H₂NCH₂CH₂NH₂ | 0 |
| Cl⁻, OH⁻ | −1 |
| C₂O₄²⁻ | −2 |
| EDTA⁴⁻ | −4 |
Worked example: the charge on a complex
Worked example: the oxidation state of the metal
Step 1. In [CoCl₄]²⁻, four chloride ligands contribute 4 × (−1) = −4.
Answer. Co + (−4) = −2, so cobalt is +2.
Step 1. In [Cr(NH₃)₆]³⁺, the six ammonia ligands are neutral.
Answer. Chromium is +3, the same as the overall charge.
Answer. Pt + (−2) = 0, so platinum is +2.
Step 1. In [Cr(C₂O₄)₃]³⁻, three ethanedioate ligands contribute 3 × (−2) = −6.
Chloride inside and outside a complex
- In a compound such as [Co(NH₃)₆]Cl₃, the chloride ions outside the square brackets are free ions, not ligands.
- Aqueous silver nitrate precipitates only the free chloride ions, as AgCl.
- Cr(NH₃)₅Cl₃ gives 2 mol of AgCl per mole, so it is [Cr(NH₃)₅Cl]Cl₂: one chloride ion is a ligand.
Exam focus: Check a formula by adding the charges. For [CuCl₄]²⁻: +2 + 4 × (−1) = −2. If the sum does not match the charge outside the brackets, the oxidation state is wrong.
Check: Co-ordination number, Charge and Oxidation state
Work out the co-ordination number, the overall charge and the oxidation state of the metal in complexes not used above.
Quick Check: Inside or Outside the Brackets?
Work out each answer on paper, then flip the card to compare.
Shapes of Complex Ions
The shape depends on the co-ordination number and on the size of the ligands. The bonding pairs repel each other and take up positions as far apart as possible.
| Shape | Co-ordination number | Bond angle | Typical ligands | Examples |
|---|---|---|---|---|
| octahedral | 6 | 90° | small: H₂O, NH₃ | [Cu(H₂O)₆]²⁺, [Fe(H₂O)₆]²⁺, [Cr(NH₃)₆]³⁺ |
| tetrahedral | 4 | 109.5° | large: Cl⁻ | [CuCl₄]²⁻, [CoCl₄]²⁻ |
| square planar | 4 | 90° | mostly complexes of platinum(II) | [Pt(NH₃)₂Cl₂], cisplatin |
| linear | 2 | 180° | NH₃ with silver(I) | [Ag(NH₃)₂]⁺ |
Why six bonds give an octahedron
- Six bonding pairs of electrons surround the metal ion.
- They repel each other equally and move as far apart as possible.
- The position of minimum repulsion puts the six ligands at the corners of an octahedron, with all bond angles 90°.
Why chloride gives a tetrahedron
- A chloride ion is much larger than a water or ammonia molecule.
- Only four chloride ions fit around the metal ion, and they repel each other strongly.
- Four bonding pairs spread out to 109.5°, so [CuCl₄]²⁻ is tetrahedral and the co-ordination number falls from 6 to 4.
Octahedral [Cu(H₂O)₆]²⁺, tetrahedral [CuCl₄]²⁻ and square planar cisplatin, drawn with wedges and hashed bonds.
Drawing tip: Draw an octahedral complex with two plain bonds, two wedges and two hashed bonds. Bond each ligand through its donor atom, and put square brackets and the charge around the whole ion.
Linear Complexes
Silver(I) ions form a complex with only two ligands. Two bonding pairs repel to opposite sides of the ion, giving a linear shape with a bond angle of 180°.
- [Ag(NH₃)₂]⁺ has a co-ordination number of 2.
- It is the complex ion in Tollens’ reagent, which is used to distinguish aldehydes from ketones.
- The redox reaction of this ion is covered in Redox Chemistry of Transition metals.
- The solution is colourless: Ag⁺ is 4d¹⁰, with a full d set.
![Drawing of the linear diamminesilver(I) ion, [Ag(NH₃)₂]⁺: a central silver ion with one ammonia ligand on each side, bonded through nitrogen, the three atoms in a straight line with a 180° bond angle.](https://www.onlinelearningsystem.net/wp-content/uploads/2026/09/transition-t17-linearcomplex.svg)
The linear complex ion [Ag(NH₃)₂]⁺: both ammonia ligands bond through nitrogen and the N–Ag–N angle is 180°.
Cisplatin
Cisplatin, [Pt(NH₃)₂Cl₂], is a square planar complex of platinum(II). It is neutral: the +2 of the platinum is balanced by two chloride ligands.
- The four ligands lie in one plane at 90° to each other.
- “cis” means that the two chloride ligands are next to each other, on the same side of the complex.
- In the other isomer, the trans form, the two chloride ligands are opposite each other.
- Cisplatin is used as an anti-cancer drug.
The two isomers, and how to draw them, are covered in Stereoisomerism in Complex Ions.
Remember: Cisplatin has the same co-ordination number as [CuCl₄]²⁻ but a different shape: square planar with 90° angles, not tetrahedral with 109.5° angles.
Check: Shapes and Bond Angles
Predict the shape and bond angle of complexes from their ligands and co-ordination number.
Haemoglobin
Haemoglobin is the protein that carries oxygen in red blood cells. It contains iron(II) ions, each held in a complex with a co-ordination number of 6.
- Four bonds come from nitrogen atoms in haem, a multidentate ligand that surrounds the Fe²⁺ ion in a ring.
- A fifth bond comes from a nitrogen atom in the protein, globin.
- The sixth position is taken by an oxygen molecule, which forms a co-ordinate bond to the Fe²⁺ ion.
- The oxygen is picked up in the lungs and released where it is needed, so the bond must form and break easily.
A block diagram of the six bonds to iron(II) in haemoglobin: four from haem, one from globin and one to the oxygen molecule being carried.
Carbon monoxide can take the place of oxygen in the sixth position. This reaction, and why it makes carbon monoxide toxic, is covered in Ligand substitution and the Stability of Complexes.
Remember: You do not need to learn or draw the structure of haem. Know that haemoglobin is an iron(II) complex and that oxygen bonds to the iron.
Common Exam Points
- A ligand donates a lone pair to the metal ion. Say “lone pair” and name the bond as a co-ordinate bond.
- The co-ordination number is the number of co-ordinate bonds to the metal, not the number of ligands.
- [Ni(en)₃]²⁺ has three ligands but a co-ordination number of 6.
- Small ligands (H₂O, NH₃) give octahedral complexes with 90° angles. The larger Cl⁻ gives tetrahedral complexes with 109.5° angles.
- Cisplatin is square planar, 90°, and has no overall charge.
- [Ag(NH₃)₂]⁺ is linear with a bond angle of 180°.
- Bond water through O and ammonia through N in any drawing.
- To find the oxidation state of the metal, subtract the total ligand charge from the overall charge.
- Haemoglobin is an iron(II) complex containing a multidentate ligand.
FAQs
Use these quick answers to check ligands, the co-ordination number and the shapes of complex ions.
What is a ligand?
A ligand is a molecule or ion that donates a lone pair of electrons to a central metal ion, forming a co-ordinate bond. Water, ammonia and chloride ions are monodentate: each one forms a single bond to the metal ion.
Is the co-ordination number the same as the number of ligands?
No. The co-ordination number is the number of co-ordinate bonds to the central metal ion. In [Ni(en)₃]²⁺ three bidentate ligands form six bonds, so the co-ordination number is 6. In [Cu(EDTA)]²⁻ one ligand forms all six.
Why is [CuCl₄]²⁻ tetrahedral when [Cu(H₂O)₆]²⁺ is octahedral?
Chloride ions are larger than water molecules, so only four fit round the Cu²⁺ ion. Four ligands give a tetrahedral shape with bond angles of 109.5°. Six small ligands, such as H₂O or NH₃, give an octahedral shape with 90° angles.
How do I work out the oxidation state of the metal in a complex?
The charge on the complex equals the oxidation state of the metal plus the total charge of the ligands. In [CoCl₄]²⁻ four Cl⁻ ligands total 4−, so cobalt is +2. In [Cr(NH₃)₆]³⁺ ammonia is neutral, so chromium is +3.
Which complex ions are linear?
Silver(I) forms the linear complex [Ag(NH₃)₂]⁺, with a co-ordination number of 2 and a bond angle of 180°. It is the complex ion in Tollens’ reagent, which is used to distinguish aldehydes from ketones.
Copyright notice: This OLS revision content, including the explanations, layout, diagrams, tables and embedded learning structure, is authored for Online Learning System by Dr. Mohammed Al-Fatah. It may not be copied, reproduced, redistributed or adapted without written permission.
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