Ligands, Complex Ions and Their Shapes
A concise revision guide to complex ions: ligands and the coordinate (dative covalent) bond, coordination number, monodentate, bidentate and multidentate ligands, and the octahedral, tetrahedral and square planar shapes, with cis-platin and haemoglobin.
- 5.3.1(d)-i
- 5.3.1(d)-ii
- 5.3.1(e)-i
- 5.3.1(e)-ii
- 5.3.1(e)(i)
- 5.3.1(e)(ii)-a
- 5.3.1(e)(ii)-b
What these spec points say
- 5.3.1(d)-i explanation and use of the term ligand in terms of coordinate (dative covalent) bonding to a metal ion or metal
- 5.3.1(d)-ii bidentate ligands
- 5.3.1(e)-i use of the term complex ion
- 5.3.1(e)-ii use of the term coordination number
- 5.3.1(e)(i) examples of complexes with six-fold coordination with an octahedral shape
- 5.3.1(e)(ii)-a examples of complexes with four-fold coordination with a planar shape (see also 2.2.2 g–h)
- 5.3.1(e)(ii)-b examples of complexes with four-fold coordination with a tetrahedral shape (see also 2.2.2 g–h)
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 coordinate (dative covalent) 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 elements.
Ligands and Complex Ions
A transition element 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 coordinate (dative covalent) 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 coordinate (dative covalent) 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 coordinate (dative covalent) bond is drawn as a line or as an arrow pointing from the ligand to the metal.
Coordination number
Definition: The coordination number is the number of coordinate (dative covalent) bonds to the central metal atom or ion.
The coordination number counts bonds, not ligands. The two are equal only when every ligand forms one bond.
| Complex | Number of ligands | Bonds per ligand | Coordination 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 |
- The most common coordination numbers are 6 and 4.
- A coordination 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”) |
| 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.
- 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.
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 coordination number of 6.
Check: Ligands and Complex Ions
Use the terms ligand, complex ion, coordinate (dative covalent) bond and coordination 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 numbers 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 number of the metal.
| Ligand | Charge |
|---|---|
| H₂O, NH₃, H₂NCH₂CH₂NH₂ | 0 |
| Cl⁻, OH⁻ | −1 |
| EDTA⁴⁻ | −4 |
Worked example: the charge on a complex
Worked example: the oxidation number 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.
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 number is wrong.
Check: Coordination number, Charge and Oxidation number
Work out the coordination number, the overall charge and the oxidation number of the metal in complexes not used above.
Shapes of Complex Ions
The shape depends on the coordination number and on the size of the ligands. The bonding pairs repel each other and take up positions as far apart as possible.
| Shape | Coordination number | Bond angle | Typical ligands | Examples |
|---|---|---|---|---|
| octahedral | 6 | 90° | small: H₂O, NH₃, OH⁻ | [Cu(H₂O)₆]²⁺, [Fe(H₂O)₆]²⁺, [Cr(NH₃)₆]³⁺, [Cr(OH)₆]³⁻ |
| tetrahedral | 4 | 109.5° | large: Cl⁻ | [CuCl₄]²⁻, [CoCl₄]²⁻ |
| square planar | 4 | 90° | mostly complexes of platinum(II) | [Pt(NH₃)₂Cl₂], cis-platin |
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 coordination number falls from 6 to 4.
Octahedral [Cu(H₂O)₆]²⁺, tetrahedral [CuCl₄]²⁻ and square planar cis-platin, 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.
Cis-platin
Cis-platin, [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.
- Cis-platin is used in cancer treatment.
The two isomers, and how the drug acts, are covered in Stereoisomerism in Complex Ions.
Remember: Cis-platin has the same coordination 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 coordination 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 coordination 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 coordinate (dative covalent) 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 coordinate (dative covalent) bond.
- The coordination number is the number of coordinate (dative covalent) bonds to the metal, not the number of ligands.
- [Ni(en)₃]²⁺ has three ligands but a coordination number of 6.
- Small ligands (H₂O, NH₃) give octahedral complexes with 90° angles. The larger Cl⁻ gives tetrahedral complexes with 109.5° angles.
- Cis-platin is square planar, 90°, and has no overall charge.
- Bond water through O and ammonia through N in any drawing.
- To find the oxidation number 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 coordination 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 coordinate (dative covalent) bond. Water, ammonia and chloride ions are monodentate: each one forms a single bond to the metal ion.
Is the coordination number the same as the number of ligands?
No. The coordination number is the number of coordinate (dative covalent) bonds to the central metal ion. In [Ni(en)₃]²⁺ three bidentate ligands form six bonds, so the coordination 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 number of the metal in a complex?
The charge on the complex equals the oxidation number 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.
What do I need to know about haemoglobin?
Haemoglobin is an iron(II) complex. Four bonds to Fe²⁺ come from nitrogen atoms in haem, a multidentate ligand, and one comes from the protein globin. The sixth position holds an oxygen molecule, which is how blood carries oxygen.
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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