A species with a lone pair of electrons that it donates to a central metal ion, forming a dative covalent bond.

What is a dative (coordinate) covalent bond?

A covalent bond in which both shared electrons come from one atom, here the ligand.

What is a complex ion?

A central metal ion surrounded by ligands, bonded by dative covalent bonds.

What is meant by coordination number?

The number of dative bonds from ligands to the central metal ion.

Why are H₂O, OH⁻ and NH₃ monodentate?

Each has one donor atom with a lone pair, forming one dative bond per ligand.

What shape and angle does a six-coordinate complex have?

Octahedral, with bond angles of 90°.

When do four-coordinate complexes form tetrahedral shapes?

Usually with relatively large ligands such as Cl⁻, e.g. [CuCl₄]²⁻; bond angle 109.5°.

Give an example of a square planar complex.

Cis-platin, Pt(NH₃)₂Cl₂; coordination number 4, bond angle 90°.

Why is cis-platin supplied as a single isomer?

Only the cis isomer is effective against cancer; a mixture would dilute the dose with unwanted trans-platin.

Give an example of a bidentate ligand.

Ethane-1,2-diamine, NH₂CH₂CH₂NH₂: each nitrogen donates a lone pair.

How many dative bonds does EDTA⁴⁻ form?

Six, so it is hexadentate (two N and four O donor atoms).

What type of complex is haemoglobin?

An iron(II) complex with a multidentate ligand.

How does carbon monoxide poison haemoglobin?

CO replaces O₂ by ligand exchange and bonds more strongly to Fe²⁺, so oxygen cannot be carried.

Complex ions and ligandsEdexcel A-Level Chemistry: Flashcards

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What is a ligand?

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What is a ligand?: A species with a lone pair of electrons that it donates to a central metal ion, forming a dative covalent bond.
What is a dative (coordinate) covalent bond?: A covalent bond in which both shared electrons come from one atom, here the ligand.
What is a complex ion?: A central metal ion surrounded by ligands, bonded by dative covalent bonds.
What is meant by coordination number?: The number of dative bonds from ligands to the central metal ion.
Why are H₂O, OH⁻ and NH₃ monodentate?: Each has one donor atom with a lone pair, forming one dative bond per ligand.
What shape and angle does a six-coordinate complex have?: Octahedral, with bond angles of 90°.
When do four-coordinate complexes form tetrahedral shapes?: Usually with relatively large ligands such as Cl⁻, e.g. [CuCl₄]²⁻; bond angle 109.5°.
Give an example of a square planar complex.: Cis-platin, Pt(NH₃)₂Cl₂; coordination number 4, bond angle 90°.
Why is cis-platin supplied as a single isomer?: Only the cis isomer is effective against cancer; a mixture would dilute the dose with unwanted trans-platin.
Give an example of a bidentate ligand.: Ethane-1,2-diamine, NH₂CH₂CH₂NH₂: each nitrogen donates a lone pair.
How many dative bonds does EDTA⁴⁻ form?: Six, so it is hexadentate (two N and four O donor atoms).
What is the charge on [Fe(EDTA)]²⁻ and how is it found?: 2−: (+2) + (−4) = −2.
What type of complex is haemoglobin?: An iron(II) complex with a multidentate ligand.
How does carbon monoxide poison haemoglobin?: CO replaces O₂ by ligand exchange and bonds more strongly to Fe²⁺, so oxygen cannot be carried.

Exam questions on Complex ions and ligands

A student dissolves iron(II) sulfate in water. The iron(II) ions are present as the pale green complex ion [Fe(H₂O)₆]²⁺.
Explain why water acts as a monodentate ligand.2 marks
Cis-platin, Pt(NH₃)₂Cl₂, is a square planar complex used in chemotherapy. Its isomer, trans-platin, has the same formula but is not used as a drug.
Explain why cis-platin is supplied for cancer treatment as a single isomer rather than as a mixture with trans-platin.2 marks
Nickel(II) ions form the green complex ion [Ni(H₂O)₆]²⁺ in water. Chemists can replace the water ligands with ethane-1,2-diamine, NH₂CH₂CH₂NH₂ (en), or with the ethylenediaminetetraacetate ion, EDTA⁴⁻.
Ethane-1,2-diamine is a bidentate ligand and forms the complex ion [Ni(NH₂CH₂CH₂NH₂)₃]²⁺. Explain what is meant by bidentate and deduce the coordination number of nickel in this complex.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).