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

What is a complex ion?

A central metal ion surrounded by ligands.

What is coordination number?

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

Name three monodentate ligands.

H₂O, NH₃ and OH⁻ (also Cl⁻).

What shape do six small ligands give?

Octahedral, with bond angles of 90°.

Why do chloride ligands give tetrahedral complexes?

Chloride ions are large, so only four fit around the metal ion.

Why is cisplatin supplied as one isomer?

Only the cis isomer is the active anticancer drug, so it must not be mixed with the trans form.

What is a bidentate ligand? Give an example.

A ligand forming two dative bonds, e.g. ethane-1,2-diamine, NH₂CH₂CH₂NH₂.

How many dative bonds does EDTA⁴⁻ form?

Six (hexadentate), from two N atoms and four O atoms.

Why is carbon monoxide toxic?

It replaces O₂ on the iron(II) in haemoglobin by ligand exchange, so oxygen cannot be carried.

Complex ions, ligands and shapesEdexcel International A Level Chemistry: Flashcards

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What is a ligand?

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What is a ligand?: A species that donates a lone pair of electrons to a metal ion, forming a dative covalent bond.
What is a complex ion?: A central metal ion surrounded by ligands.
What is coordination number?: The number of dative bonds from ligands to the central metal ion.
Name three monodentate ligands.: H₂O, NH₃ and OH⁻ (also Cl⁻).
What shape do six small ligands give?: Octahedral, with bond angles of 90°.
Why do chloride ligands give tetrahedral complexes?: Chloride ions are large, so only four fit around the metal ion.
Bond angle in a tetrahedral complex?: 109.5°.
Shape of cisplatin?: Square planar.
Why is cisplatin supplied as one isomer?: Only the cis isomer is the active anticancer drug, so it must not be mixed with the trans form.
What is a bidentate ligand? Give an example.: A ligand forming two dative bonds, e.g. ethane-1,2-diamine, NH₂CH₂CH₂NH₂.
How many dative bonds does EDTA⁴⁻ form?: Six (hexadentate), from two N atoms and four O atoms.
Why is carbon monoxide toxic?: It replaces O₂ on the iron(II) in haemoglobin by ligand exchange, so oxygen cannot be carried.

Exam questions on Complex ions, ligands and shapes

When an excess of aqueous ammonia is added to aqueous copper(II) sulfate, some of the water ligands around the copper(II) ion are replaced and the complex ion [Cu(NH₃)₄(H₂O)₂]²⁺ forms.
Explain why ammonia is able to act as a ligand.2 marks
Cobalt(II) ions form the complex ion [Co(H₂O)₆]²⁺ in water. When concentrated hydrochloric acid is added, the complex ion [CoCl₄]²⁻ forms.
Suggest why the cobalt(II) ion bonds to four chloride ligands but to six water ligands.2 marks
Cisplatin, [Pt(NH₃)₂Cl₂], is a neutral, square planar complex of platinum used in cancer treatment. It is supplied to hospitals as a single isomer.
State the coordination number of platinum in cisplatin and deduce the oxidation number of platinum in it. State the shape of the 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).