Lesson 15A.2

15A.2 Ligands, complex ions and coordination number Quiz: Pearson Edexcel Chemistry, Unit 15

20 questions

Written by Revision Ninja · Last reviewed 11 October 2026

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Lesson 15A.2, Ligands, complex ions and coordination number: 20 multiple choice questions for the Pearson Edexcel Chemistry (9CH0), Unit 15: Transition Metals, written with Revision Ninja.

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The 20 questions

  1. What is a ligand?

    • A molecule that has no lone pairs of electrons and so cannot bond to any metal ion at all
    • A metal ion that accepts electrons from a non-metal atom to form a covalent bond with it
    • A positive ion that bonds covalently to a halide and so is attached to the metal by a dative bond
    • A molecule or ion that donates a lone pair of electrons to a central metal ion to form a dative bond

  2. What type of bond forms between a ligand and the central metal ion?

    • A dative (coordinate) covalent bond, where both electrons come from the ligand
    • An ionic bond formed by transfer of an electron from the metal to the ligand atom
    • A metallic bond formed by delocalised electrons shared between the metal ion and the ligand
    • A hydrogen bond between water molecules and the ligand, with the metal at its centre

  3. What is a complex ion?

    • A salt containing two different metal cations
    • An ion made only of non-metal atoms
    • A metal atom held in a giant lattice with anions
    • A central metal ion surrounded by ligands bonded to it

  4. What is the coordination number of a complex ion?

    • The number of d electrons in the metal ion, which determines the colour and shape of the complex
    • The total charge on the complex ion, which is found by adding the charges of the metal and ligands
    • The number of dative bonds formed between the ligands and the central metal ion
    • The number of different ligands in the complex, counting each type of ligand only once

  5. What is a monodentate ligand?

    • A ligand that forms one dative bond to the central metal ion, such as H2O, NH3 or Cl-
    • A ligand with more than four donor atoms, such as EDTA4-, which binds the metal at many sites
    • A ligand that cannot donate any electrons to a metal, so it cannot form any bond to the metal ion
    • A ligand that forms two dative bonds at once, such as NH2CH2CH2NH2, binding two sites on the metal

  6. Which is an example of a bidentate ligand?

    • EDTA4-, a hexadentate ligand that binds through several donor atoms at once
    • NH2CH2CH2NH2
    • Cl-, a monodentate halide ion that binds through one chlorine atom
    • H2O, a neutral monodentate ligand that binds through one oxygen atom

  7. Why do complexes with six-fold coordination, such as those with H2O, OH- and NH3 ligands, usually have an octahedral shape?

    • The metal ion is too small to hold more than two ligands, so the complex must be linear in shape
    • Six ligands arrange themselves as far apart as possible around the metal ion, giving an octahedral geometry
    • Each ligand must be bidentate, forming a linear structure in which every pair of ligands is opposite
    • Six ligands always form a planar hexagon around the metal, with all bond angles equal to 120 degrees

  8. What is the coordination number of the cobalt ion in [CoCl4]2-?

    • 6
    • 8
    • 4
    • 2

  9. What is the coordination number of the copper ion in [Cu(NH3)4(H2O)2]2+?

    • 2
    • 6
    • 8
    • 4

  10. What shape do tetrahedral complexes with large ligands such as Cl- usually take?

    • Trigonal bipyramidal, because five ligands always form a stable arrangement
    • Linear, because two ligands are always opposite each other
    • Tetrahedral, because four large ligands give a coordination number of 4
    • Octahedral, because the metal always has six bonds

  11. What is the oxidation state of cobalt in [CoCl4]2-?

    • +2
    • +6
    • +4
    • -2

  12. Haemoglobin contains a metal centre bound to a multidentate ligand. Which description is correct?

    • An iron(III) complex that carries carbon dioxide
    • A zinc(II) complex in which carbon monoxide is a permanent ligand
    • A copper(II) complex with a monodentate ligand
    • An iron(II) complex containing a multidentate ligand within the haem group

  13. Which statement about cisplatin is correct?

    • Cisplatin is a square planar complex with coordination number 4
    • Cisplatin is a linear complex with coordination number 2
    • Cisplatin is an octahedral complex with coordination number 6
    • Cisplatin is a tetrahedral complex with coordination number 8

  14. Why does EDTA4- form a more stable complex than six separate monodentate ligands such as NH3?

    • EDTA4- has a higher charge, so it is always more covalent and so forms a stronger metal bond
    • EDTA4- donates more electrons per bond than NH3, which makes each bond much stronger than before
    • A multidentate ligand gives a large positive entropy change for the system, making the complex more stable
    • EDTA4- lowers the entropy of the system when it binds, because it fixes the solvent molecules

  15. What change occurs when [Co(H2O)6]2+ is converted to [CoCl4]2- in concentrated chloride solution?

    • The coordination number stays at 6, and the colour stays pink
    • The coordination number rises from 4 to 6, and the colour changes from blue to pink
    • The coordination number falls from 6 to 4, and the colour changes from pink to blue
    • The coordination number falls from 6 to 2, and the solution turns colourless

  16. Which species is a multidentate ligand with six donor atoms?

    • Cl-
    • H2O
    • EDTA4-
    • NH2CH2CH2NH2

  17. Which complex is more stable: [Cu(H2O)4]2+ or [Cu(NH2CH2CH2NH2)2]2+ in terms of ligand type?

    • [Cu(NH2CH2CH2NH2)2]2+, because it has fewer ligands and so is less stable
    • [Cu(NH2CH2CH2NH2)2]2+, because the bidentate ligands chelate and increase the entropy of the system
    • They are equally stable, because both complexes have four bonds to copper and the same charge
    • [Cu(H2O)4]2+, because water is neutral and so binds the copper more strongly than the diamine

  18. Why can replacing H2O by Cl- reduce the coordination number of a transition metal?

    • Cl- is smaller than water, so more chloride ions fit round the metal and the coordination number rises
    • Cl- ions always bind as bidentate ligands, so they fill two sites on the metal at once every time
    • Larger, charged Cl- ions cannot pack as many around the metal as small, uncharged H2O molecules
    • Water is larger, so it binds more strongly and blocks fewer sites around the metal in every complex

  19. Why does a complex ion such as [Co(H2O)6]2+ appear coloured?

    • The complex contains no d electrons, so it reflects all light
    • Cobalt is a Group 2 metal, which always gives a pink colour
    • Ligands split the d orbital energies
    • The colour comes from electron transfer from water into cobalt only

  20. Why does carbon monoxide bind to haemoglobin so damagingly?

    • O2 forms a covalent bond with the protein only, leaving the iron free
    • CO displaces oxygen from the iron(II) centre, forming carboxyhaemoglobin
    • CO displaces the haem group from the iron, releasing it into the blood
    • O2 displaces CO, which increases the oxygen carried in blood

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