CIE A Level Chemistry (9701) 2019-2021

Revision Notes

6.2.4 Ligands

Define Ligand

  • A ligand is a molecule or ion that has one or more lone pairs of electrons
  • These lone pairs of electrons are used to form dative covalent bonds to a central transition metal atom or ion
  • Depending on whether the ligands can form one, two or more than two dative bonds from each ion or molecule to the transition metal ion they can be classified as monodentate, bidentate and polydentate ligands

Monodentate ligands

  • These are ligands that can form only one dative bond to the complex ion
  • Examples of monodentate ligands are:
    • Water (H2O) molecules
    • Ammonia (NH3) molecules
    • Chloride (Cl) ions
    • Cyanide (CN) ions

Chemistry of Transition Elements - Monodentate Ligands, downloadable AS & A Level Chemistry revision notes

Examples of complexes with monodentate ligands

Bidentate ligands

  • Bidentate ligands can form two dative bonds to the complex ion
  • This is because each ligand contains two atoms with lone pairs of electrons
  • Examples of bidentate ligands are:
    • 1,2-Diaminoethane (H2NCH2CH2NH2) which is also written as ‘en’
    • Ethanedioate (C2O42- ) ion which is also written as ‘ox’

Chemistry of Transition Elements - Bidentate Ligands, downloadable AS & A Level Chemistry revision notes

Examples of complexes with bidentate ligands

Polydentate ligands

  • Some ligands contain more than two atoms with lone pairs of electrons
  • These ligands can form more than two dative bonds to the complex ions and are said to be polydentate ligands
  • An example of a polydentate ligand is EDTA4- which is a hexadentate ligand as it forms 6 dative covalent bonds to the complex ion to form an octahedral complex

Chemistry of Transition Elements - Polydentate Ligands_1, downloadable AS & A Level Chemistry revision notes

Example of a polydentate ligand complex

Complex

  • A complex is a molecule or ion formed by a central metal atom or ion surrounded by one or more ligands
    • The central metal atom/ion is also known as the complex ion
    • The ligands are molecular or ions with one or more lone pairs of electrons
  • The ligands form a dative covalent bond with the complex ion by using its lone pair of electrons

Chemistry of Transition Elements - Ligand Complex Example, downloadable AS & A Level Chemistry revision notes

Example of a complex ion formed by a central iron(III) ion and six water molecule ligands

Geometry of transition element complexes

  • Depending on the size of the ligands and the number of coordinate (dative) bonds to the central metal ion, transition element complexes have different geometries

Linear

  • Metal ions with two coordinate bonds form linear complexes
  • The bond angles in these complexes are 180o
  • The most common example is a copper metal ion (Cu+) forming two covalent bonds with ammonia ligands

Chemistry of Transition Elements - Linear Complexes, downloadable AS & A Level Chemistry revision notes

Example of a linear complex

Tetrahedral

  • When there are four coordinate bonds the complexes often have a tetrahedral shape
    • Complexes with four chloride ions most commonly adopt this geometry
    • The bond angles in tetrahedral complexes are 109.5o

Chemistry of Transition Elements - Tetrahedral Complexes, downloadable AS & A Level Chemistry revision notes

Example of a tetrahedral complex

Square planar

  • Sometimes, complexes with four coordinate bonds may adopt a square planar geometry instead of a tetrahedral one
    • Cyanide ions (CN) are the most common ligands to adopt this geometry
    • An example of a square planar complex is cisplatin
  • The bond angles in a square planar complex are 90o

Chemistry of Transition Elements - Square Planar Complexes, downloadable AS & A Level Chemistry revision notes

Cisplatin is an example of a square planar complex

Octahedral

  • Octahedral complex are commonly formed when a metal forms six-coordinate bonds with smaller ligands
    • Examples of such ligands are water and ammonia molecules and hydroxide and thiocyanate ions
  • Larger ligands can also form octahedral complexes such as:
    • Bidentate ligands like 1,2-diaminoethane and
    • Polydentate ligands such as EDTA4-
  • The bond angles are 90o

Chemistry of Transition Elements - Octahedral Complexes, downloadable AS & A Level Chemistry revision notes

Examples of octahedral complexes

Types of ligands table

Chemistry of Transition Elements - Types of ligands table, downloadable AS & A Level Chemistry revision notes

Coordination number

  • The coordination number is the number of coordinate bonds that are formed by ligands with a transition metal ion in a complex
  • Some ligands can form only one coordinate bond with the central metal ion (monodentate ligands) whereas others can form two (bidentate ligands) or more (polydentate ligands)

Predicting complex ion formula & charge

  • The formula and charge of a complex ion can be predicted if the following are known:
    • The charge/oxidation state of the metal ion
    • The coordination number/geometry of the ligand

Worked Example

Predicting the formula and charge of a nickel complex

Give the formula and overall charge of the complex formed between Ni2+ and the hexadentate EDTA4- ligand

Answer

Step 1: Determine the number of ligands in the complex

Since EDTA4- is a hexadentate ligand (and forms six dative bonds with the Ni2+ ion) , one EDTA4-  ligand will form an octahedral complex with the metal ion

 

Step 2: Determine the overall charge on the complex

Since EDTA4- has a 4- charge and the charge on the Ni is 2+ so the overall charge of the complex is 2-

The formula of this transition metal complex will therefore be [Ni(EDTA)]2-

Worked Example

Predicting the formula and charge of a copper complex

Give the formula and overall charge of the complex formed between Cu2+ and water molecules with octahedral geometry

Answer

Step 1: Determine the number of ligands in the complex

Since H2O has octahedral geometry, this means that there are six water molecules in the complex each forming one dative bond with the Cu2+ ion

 

Step 2: Determine the overall charge on the complex

Water molecules are overall neutral and since the Cu2+ has a charge of 2+, the overall charge on the complex will be 2+

The formula of this transition metal complex will therefore be [Cu(H2O)6]2+

Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.
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