Coordination Bonds (HL) (DP IB Chemistry): Revision Note

Coordination bonds

• Transition element ions can form complexes which consist of a central metal ion and ligands

• A ligand is a molecule or ion that forms a coordinate bond with a transition metal

• It does this by donating a lone pair of electrons to the metal ion

• Ligands are:

  • Lewis bases because they donate a lone pair of electrons

  • Nucleophiles because they are electron-rich and can donate a lone pair to an electron-deficient centre

Types of ligand

• Ligands can be classified based on the number of coordination bonds they can form to the central metal ion in a complex

  • Monodentate ligands can form one coordinate bond to the central metal ion

  • Bidentate ligands can form two coordinate bonds

  • Multidentate ligands can form multiple coordinate bonds

Common monodentate ligands

• Here are some examples of common monodentate ligands:

  • Water (H₂O)

    • Neutral molecule

    • Lone pair donated by oxygen

  • Ammonia (NH₃)

    • Neutral molecule

    • Lone pair donated by nitrogen

  • Chloride (Cl^-)

    • Negatively charged ion

    • Lone pair donated by chlorine

  • Cyanide (CN^-)

    • Negatively charged ion

    • Lone pair donated by carbon

  • Hydroxide (OH^-)

    • Negatively charged ion

    • Lone pair donated by oxygen

Bidentate ligands

• Bidentate ligands can two coordinate bonds with the central metal ion

  • This is because they contain two donor atoms, each with a lone pair of electrons

• Examples of bidentate ligands are:

  • 1,2-diaminoethane (H₂NCH₂CH₂NH₂), also known as ‘en’

  • Ethanedioate ion (C₂O₄^2- ), sometimes written as ‘ox’ (from the common name of oxalate)

Multidentate ligands

• Some ligands contain more than two donor atoms, each with a lone pair of electrons

• These ligands can form more than two coordinate bonds and are called multidentate or polydentate ligands

• An example of a multidentate ligand is EDTA^4-

  • EDTA is a hexadentate ligand

  • This means that it forms six coordinate bonds with the central metal ion

• EDTA stands for EthyleneDiamineTetraAcetic acid, which is why it is abbreviated to EDTA

Representing complex ions

• Square brackets are used to group the metal ion and ligands in a complex ion

• The overall charge on the complex ion is the sum of the oxidation states of all species present

• If the ligands are neutral, the charge is the same as the oxidation state of the metal ion

 Complexes with monodentate ligands

Coordination number

• Coordination number is the number of coordinate bonds to the central metal ion

• For monodentate ligands, this equals the number of ligands

• For bidentate or multidentate ligands, the number of bonds may differ from the number of ligands

• Certain coordination numbers are associated with certain geometries

  • Coordination number 2:

    • Linear

  • Coordination number 4:

    • Tetrahedral (most commonly)

    • Square planar

  • Coordination number 6:

    • Octahedral

Charges of complex ions

• The charge of a complex ion depends on:

  • The charge of the central metal ion

  • The charge on each ligand

  • The coordination number

• If these factors are known, the charge on the complex ion can be calculated

Table showing the charges of complex ions