Ligands

• Ligands are molecules or ions that bond to a central metal ion to form a complex. They do this by donating a pair of electrons into the empty d-orbital of the metal ion.
• Typical examples include water (H2O), ammonia (NH3), cyanide (CN-) and chloride ions (Cl-).
• These entities are also termed as ‘Lewis bases’, as these are electron-pair donors.
• Ligands possess at least one unshared or lone pair of electrons for bond formation.
• They may form one or more coordinate bonds with the central metal ion. Those that form one bond are called ‘monodentate’ ligands, such as Cl-, whereas those that can form two or more bonds are termed ‘polydentate’ ligands, such as the ethylenediamine (en).
• Ligand substitution is a common reaction for transition metal complexes and it involves the replacement of one ligand by another.
• Spectroscopic properties of metal complexes can allow scientists to determine the type of ligands attached to the metal ion.
• Ligands also play a crucial role in determining the shape of a complex. The number of ligand attachments and the size of the ligands influence the geometry of the complex.
• They also influence the colour of a transition metal complex. This is because the energy gap between the d-orbitals varies with the type of ligand attached, changing the frequencies of light absorbed and the complementary colour seen.
• Ligands can cause splitting of d-orbitals leading to crystal field theory.
• Charged ligands will affect the overall charge of the complex. The charge of a complex ion is the sum of the charge on the metal ion plus the total charge on the ligands.
• Understanding different types of ligands such as neutral, negative and positive ligands, and their effects on observable properties, can help in predicting and understanding the behaviour of various transition metal complexes.