Transition Metal Complexes (Edexcel International A Level Chemistry)

• 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 dative covalent / co-ordinate bond with a transition metal by donating a pair of electrons to the bond
  • This is the definition of a Lewis base - electron pair donor
• This means ligands have a negative charge or a lone pair of electrons capable of being donated
  
  • This definition may seem familiar: a ligand is the same as a nucleophile
• Different ligands can form different numbers of dative covalent / co-ordinate bonds to the central metal ion in a complex
  • Some ligands can form one dative covalent / co-ordinate bond to the central metal ion
  • Other ligands can form two dative covalent / co-ordinate bonds, and some can form multiple dative covalent / co-ordinate bonds
• Co-ordination number is number of dative covalent / co-ordinate  bonds to the central metal atom or ion

Common Ligands

• Water molecules frequently act as ligands. Each water molecule makes a single bond with the metal ion using one of the lone pairs on the oxygen atom
• The lone pair is donated to the partially filled d-subshell of the transition metal ion

Table Showing Examples of Common Monodentate Ligands

Representing complex ions

• Square brackets are used to group together the ligands and metal ion in a representation of the geometrical arrangement
• The overall charge on the complex ion is the sum of the oxidation states of all the species present
• If the ligands are neutral then the overall charge will be the same as the oxidation state of the metal ion

Examples of complexes with monodentate ligands

Naming complexes

• Complexes are named in the following way
• If the overall ion is a cation then the nomenclature is:

Prefix for number of ligands/ligand name/element/oxidation number

• The prefixes are the same ones used in organic chemistry: di, tetra, hexa for 2, 4 & 6 respectively (3 & 5 are rarely encountered except in mixed ligand complexes)
• If the overall ion is an anion, the name of element is modified to have the name ending 'ate' and sometimes Latin word stems are used
  • tetrachlorcuprate(II)
  • hexaaquairon(II)
  • hexaamminecobalt(II)
  • tetracyanonickelate(II)
• Using the examples in the illustration above, the names are:
• Notice in these examples that
  • cuprate( Latin - cuprum) and nickelate are used in place of copper and nickel as they are anions
  • Ammonia takes the prefix ammine as a ligand, which is spelt with a double 'm' unlike the functional group amine