13.1.3 Complex Ions

• 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 co-ordinate bond with a transition metal by donating a pair of electrons to the bond
  • This is also the definition of a Lewis base

• 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 bonds to the central metal ion in a complex
  • Some ligands can form one co-ordinate bond to the central metal ion
  • Other ligands can form two co-ordinate bonds, and some can form multiple dative bonds

• Co-ordination number is number of 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

Co-ordination number

• The coordination number is the number of coordinate bonds to the metal ions
• This number can be the same as the number of ligands if they are monodentate, but it could be different if bi- or multidentate ligands are present

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
• Using the examples in the illustration above, the names are:
  • tetrachlorcuprate(II)
  • hexaaquairon(II)
  • hexaamminecobalt(II)
  • tetracyanonickelate(II)

• 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

Bidentate Ligands

• Bidentate ligands can each form two co-ordinate bonds to the central metal 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 ion (C₂O₄^2- ) which is sometimes written as ‘ox’ (coming from the common name of oxalate)

Examples of complexes with bidentate ligands

Multidentate Ligands

• Some ligands contain more than two atoms with lone pairs of electrons
• These ligands can form more than two dative bonds and are said to be multidentate or polydentate ligands
• An example of a multidentate ligand is EDTA^4-, which is a hexadentate ligand as it forms 6 dative covalent bonds to the central metal ion
• EDTA comes from ethylenediaminetetraacetic acid, which is rather a mouthful so EDTA is easier!

Example of a polydentate ligand complex

• You can deduce the charge on a complex ion if you know the charges on the ligands and the oxidation state of the transition metal ion

Answer

The correct option is D

• • The water molecules do not contribute to the charge
  • The chloride ion, Cl^-, outside the square brackets must balance against the charge on the complex
  • [Cr(H₂O)₆]Cl₃  contains three chloride ions, so the charge on the complex is 3+
  • [CrCl(H₂O)₅]Cl₂.H₂O contains two chloride ions outside the square bracket, so the charge on the complex is 2+
  • [CrCl₂(H₂O)₄]Cl.2H₂O contains one chloride ion outside the square bracket, so the charge on the complex is 1+