Ligands and Complex Ions

A complex ion is a central metal ion (usually a transition metal) surrounded by a number of molecules or ions called ligands, all held together by dative covalent bonds. Because transition metal ions have empty low-energy orbitals, they act as electron pair acceptors, while ligands act as electron pair donors. This lesson focuses on identifying ligands, describing how they bond, and working out the coordination number and overall charge of a complex ion.

A ligand is any molecule or ion that has at least one lone pair of electrons available to donate into an empty orbital on a central metal ion. The bond formed is a dative covalent bond (also called a coordinate bond), where both electrons in the shared pair come from the ligand rather than one from each atom.

Common examples of ligands include:

• Water, \( H_2O \)
• Ammonia, \( NH_3 \)
• Chloride ion, \( Cl^- \)
• Cyanide ion, \( CN^- \)
• Hydroxide ion, \( OH^- \)

Each of these has an atom with a lone pair (oxygen, nitrogen, chlorine or carbon) that can be donated to the central metal ion.

Denticity describes how many donor atoms on a single ligand bond to the central metal ion at the same time.

• Monodentate ligands have one donor atom, so they form only one dative bond per ligand. Examples: \( H_2O \), \( NH_3 \), \( Cl^- \), \( CN^- \).
• Bidentate ligands have two donor atoms and form two dative bonds to the same metal ion. Ethylenediamine, often abbreviated \( en \), \( NH_2CH_2CH_2NH_2 \), donates through both nitrogen atoms. 1,10-phenanthroline (\( phen \)) is another common bidentate ligand, donating through two nitrogen atoms in its ring system.
• Polydentate (multidentate) ligands have several donor atoms. EDTA (ethylenediaminetetraacetic acid), usually present as the \( EDTA^{4-} \) ion, is hexadentate: it donates through two nitrogen atoms and four oxygen atoms, forming six dative bonds to a single metal ion.

A ligand that forms multiple bonds to the same metal ion, like a bidentate or polydentate ligand, creates a ring structure called a chelate, and the metal complex is described as a chelate complex.

The coordination number of a complex ion is the total number of dative bonds formed between the central metal ion and its surrounding ligands. This is not the same as the number of ligands, since bidentate and polydentate ligands each contribute more than one bond.

For example:

• \( [Cu(NH_3)_4]^{2+} \) has four monodentate ligands, giving a coordination number of 4.
• \( [Ni(en)_3]^{2+} \) has three bidentate ligands, each forming two bonds, giving a coordination number of \( 3 \times 2 = 6 \).
• \( [Ca(EDTA)]^{2-} \) has one hexadentate ligand, giving a coordination number of 6.

The coordination number largely determines the shape of the complex ion. A coordination number of 6 usually gives an octahedral shape, a coordination number of 4 can give either tetrahedral or square planar shapes, and a coordination number of 2 gives a linear shape.

The overall charge of a complex ion is the sum of the charge on the central metal ion and the total charge contributed by all the ligands. Neutral ligands such as \( H_2O \) and \( NH_3 \) contribute zero charge, while charged ligands such as \( Cl^- \) or \( CN^- \) contribute their own charge.

Worked example: Find the oxidation state of iron in \( [Fe(CN)_6]^{4-} \).

Each cyanide ligand carries a charge of \( -1 \), and there are six of them, contributing \( 6 \times (-1) = -6 \). Let the iron oxidation state be \( x \):

\( x + (-6) = -4 \)

\( x = +2 \)

So iron is present as \( Fe^{2+} \) in this complex.

Once you can identify ligands, denticity, and coordination number, the next step is understanding how one ligand can be swapped for another around the same metal ion, which is covered in ligand exchange reactions. The ability of transition metals to form complex ions with different ligands also underpins their role in transition metal catalysts, where ligand binding and exchange at the metal surface or active site drives catalytic activity.

Related lessons

• Uk A Level Chemistry