Complex ions and ligandsEdexcel A-Level Chemistry: Revision notes

A ligand is a species (an atom, ion or molecule) with a lone pair of electrons that it donates to a metal ion, forming a dative (coordinate) covalent bond. Both electrons in the bond come from the ligand.

A complex ion is a central metal ion surrounded by ligands, held by dative bonds, e.g. $[\mathrm{Fe(H_2O)_6}]^{2+}$. The overall charge is the charge on the metal ion plus the charges on the ligands.

Worked example. $[\mathrm{CuCl_4}]^{2-}$: $(+2) + 4 \times (-1) = -2$.

The coordination number is the number of dative bonds formed to the central metal ion (not the number of ligands, if some ligands form more than one bond).

$\mathrm{H_2O}$, $\mathrm{OH^-}$ and $\mathrm{NH_3}$ are monodentate ligands: each has one donor atom (O or N) which forms one dative bond per ligand. Cl⁻ is also monodentate.

The shape depends on the coordination number and the size of the ligands.

• Octahedral, coordination number 6, bond angles 90°: with small ligands such as $\mathrm{H_2O}$, $\mathrm{NH_3}$ and $\mathrm{OH^-}$, e.g. $[\mathrm{Cu(H_2O)_6}]^{2+}$.
• Tetrahedral, coordination number 4, bond angles 109.5°: usually with relatively large ligands such as $\mathrm{Cl^-}$, which are too big for six to fit round the metal ion, e.g. $[\mathrm{CuCl_4}]^{2-}$.
• Square planar, coordination number 4, bond angles 90°: e.g. cis-platin, $\mathrm{Pt(NH_3)_2Cl_2}$.

Cis-platin is a square planar complex of platinum(II): two $\mathrm{NH_3}$ and two $\mathrm{Cl^-}$ ligands, with the like ligands next to each other (at 90°). In trans-platin the like ligands are opposite each other (at 180°).

Cis-platin is used in cancer treatment, where it binds to DNA in tumour cells. The trans isomer does not have the same biological activity, so cis-platin is supplied as a single isomer, not a mixture: a mixture would contain less active drug and unwanted material.

A bidentate ligand has two donor atoms and forms two dative bonds, e.g. ethane-1,2-diamine (en), $\mathrm{NH_2CH_2CH_2NH_2}$ (each N has a lone pair). Three en ligands give an octahedral complex $[\mathrm{Ni(en)_3}]^{2+}$ with coordination number 6.

A multidentate ligand has several donor atoms. EDTA⁴⁻ is hexadentate: two N and four O⁻ donors give six dative bonds, so one EDTA⁴⁻ ion fully surrounds a metal ion, e.g. $[\mathrm{Fe(EDTA)}]^{2-}$ with charge $(+2) + (-4) = 2-$.

Haemoglobin is an iron(II) complex with a multidentate ligand (the haem group; you do not need to know its structure). Oxygen binds to Fe²⁺ as a ligand by a dative bond and is carried round the body, then released: the bonding is reversible.

Carbon monoxide is also a ligand with a lone pair. It undergoes ligand exchange, replacing O₂, and forms a stronger dative bond with iron, so it is hard to displace. Haemoglobin can no longer carry oxygen and tissues are starved of oxygen, which is why CO is poisonous.

A typical preparation is the copper(II) complex tetraamminecopper(II) sulfate, made by ligand exchange: add aqueous ammonia to copper(II) sulfate solution until the pale blue precipitate dissolves to give a deep blue solution, then add ethanol, in which the complex is less soluble, so crystals form.

Separate the product by filtration (suction), wash with a little ethanol, dry, and calculate the percentage yield. Avoid strong heating, since ammonia is lost from the complex on heating.