Complex ions, ligands and shapesEdexcel International A Level Chemistry: Revision notes

A ligand is a species that donates a lone pair of electrons to a central 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. The coordination number is the number of dative bonds from ligands to the central metal ion.

Examples: [Cu(H₂O)₆]²⁺ and [Cu(NH₃)₄(H₂O)₂]²⁺ both have coordination number 6.

The overall charge is the charge on the metal ion plus the charges on the ligands: [CoCl₄]²⁻ is Co²⁺ + 4(−1) = 2−, and [Cr(OH)₆]³⁻ is Cr³⁺ + 6(−1) = 3−.

Monodentate ligands form one dative bond each. H₂O, NH₃ and OH⁻ are all monodentate: each has a lone pair on its oxygen or nitrogen atom.

These small ligands normally surround a metal ion in six-fold coordination, giving an octahedral shape with bond angles of 90°.

Examples: [Fe(H₂O)₆]²⁺, [Ni(NH₃)₆]²⁺ and [Cr(OH)₆]³⁻.

Larger ligands such as Cl⁻ cannot fit six around a metal ion, so they form complexes with coordination number 4, usually tetrahedral with bond angles of 109.5°, e.g. [CoCl₄]²⁻ and [CuCl₄]²⁻.

A ligand exchange from water to chloride changes the coordination number:

[Cu(H₂O)₆]²⁺ + 4Cl⁻ → [CuCl₄]²⁻ + 6H₂O

Some four-coordinate complexes are square planar, with 90° angles. Cisplatin, [Pt(NH₃)₂Cl₂], is square planar. In the cis isomer the two chloride ligands are adjacent; in the trans isomer they are opposite. Only the cis isomer is used to treat cancer, and it is supplied as a single isomer, not mixed with the trans form.

A bidentate ligand has two donor atoms and forms two dative bonds. Ethane-1,2-diamine, NH₂CH₂CH₂NH₂ (en), donates a lone pair from each nitrogen atom. Three en ligands give coordination number 6, e.g. [Ni(en)₃]²⁺.

EDTA⁴⁻ is hexadentate: it forms six dative bonds using lone pairs on two nitrogen atoms and four oxygen atoms. One EDTA⁴⁻ ion therefore gives coordination number 6.

Pb²⁺ + EDTA⁴⁻ → [Pb(EDTA)]²⁻

Ions such as these that bond through several donor atoms are polydentate. EDTA is used to remove toxic metal ions from the body.

Ligand exchange is a reaction in which one ligand is replaced by another.

Haemoglobin is an iron(II) complex with a polydentate ligand (the structure of haem is not needed). An O₂ molecule bonds to the iron(II) ion as a ligand and is carried round the body.

Carbon monoxide is also a ligand and bonds more strongly than O₂. It replaces O₂ by ligand exchange:

Hb(O₂) + CO → Hb(CO) + O₂

Haemoglobin can then no longer carry oxygen, so cells are starved of oxygen and carbon monoxide is poisonous.