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*Revision Materials and Past Papers* 2.1.1 Atomic structure and isotopes 2.1.2 Compounds, formulae and equations 2.1.3 Amount of substance 2.1.4 Acids 2.1.5 Redox 2.2.1 Electron structure 2.2.2 Bonding and structure 3.1.1 Periodicity 3.1.2 Group 2 3.1.3 The halogens 3.1.4 Qualitative analysis 3.2.1 Enthalpy 3.2.2 Reaction Rates 3.2.3 Chemical equilibrium 4.1 Basic concepts and hydrocarbons 4.1.2 Alkanes 4.1.3 Alkenes 4.2.1 Alcohols 4.2.2 Haloalkanes 4.2.3 Organic synthesis 4.2.4 Analytical techniques 5.1.1 How fast? 5.1.2 How far? 5.1.3 Acids, bases and buffers 5.2.1 Lattice enthalpy 5.2.2 Enthalpy and entropy 5.2.3 Redox and electrode potentials 5.3.1 Transition elements 5.3.2 Qualitative analysis 6.1.1 Aromatic compounds 6.1.2 Carbonyl compounds 6.1.3 Carboxylic acids and esters 6.2.1 Amines 6.2.2 Amino acids, amides and chirality 6.2.3 Polyesters and polyamides 6.2.4 Carbon–carbon bond formation 6.2.5 Organic synthesis 6.3.1 Chromatography and qualitative analysis 6.3.2 Spectroscopy Required Practicals

5.3.1 Transition elements

Ligand substitutionLigands and complex ionsPrecipitation reactionsPropertiesRedox reactions

Ligands and Complex Ions

Specification Reference 5.3.1 (d)–(g)

Quick Notes

  • Ligands: Molecules/ions that donate lone pairs to form co-ordinate (dative covalent) bonds to a metal ion (e.g., H2O, NH3).
    • Monodentate ligands can form one co-ordinate bond.
    • Bidentate ligands form two co-ordinate bonds.
  • A complex consists of a central metal atom or ion surrounded by ligands.
  • Co-ordination number refers to the number of co-ordinate bonds to a central metal atom or ion in a complex.
    • Common geometries: octahedral (co-ordination number 6), tetrahedral or square planar (co-ordination number 4).
  • Stereoisomerism: Complex ions can exhibit stereoisomerism if the ligands can be arranged in more than one way.
    • Cis–trans in square planar and octahedral complexes.
      • OCR (A) A-Level Chemistry diagram showing cis-trans isomerism in octahedral complexes. OCR (A) A-Level Chemistry diagram showing cis-trans isomerism in square planar complexes.
    • Optical in octahedral with bidentate ligands.
      • OCR (A) A-Level Chemistry diagram of optical isomers in complexes with bidentate ligands.
  • Cisplatin is a square planar Pt(II) complex used in cancer therapy by binding to DNA.

Full Notes

What Is a Ligand?

A ligand is a molecule or ion that donates a pair of electrons to a central metal ion to form a co-ordinate bond.

Example: Water molecules (H2O) act as ligands as the oxygen atom can use one of its lone pairs of electrons to form a co-ordinate bond to a central metal atom or ion.

OCR (A) A-Level Chemistry diagram showing H2O acting as a monodentate ligand donating a lone pair to a metal centre.

Types of Ligand

(See ligand substitution for more detail)

Monodentate ligands can donate one lone pair to the central metal ion and form one co-ordinate bond.

OCR (A) A-Level Chemistry diagram of monodentate ligands including H2O, NH3 and Cl−.

Common examples include H2O (aqua), NH3 (ammine) and Cl− (chloride)

Bidentate ligands form two co-ordinate bonds, with two atoms donating one lone pair of electrons.

Common examples include ethan-1,2-diamine (H2NCH2CH2NH2, “en”) and ethanedioate (C2O42−):

OCR (A) A-Level Chemistry diagram of bidentate ligands including ethan-1,2-diamine and ethanedioate.

What Is a Complex Ion?

A complex ion consists of a central transition metal ion surrounded by ligands via co-ordinate bonds.

Example: [Cu(H2O)6]2+, the hexaaquacopper(II) ion.

OCR (A) A-Level Chemistry diagram of the [Cu(H2O)6]2+ complex ion.

The formulas of complex ions are written in square brackets with the overall charge of the complex ion shown as a superscript.

OCR (A) A-Level Chemistry diagram showing notation of complex ion charges.

Co-ordination Number

Co-ordination number refers to the number of co-ordinate bonds around a central metal ion and determines the geometry (shape) of the complex.

OCR (A) A-Level Chemistry diagram showing shapes of complexes depending on coordination number.
Co-ordination Number Shape Example
6 Octahedral [Fe(H2O)6]3+
4 Tetrahedral [CuCl4]2−
4 Square Planar [Pt(NH3)2Cl2]
2 Linear [Ag(NH3)2]+

Most complexes have a co-ordination number of 6 (octahedral) or 4 (tetrahedral or square planar).

Photo of Matt
Matt’s exam tip

Don’t confuse co-ordination number with the number of ligands in a complex ion! Sometimes the number of ligands can be different to the co-ordination number (for complexes with bidentate and multidentate ligands).

Stereoisomerism

Complexes can sometimes show stereoisomerism, depending on the shape of the complex and the ligands present:

Cis-Trans Isomerism

Cis-Trans isomerism occurs when two identical ligands are arranged either next to each other (cis) or opposite each other (trans).

Can happen in both octahedral and square planar complexes.

OCR (A) A-Level Chemistry diagram showing cis-trans isomerism in octahedral complexes. OCR (A) A-Level Chemistry diagram showing cis-trans isomerism in square planar complexes.

Example:[Cu(NH3)4(H2O)2]+

OCR (A) A-Level Chemistry diagram showing cis and trans forms of [Cu(NH3)4(H2O)2]+.

Example:Cisplatin ([Pt(NH3)2Cl2])

OCR (A) A-Level Chemistry diagram of cis and trans forms of cisplatin.

Optical Isomerism

Optical isomerism can occur when an octahedral complex has bidentate ligands (e.g., ethane-1,2-diamine).

OCR (A) A-Level Chemistry diagram of optical isomers in complexes with bidentate ligands.

Example: [Ni(H2NCH2CH2NH2)3]2+

OCR (A) A-Level Chemistry diagram of optical isomers of [Ni(en)3]2+.

Contains three bidentate ligands (en), forming a 3D structure. Two optical isomers are mirror images that cannot be superimposed. They rotate plane-polarised light in opposite directions.

Cis-Platin as an Anti-Cancer Drug

Cis-platin ([Pt(NH3)2Cl2]) has Cl− ligands on the same side (cis isomer).

OCR (A) A-Level Chemistry diagram showing cis-platin structure.

It binds to DNA in cancer cells, blocking replication.

OCR (A) A-Level Chemistry diagram showing cisplatin binding to DNA.

Trans-platin is not effective, as it doesn't bind DNA in the same way.

Summary