General Properties of Transition Metals

Specification Reference Inorganic chemistry, Transition metals 3.2.5.1

Quick Notes

Transition metals have an incomplete d sub-shell in their atoms or ions (first row Ti–Cu).
Key properties of transition metals include:
- Complex formation – transition metals form complexes with ligands.
- Formation of coloured ions – due to d-orbital electron transitions.
- Variable oxidation states – they can form multiple oxidation states.
- Catalytic activity – can often act as catalysts.
Ligands are molecules or ions that donate a pair of electrons to a metal ion to form a co-ordinate (dative covalent) bond.
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.

Full Notes

Transition metals and complex ions have been outlined in more detail here.
This page is just what you need to know for AQA A-level Chemistry :)

What Are Transition Metals?

Transition metals are elements in the d-block that have an incomplete d sub-shell in their atoms or ions.

AQA A-Level Chemistry overview diagram of first-row transition metals Ti to Cu with headline properties

For A-Level students, the AQA syllabus focuses on Ti–Cu.

Titanium (Ti)
Vanadium (V)
Chromium (Cr)
Manganese (Mn)
Iron (Fe)
Cobalt (Co)
Nickel (Ni)
Copper (Cu)

Key Properties of Transition Metals

Complex Formation

Transition metals form complex ions when surrounded by ligands.

A complex ion consists of a central metal ion bonded to ligands via co-ordinate (dative covalent) bonds.

Formation of Coloured Ions

Transition metal ions are often coloured in solution.

Variable Oxidation States

Transition metals can form multiple oxidation states.

This happens because d-electron orbitals are close in energy, allowing different numbers of electrons to be lost.

Element	Common oxidation states (examples)
Ti	+2, +3, +4
V	+2, +3, +4, +5
Cr	+2, +3, +6
Mn	+2, +4, +7
Fe	+2, +3
Co	+2, +3
Ni	+2, +3
Cu	+1, +2

Catalytic Activity

Transition metals and their compounds act as catalysts due to their ability to change oxidation states and adsorb reactants onto their surface.

Catalyst	Industrial / reaction use
Fe (solid iron)	Haber process (N₂ + 3H₂ ⇌ 2NH₃)
V₂O₅	Contact process (2SO₂ + O₂ ⇌ 2SO₃)
Ni (finely divided)	Hydrogenation of alkenes
MnO₂	Catalytic decomposition of H₂O₂

Remember – Catalysts lower activation energy and speed up reactions.

Ligands, Complex Ions, and Co-ordination Number

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) are able to 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.

AQA A-Level Chemistry diagram showing H2O acting as a monodentate ligand via oxygen lone pair

Types of Ligand (see ligand substitution for more detail):

Monodentate: Donates one pair of electrons (e.g., H₂O, NH₃, Cl⁻).
Bidentate: Donates two pairs of electrons (e.g., ethane-1,2-diamine).
Multidentate: Donates multiple pairs of electrons (e.g., EDTA⁴⁻).

What Is a Complex Ion?

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

Example Example complex ion: [Cu(H₂O)₆]²⁺ (hexaaquacopper(II) ion)

AQA A-Level Chemistry diagram of the hexaaquacopper(II) complex [Cu(H2O)6]2+

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

AQA A-Level Chemistry annotation showing complex ion written in brackets with overall charge as superscript

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.

AQA A-Level Chemistry overview of common complex geometries: octahedral, tetrahedral, square planar, linear

Co-ordination number	Common geometry	Typical example
6	Octahedral	[M(H₂O)₆]ⁿ⁺
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).

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 in).

Summary Table

Property	What it means
Complex formation	Ligands donate lone pairs to a central metal ion, forming co-ordinate bonds
Coloured ions	d-electron transitions absorb visible light
Variable oxidation states	Similar energy d-orbitals allow different numbers of electrons to be lost
Catalysis	Change oxidation state and/or adsorb reactants to lower E_a
Co-ordination number	Number of co-ordinate bonds around the metal centre