Complex Ions


Complex ions form when a metal ion is surrounded by ligands that coordinately bond to the metal ion forming a complex.

The transition metals are able to form complexes, where a metal ion is at the centre of multiple other compounds bonded to them. This happens as the central metal ion can accept electrons being donated to it to form a bond. This type of bond is known as a coordinated bond and is a form of covalent interaction. It forms when compounds with unbonded electrons donate their electrons to the central metal ion, these compounds or elements that donate the lone pairs are commonly called ligands. The number of ligands that can bond to a metal to form a complex varies because of their size and charge and also the size of the metal ion and the charge on the ion.

Ligands can also sometimes donate more than one pair of electrons to form more than one coordinated bond, these are often called multidentate ligands. For example water has two lone pars but as it is so small is only able to donate one lone pair and form one coordinated bond. However something like EDTA has many lone pairs and is a very big molecules and it can donate many lone pairs to form 6 coordinated bonds.

Depending on the number of ligands that are attached means the complex will have a different shape. The number of ligands that are bonded to a metal ion is called the coordination number. This determines the shape that the complex has and can also influence the colour of the complex. Different complexes have different bond angles based on the number and size of ligands attached to the metal ion. For example a complex formed from a copper ion and 6 water ligands would have a coordination of 6 and would have an octahedral shape with a 90 degree bond angle between the 6 ligands of water. But a complex of a copper 2+ ion with 4 chloride ions would have a coordination number of 4 as the chloride ions are much larger and you can fit fewer of them around the copper metal central ion due to their larger size this means it has a lower coordination number and a larger bond angle between the ligands of 109.5 degrees.

The charge and number of ligands can also influence the overall charge of the complex. These complexes are important in industry and nature. For example Iron is held in a complex in the protein structure of haemoglobin, industrial catalysts and also drugs.

Terms in section
Transition metals

Transition metals are the central section of the periodic table containing the majority of the metals. Also have d sub orbitals producing certain chemical properties


A complex is the term given to a central metal ion that is surrounded by atoms that are able to donate lone pairs of electrons to form bonds to the central metal ion. Can have a different number of atoms/molecules bonded to it.

Central metal ion

The central metal ion is the metal ion at the centre of a complex usually with a positive charge and a transition metal element


The electron is the smallest sub atomic particle that make up the atom. Has a negative charge and is located in shells that orbit the nucleus

Coordinated bond

A coordinated bond is the term given to a lone pair of electrons is donated from one atom/molecule to another which can accept the lone pair forming a covalent bond

Lone pair

A lone pair is a pair of electrons on an atom that are not involved in bonding


A ligand is a molecule or atom that can form a bond with a central metal ion, usually by donating a lone pair of electrons to form a coordinated bond


A multidentate ligand can form when more than 2 bonds with a central metal ion by donating more than one pair of electrons i.e EDTA

Coordination number

Coordination number is the number of ligands/molecules bonded to a central metal ion. This number determines the geometry, shape and other chemical properties of the complex

Bond angle

The bond angle is a measure of the angle between two bonds that is changed depending on the repulsion between lone pairs and bonded pairs helps to determine the shape of covalent compounds


The Chemistry of Transition Metal


Transition Metals as Colored Compounds