A complex ion is formed when one or more ligands bond to a central metal ion through coordinate bonds. It consists of the central metal ion and the surrounding ligands.

An example of a complex ion is [Cr(H₂O)₆]³⁺; the oxygen atom of each water molecule donates a lone pair of electrons to the central Cr³⁺ ion:

A ligand is a molecule or ion that donates a pair of electrons to a central metal ion, forming a coordinate bond.

Transition elements form complex ions because their partially filled, energetically accessible d-orbitals can accept lone pairs of electrons from ligands.

The bond that forms between a ligand and a central metal ion is called a coordinate bond or dative covalent bond. 

The coordination number indicates the number of coordinate bonds attached to the central metal ion in a complex ion.

A complex ion is enclosed inside square brackets, with the overall charge of the complex shown outside the brackets.

Ligands are often represented inside round brackets, with the number of ligands shown outside the brackets.

An example of a complex ion is [Cu(NH₃)₄]²⁺.

A monodentate ligand can donate one pair of electrons to the central metal ion, forming a single coordinate bond.

In contrast, a bidentate ligand can donate two pairs of electrons, forming two coordinate bonds with the central metal ion.

A complex ion with a coordination number of six has an octahedral shape with bond angles of 90°.

The octahedral shape of [Mn(H₂O)₆]²⁺ is shown below.

Complex ions with a coordination number of four can adopt either a tetrahedral shape (with bond angles of 109.5°), or a square planar shape (with bond angles of 90°).

Smaller central metal ions can accommodate a higher coordination number as they can fit more ligands around them, while larger metal ions tend to have lower coordination numbers.

Oxidation state of metal cation = Total oxidation state − Sum of oxidation states of all ligands