Transition metals tend to produce small highly charged positive ions. As a result of this characteristic transition metal ions tend to strongly attract polar molecules and negative ions. In water, metal ions such as Cu²⁺ and Zn²⁺ have a definite number of water molecules attracted to each ion. For example the copper ion has 6 water molecules geometrically spaced around it to form the complex ion Cu(H₂O)₆²⁺, while zinc has 4 water molecules around the ion to form the complex ion Zn(H₂O)₄²⁺ .

The water molecule attracted to the metal ion is known as a ligand and the combination of metal ion and ligands is known as a complex ion. For example Cu(H₂O)₆²⁺ has 6 water molecules (ligands) attracted to the ion by ion-dipole bonding.

Ligands can be ions as in the complex ion CoCl₄^2- In this case four chloride ligands are bonded to the metal ion via ion-ion bonding.