The coordination number of an ion refers to the number of ligands molecules or ions that are directly bonded to the central metal ion. In the case of the actinide ion [Pu NO3 6]2-, the central metal ion is plutonium Pu .There are six nitrate NO3- ligands surrounding the plutonium ion, each forming a bond with it. Therefore, the coordination number of the actinide ion [Pu NO3 6]2- is 6.