What is the co-ordination number of Ni in Ni(CO)4 ?

Ni(CO)4 is an complex compound having Ni as central atom and CO as ligands.According to Werner’s theory-complex compounds show two types of valencies-
1.Primary and
2.Secondary.
Secondary valency is equal to the coordination number which is defined as number of ligand donor atoms to which the metal is directly bonded.
In the above complex CO i.e carbonyl is an neutral ligand and the coordination number of the complex is 4.