why sigma bonds are taken into account for the bond formation between central metal ion and ligand?

Werner's postulates of cordination theory  tells that central metal ion hasa fixed no. of secondary valency i.e,  a fixed cordination no. 

codination no. is the no. of ligands the central metal ion can bond to.secondary valencies of the central metal ion are satisfied by sigma bonds that is why sigma bonds are involved in bonding between metal ion and the ligand.