Here, Cr has zero oxidation state, as has zero oxidation state in the compound. So, at zero oxidation state the electronic configuration of Cr is [Ar] 4s1,3d5 that means Cr has 6 valencies, as Cr in this state is stable due to half-filled orbitals. Now, for 6 valencies, 6CO is required. Therefore, x=6 The compound is Cr(CO)6.