Following Sidgwick's rule of $EAN$,the formula of the metal carbonyl $Co(CO)_x$ is:

What is the $EAN$ (Effective Atomic Number) of the metal ion in the hexacyanoferrate$(II)$ ion?

Among $V(CO)_6$,$Cr(CO)_5$,$Cu(CO)_3$,$Mn(CO)_5$,$Fe(CO)_5$,$[Co(CO)_3]^{3-}$,$[Cr(CO)_4]^{4-}$,and $Ir(CO)_3$,the total number of species isoelectronic with $Ni(CO)_4$ is. . . . . [Given atomic number : $V=23$,$Cr=24$,$Mn=25$,$Fe=26$,$Co=27$,$Ni=28$,$Cu=29$,$Ir=77$ ]

In $[Ni(NH_3)_4]SO_4$,the $E.A.N.$ of $Ni$ is

What is the $EAN$ of $Zn$ in $[Zn(NH_3)_4]^{2+}$?

The effective atomic number of $[A(NH_3)_6]Cl_3$ is $33$. What is the atomic number of element $A$?