For the complex $Cr(CO)_x$,find the value of $x$?

The $EAN$ of platinum in potassium hexachloroplatinate $(IV)$ is :

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

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

The Effective Atomic Number $(EAN)$ of $Ni$ in $Ni(CO)_4$ is:

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 : $V=23$,$Cr=24$,$Mn=25$,$Fe=26$,$Co=27$,$Ni=28$,$Cu=29$,$Ir=77$ ]