The effective atomic number $(EAN)$ of $Co(CO)_4$ is $35$ and hence it is less stable. It attains stability by

What is the Effective Atomic Number $(EAN)$ for the metal ion in $[Fe(CN)_6]^{3-}$?

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

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

What is the $EAN$ of nickel in $Ni(CO)_4$?

The atomic number of cobalt is $27$. The $EAN$ of cobalt in $Na_{3}[Co(NO_{2})_{4} Cl_{2}]$ is