The complex that violates the Sidgwick's rule of $EAN$ 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 : $V=23$,$Cr=24$,$Mn=25$,$Fe=26$,$Co=27$,$Ni=28$,$Cu=29$,$Ir=77$ ]

During the race of complex compounds,the finish line is fixed by achieving an $E.A.N.$ value equal to the atomic number of the corresponding inert gas. Which of the following complexes is present at the finish line?

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

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

Which of the following complex compounds/ions obeys the Sidgwick $EAN$ rule?