The Effective Atomic Number $(EAN)$ rule is less likely to be followed if the metal-ligand bond is of which type?

The $EAN$ of platinum in potassium hexachloroplatinate $(IV)$ 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 : $V=23$,$Cr=24$,$Mn=25$,$Fe=26$,$Co=27$,$Ni=28$,$Cu=29$,$Ir=77$ ]

What is the $EAN$ of $Co$ in $[Co(NH_3)_6]^{3+}$?

The $EAN$ of $Ni$ in $[Ni(CO)_4]$ is ....

Which of the following complexes does $NOT$ obey the $EAN$ rule?