Effective Atomic Number $(EAN)$ of $Co$ in $Co_2(CO)_8$ will be

What is the effective atomic number $(EAN)$ of $Zn$ in $[Zn(NH_3)_4]SO_4$? (Atomic number of $Zn = 30$)

$a$. Answer the following questions:
$i$. Which element of the first transition series has the highest second ionisation enthalpy?
$ii$. Which element of the first transition series has the highest third ionisation enthalpy?
$iii$. Which element of the first transition series has the lowest enthalpy of atomisation?
$b$. Identify the metal and justify your answer:
$i$. Carbonyl $M(CO)_5$
$ii$. $MO_3F$

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

What is the oxidation state of cobalt in a coordination complex if its $EAN$ is $36$ and the value of $C.N.$ is $6$? (Given: Atomic number of cobalt = $27$).

The complex that violates the Sidgwick's rule of $EAN$ is