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

The effective atomic number $(EAN)$ of iron in the brown ring complex is:

$EAN$ of the central metal in the complexes $K_2[Ni(CN)_4]$,$[Cu(NH_3)_4]SO_4$ and $K_2[PtCl_6]$ are respectively.

The effective atomic number $(EAN)$ of cobalt in the complex $[Co(NH_3)_6]^{3+}$ is

Choose the correct option regarding the following complex compounds which follow $(F)$ and do not follow $(NF)$ the Sidgwick $EAN$ rule:
$I. [(Ph_3P)_2PdCl_2PdCl_2]$
$II. [NiBrCl(en)]$
$III. Na_4[Fe(CN)_5NOS]$
$IV. Cr(CO)_3(NO)_2$
$I-II-III-IV$

$a$. 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 $M(CO)_5$
$ii$. $MO_3F$