(b) If cation is complex, then we first name ligand in alphabetical order and then central atom with oxidation state remain numeral in common brackets.
$\left[\mathrm{Cr}(\mathrm{en})_{2} \mathrm{Br}_{2}\right] \mathrm{Br}-$ is named as dibromidobis (ethylenediamine) chromium(III) bromide:
Ligands are : Br (two), ethyenediammine (two).
Central ion: $\mathrm{Cr}(\mathrm{III})$ or $\mathrm{Cr}^{3+}$
Counter ion: $\mathrm{Br}^{-}$
$(a)$ $\left( NH _{4}\right)_{2}\left[ Ce \left( NO _{3}\right)_{6}\right]$
$(b)$ $Gd \left( NO _{3}\right)_{3}$ અને
$(c)$ $Eu \left( NO _{3}\right)_{3}$
$(en =$ ઇથેન $-1, 2-$ ડાયએમાઇન$)$
(Atomic nos. $Mn = 25,\,Fe = 26,\,Co = 27$)