[પ.ક્ર.: $Cr= 24,\,Mn= 25, \,Fe= 26, \,Co= 27$]
$C r^{2+}$ and $F e^{2+}$ has same number of unpaired electrons
$C r^{2+}=>3 d^{4}$ four unpaired electrons
$F e^{2+}=>3 d^{6}$ four unpaired electrons
$A: \left[ Ni ( en )_{3}\right]^{2+}, B :\left[ Ni \left( NH _{3}\right)_{6}\right]^{2+}, C :\left[ Ni \left( H _{2} O \right)_{6}\right]^{2+}$
$(i)$ $[Ni(CO)_4], \,sp^3$
$(ii)$ $[Ni(CN)_4]^{2-}, \,sp^3$
$(iii)$ $[CoF_6]^{3-}, \,d^2sp^3$
$(iv)$ $[Fe(CN)_6]^{3-}, \,sp^3d^2$