Number of unpaired electron \(=5\)
So, \(Mn\) must be in \(+2\) oxidation state \(\left( Mn ^{+2}\right)\)
\(\Rightarrow 2+(-6)=-x\)
\(\Rightarrow-4=-x\)
\(\Rightarrow x=4\)
$(i)$ $\left[ FeF _{6}\right]^{3-}$
$(ii)$ $\left[ Co \left( NH _{3}\right)_{6}\right]^{3+}$
$(iii)$ $\left[ NiCl _{4}\right]^{2-}$
$(iv)$ $\left[ Cu \left( NH _{3}\right)_{4}\right]^{2+}$