$3\left( {\mathop K\limits^{ + 1} } \right) + \left( {Fe} \right) + 6\left( {C{N^ - }} \right) = 0$
$\therefore 3\left( { + 1} \right) + X + 6\left( { - 1} \right) = 0$
$\therefore X = + 3$
$(i)$ $\left[ FeF _{6}\right]^{3-}$
$(ii)$ $\left[ Co \left( NH _{3}\right)_{6}\right]^{3+}$
$(iii)$ $\left[ NiCl _{4}\right]^{2-}$
$(iv)$ $\left[ Cu \left( NH _{3}\right)_{4}\right]^{2+}$