The electronic configuration of $Mn$ is $[ Ar ] 3 d^{5} 4 s ^{2} .$ In $MnO _{4}^{-}$, oxidation number of $Mn$ is +7 i.e. all

the $3 d$ and $4 s$ electrons are lost to form $\left[M n O_{4}\right]^{-}$ complex. Hence, it has no $d$ -electron.

The electronic configuration of $Mn ( V \|)$ is $[A r] 3 d^{0} 4 s^{0} .$

The complex ions $\left[ Co \left( NH _{3}\right)_{6}\right]^{3+},\left[ Fe ( CN )_{6}\right]^{3-}$ and $\left[ Cr \left( H _{2} O \right)_{6}\right]^{3+}$ have 6, 5 and 3 $d$ -electrons

respectively in the central metal atom.