a
The complex ion \(\left[M n O_{4}\right]^{-}\) has no \(d\) -electron in the central metal atom.

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.