b
\(\left[ Co \left( H _{2} O \right)_{6}\right]^{2+}+ NH _{3}(\) excess \() \rightarrow\left[ Co \left( NH _{3}\right)_{6}\right]^{3+}+6 H _{2} O\)

Diamagnetic \(\rightarrow\) Low spin complex

\(Co ^{3+} \Rightarrow 3 d ^{6} 4 s ^{0}\)

\(\Rightarrow t _{2 g }{ }^{6} e _{ g }{ }^{0}\)

Total number electrons \(=6\)