Considering ${\mathrm{H}}_2\mathrm{O}$ as a weak field ligand, find the number of unpaired electrons in $[\mathrm{Mn}{\left({\mathrm{H}}_2\mathrm{O}\right)}_6{]}^{2+}$ .
(At. no. of Mn = 25)

Since ${\mathrm{H}}_2\mathrm{O}$ is a weak ligand, it will not cause pairing of electrons in the metal ion ${\mathrm{Mn}}^{2+}$. Thus electronic configuration of the metal (${\mathrm{Mn}}^{2+}$) in the complex
will be

${{}_{25}\mathrm{Mn}}^{2+}:1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}3d^5$

i.e. 5 unpaired electrons.