Why hexaquamanganese(II)ion contains five unpaired electrons, while the hexacyno manganese (II)ion contains only one unpaired electron ?

Explanation

`H_2O` is a weak field ligand and `CN^-` is a strong field ligand. Thus in `[Mn(H_2O)_6]^(2+)`, pairing of unpaired electrons is not possible and it possesses five unpaired electrons.
In `[Mn(CN)_6]^(4-)` pairing of four unpaired electrons takes place and it possesses one unpaired electron.