A solution of Sodium metal in liquid ammonia is strongly reducing agent due to

When sodium metal is dissoloved in liquid ammonia , 

$$\begin{gathered} \mathrm{Na}\hspace{0.17em}+\hspace{0.17em}(\mathrm{x}+\mathrm{y}){\mathrm{NH}}_3\to \left[{\mathrm{Na}}^{+}{\left({\mathrm{NH}}_3\right)}_{\mathrm{x}}\right] +\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\left[{\mathrm{e}}^{-}{\left({\mathrm{NH}}_3\right)}_{\mathrm{y}}\right] \\ \mathrm{Solvated} \mathrm{electrons} \end{gathered}$$

The ammoniated ( slovated electrons ) are responsible for reducing behaviour. 

Hence sodium metal in liquid ammonia is a strong reducing agent.