The given compound precipitate $A g C l$ when treated with $\mathrm{AgNO}_3$ this shows that atleast one $\mathrm{Cl}$ ion is present outside the coordination sphere. As only two ions are obtained in solution, so only one $\mathrm{Cl}^{-}$is present outside the sphere. Thus, the formula of the complex is $\left[\mathrm{Co}\left(\mathrm{H}_2 \mathrm{O}\right)_4 \mathrm{Cl}_2\right] \mathrm{Cl}$ and its IUPAC name is Tetraaquadichloridocobalt(III) chloride.