$ N{{H}_{3}} $ (ammonia) is a Lewis base (as it has a lone pair to donate), hence, it can form adduct with $ BaC{{l}_{3}} $ (a Lewis acid) by dative or co-ordinate bond. $ H--\underset{H}{\overset{H}{\mathop{\underset{|}{\overset{|}{\mathop{N}}}\,}}}\,:\xrightarrow{{}}\underset{Cl}{\overset{Cl}{\mathop{\underset{|}{\overset{|}{\mathop{B}}}\,}}}\,--Cl $