Why does Ammonia(NH₃) acts as a ligand ?

Beacause it is having one lone pair of electrons on nitrogen............so it can donate one lone pair of electrons to lewis acid.......that is why it is acting as ligand

ammonia acts as a ligand since it has one lone pair on it as shown in the figure. thus, it can use this lone pair to form coordinate covalent bond with the transition elements to make coordintion compounds.

Legands are Lewis bases as they donate one or more lone pair of electrons to the central metal ion.Structure of Ammonia(NH3) is Trigonal pyramidal...as Ammonia has one lone pair of electrons on it it can donate that lone pair of electrons to Central metal ion which are Lewis acids as they are acceptors which have empty orbitals...the transition metals having empty d-orbitals can form coordinative covalent bonds very readily..thus ammonia acts as a ligand.