Ammonia acts as a ligand but ammonium ion does not form complexes because

• A
  $NH_3$ is a gas while $NH_4^+$ is in liquid form
• B
  $NH_3$ undergoes $sp^3$ hybridisation while $NH_4^+$ undergoes $sp^3d$ hybridisation
• C
  $NH_4^+$ ion does not have any lone pair of electrons
• D
  $NH_4^+$ ion has one unpaired electron while $NH_3$ has two unpaired electrons