The adsorption of NH3 onto Cu(110) at 32 K results in the trapping of a metastable state with a distinctive bonding structure compared to chemisorbed NH3. Heating to 55 K causes the metastable NH3 species to convert to the more stable chemisorbed species. The binding energy for the metastable state is estimated to be 0.2 eV, compared to 0.77 eV for the chemisorbed state. (C) 1997 American Institute of Physics.