Bare iron clusters react in the gas phase with ammonia to form Fe n (NH 3 ) m complexes. In the present study, the iron cluster-ammonia interaction within Fe n (NH 3 ) m and Fe n (ND 3 ) m complexes (n=7-16) are investigated by molecular beam infrared depletion spectroscopy and density functional theory (DFT) calculations. Experimentally, we observe an absorption band within Fe n (ND 3 ) m complexes in the 880-890 cm - 1 range, which is attributed to the ν 2 inversion mode of ND 3 . DFT calculations performed for Fe n (NH 3 ) m and Fe n (ND 3 ) m model complexes (n=1, 4, 7, and 13) predict that three of the four vibrational fundamentals of ammonia are only slightly shifted from their gas phase values, but that the symmetric ν 2 fundamental shifts substantially upward in the complex. For ND 3 , ν 2 is predicted to shift from 748 cm - 1 to the 850-900 cm - 1 range when adsorbed to Fe n , as is observed experimentally in both the present cluster study and in iron single-crystal surface studies. DFT calculations of vertical ionization potentials of Fe n /Fe n (NH 3 ) m species and of Fe n -NH 3 binding energies are found to be in near-quantitative agreement with previously measured values.