The infrared photodissociation spectra of mass-selected Mg+(NH3)(n) (n = 3-6) are measured and analyzed with the aid of density functional theory calculations. No large frequency reduction is observed for the NH stretches of ammonia, suggesting that either all the ammonia molecules coordinate directly to the Mg+ ion or an additional ammonia in the second shell bridges two ammonias in the first shell through hydrogen bonds. Four or possibly five ammonia molecules are allowed to occupy the first shell, in striking contrast to the closure of the first shell in Mg+(H2O)(3). (C) 2004 Elsevier B.V. All rights reserved.