High-resolution infrared laser spectroscopy has been used to determine the structures of HCN-Mg-n complexes formed in helium nanodroplets. The magnesium atoms are first added to the droplets to ensure that the magnesium complexes are preformed before the HCN molecule is added. The vibrational frequencies, structures, and dipole moments of these complexes are found to vary dramatically with cluster size, illustrating the nonadditive nature of the HCN-magnesium interactions. All of the complexes discussed here have the nitrogen end of the HCN pointing towards the magnesium clusters. For Mg-3, the HCN binds to the "threefold" site, yielding a symmetric top spectrum. Although the HCN-Mg-4 complex also has C-3v symmetry, the HCN sits "on-top" of a single magnesium atom. These structures are confirmed by both ab initio calculations and measurements of the dipole moments. Significant charge transfer is observed in the case of HCN-Mg-4, indicative of charge donation from the lone pair on the nitrogen of HCN into the lowest unoccupied molecular orbital of the Mg-4. (C) 2004 American Institute of Physics.