The structures, harmonic vibrational frequencies, amino rotational barriers and binding energies of complexes formed by Li+, Na+, Mg+ and Al+ association to guanidine and acetamidine have been investigated by means of the B3LYP density functional approach. Both neutrals are predicted to be stronger bases than ammonia when the reference acids are the aforementioned metal cations. The basicity enhancement with respect to Mg+ and Al+ is only slightly smaller than that reported before when the reference acid is H+. Metal cation association leads to a significant increase in the corresponding amino rotational barriers and to sizeable shiftings of different vibrational modes, in particular those associated with the amino groups. For Li+ and Na+ complexes, the ion-neutral interactions are essentially electrostatic, while the bonds involving Al+ have a significant covalent character. For this reason the Al+ complexes closely resemble the corresponding protonated species. Mg+ represents an intermediate situation between alkali cations and Al+. The empty 3p orbitals of the metal cation play an important role in this respect.