A topological analysis of the electron density (rho) and its associated Laplacian has been carried out for P(NH3)n+, P(OH2)n+ (n = 1, 2), and P(NH3)(OH2)+ ion-molecule complexes. The shape of the Laplacian of rho shows that the X-H bond is a shared interaction. For P-N bonds, however, a variation in the characteristics of the Laplacian is observed, from one showing characteristics common to shared interactions in P(NH3)+ to one with the shape of a closed shell interaction in the P(NH3)2+ complex. In P-O bonds, the Laplacian retains the form of a closed shell interaction for all the complexes studied. The value of the energy density at the bond critical point (H(r(e))) has been taken as a measure of the degree of covalency. An important degree of covalency has been detected in the one-ligand complexes for both ammonia and water, which decreases when a second ligand is added to the ion, so we suggest that this decrease in covalency is responsible for the large decrease in binding energies when a second ligand adds. As expected, the covalency is larger for ammonia complexes than for water ones, a fact that explains the larger binding energy obtained for the former.