The synthesis of (Dien)Pt(PMEA-N7), where Dien = diethylenetriamine and PMEA^2- = dianion of 9-[2-(phosphonomethoxy)ethyl]adenine, is described. No useful biological activity could be discovered for this complex which is in contrast to the known antiviral properties of PMEA itself. The acidity constants of the twofold protonated H₂[(Dien)Pt(PMEA-N7)]²⁺ complex were determined (UV spectrophotometry and potentiometric pH titration): The release of the proton from the -P(O)₂(OH)^- group is only slightly affected by the N7-coordinated (Dien)Pt²⁺ unit, whereas the acidity of the (N1)H⁺ site is strongly enhanced. The stability constants of the M[(Dien)Pt(PMEA-N7)]²⁺ complexes with the metal ions M²⁺ = Mg²⁺, Ca²⁺, Mn²⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, and Cd²⁺ were measured by potentiometric pH titrations in aqueous solution at 25 °C and I = 0.1 M (NaNO₃). Application of previously determined straightline plots of log K_M(R-PO3) ^M versus K_H(R-PO3) ^H for simple phosph(on)ate ligands, R-PO₃ ^2- where R represents a non-inhibiting residue without an affinity for metal ions, proves that the primary binding site of the complex-ligand, (Dien)Pt(PMEA-N7), with all the metal ions studied is the phosphonate group; in most instances the expected stability is actually reduced by about 0.4 log units due to the N7-bound (Dien)Pt²⁺ unit. Only for the Cu[(Dien)Pt(PMEA-N7)]²⁺ and the Zn[(Dien)Pt(PMEA-N7)]²⁺ systems the formation of some 5-membered chelates involving the ether oxygen atom of the -CH₂-O-CH₂-PO₃ ^2- residue could be detected; the formation degrees are 52 ± 9% and 32 ± 14%, respectively. The metal ion-binding properties of (Dien)Pt(PMEA-N7) differ considerably from those of PMEA^2-, yet they are relatively similar to those of pyrimidine-nucleoside 5'-monophosphates. The structures of the various complex species in solution are discussed and compared.