Alkyldiamine-tethered derivatives of 2,6-diaminopurine, ethylenediamine-N9-propyl-2,6-diaminopurine, L1, and ethylenediamine-N9-ethyl-2,6-diaminopurine, L2, react with Pd(II) to give N3-coordinated complexes. However, the exact nature of the resulting complex is dependent on the reaction conditions. With PdCl(2)(MeCN)(2) in MeCN/H(2)O the expected [PdCl(N3-2,6-DAP-alkyl-en)](+) complex, 1, is formed with L1 chelating the metal center in a tridentate manner through the diamine function and N3 of the purine base. However, under the same conditions the shorter, ethyl-tethered, L2 gives a complex dication, 2, containing a tetradentate ligand forming simultaneously 5-, 6-, and 7-membered chelate rings. This resulting acetamidine, derived by addition to coordinated MeCN, appears to be the first such case involving the 2-amino group of a purine. The ethyl-analogue of 1, [PdCl(N3-2,6-DAP-Et-en)](+) 3, was prepared by reaction of L2 with K(2)PdCl(4) in aqueous media.