The synthesis and characterization of the guanine complexes cis-[(PMe(3))(2)Pt(9-MeGu)(2)](NO3)(2) and cis-[(PMe(3))(2)Pt(9-MeGu(-H))]6NO3)(6) (9-MeCu and 9-MeGu(-H) are the neutral and the N(1)-deprotonated 9-methylguanine, respectively) are reported. The mononuclear derivative is formed by reacting 2 equiv of nucleobase with an aqueous solution of cis-[(PMe(3))(2)Pt(NO3)(2)] at autogenous pH, and it crystallizes from methanol in the tetragonal system, space group P4(1)2(1)2, with the following cell dimensions : a = b = 11.106(4) Angstrom, 48.89(3) Angstrom, Z = 8. The structure was solved using 3562 observed reflections and refined to R = 3.98 and R(w) = 5.02%. The nucleobases are N(7)-bonded to the metal center of the cis-(PMe(3))(2)Pt unit (Pt-P = 2.250(4) Angstrom (average); Pt-N(7) = 2.11(1) Angstrom P-Pt-P = 96.2(2)degrees; N-Pt-N = 83.2(5)degrees) in a head-to-tail arrangement. The two purine rings are almost perpendicular to the metal-coordination plane (dihedral angles 83.6 and 101.7 degrees) and form an intraligand dihedral angle of 77.6 degrees. The hexanuclear complex is formed when equimolar amounts of cis-[(PMe(3))(2)Pt(NO3)(2)] and 9-MeCu are reacted at neutral pH. Crystallization of the crude product from water affords colorless prisms which crystallize as cis-[(PMe(3))(2)Pt(9-MeGu(-H))](6)(NO3)(6) . 18H(2)O, in the hexagonal system, space group R (3) over bar, with the following cell dimensions : a = b = 23.178 (3) Angstrom, c = 24.937(3) Angstrom, Z = 3. The structure was solved using 2094 independent observed reflections and refined to R = 5.64 and R(w) = 5.97%. The compound contains the cyclic cation cis-[(PMe(3))(2)Pt(9-MeGu(-H))](6+)(6) in which six cis-(PMe(3))(2)Pt units are symmetrically bridged by deprotonated guanine ligands through their N(1) and N(7) atoms. The hexamer, with an S-3 symmetry, exhibits the purine rings alternatively disposed above and below the mean plane passing through the metal atoms and forming with this plane an angle of 50 degrees. The adjacent platinum atoms are disposed alternatively in two parallel planes, slightly separated (0.34 Angstrom), at a minimum distance of 6.5 Angstrom. Each metal atom is in a distorted square-planar arrangement defined by the phosphorus atoms of two PMe(3) moieties [(Pt-P = 2.245(7) Angstrom (average); P-Pt-P = 94.9(2)degrees] and the N(1) [Pt-N(1) = 2.12(2)Angstrom] and N(7)’ [Pt-N(7)’ = 2.09(1) Angstrom; N(1)-Pt-N(7) = 83.4(5)degrees] atoms of two deprotonated guanine ligands. Both complexes have been further characterized by H-1 and P-31 NMR spectroscopy.