The molecular structure of trans-[(NH3)(2)Pt(mu-1-MeC(-))(2)Pd(NH3)][Pd(NH3)(4)](0.5)(NO)3 . 3H(2)O (1) has been determined by X-ray crystallography. The compound crystallizes in the triclinic space group P $($) over bar$$ 1 (No. 2), with a = 7.476(2) Angstrom, b = 11.373(3) Angstrom, c = 16.397(4) Angstrom, alpha 87.27(2)degrees, beta = 85.32(2)degrees, gamma = 88.74(2)degrees, and Z = 2. The structure was refined to R = 0.034 and R(w) = 0.038 for 5045 independent reflections. The complex is best described by a square planar coordination of Pd and a square pyramidal one of Pt with the Pd in the apical position (T over square geometry, TSQ). The Pt and Pd atoms are bridged by two nearly coplanar 1-methylcytosinate anions with a Pt-Pd distance of 2.511(1) Angstrom. A theoretical analysis, based on EHMO calculations, highlights the nature of the single M-M’ bond in d(8)-d(8) dimers of this type (4:3) and points out the correlations with the better known d(8)-d(8) dimers (4:4) characterized by two parallel square planar coordination geometries. The M-M’ bond order in the latter is close to zero, but not null. Finally, the fate of the M-M’ linkage for adding one or two electrons to the above TSQ species is formally analyzed. A correlation is made with the structural data available for dimers with d(8)-d(9) and d(8)-d(9) electron counts [e.g. Pt(II)-Cu(II) and Pt(II)-Hg(II) species with bridging nucleobases]. While one additional electron weakens the M-M’ linkage without destroying the primary TSQ geometry, two extra electrons force the expulsion of the ligand formerly coaxial with the metals (4:2 adducts). The M-M’ interaction in the 4:2 adducts is comparable with that proposed for the 4:4 species.