Theoretical Aspects of the Heterobimetallic Dimers with the T over Square Structural Motif. Synthesis and Structure of a Heteronuclear Platinum and Palladium Complex with 1-Methylcytosinato Bridging Ligands

The molecular structure of frans-[(NH₃)₂Pt(μ-l-MeC⁻)₂Pd(NH₃)] [Pd(NH₃)₄]_0.5(NO₃)3·3H₂O (1) has been determined by X-ray crystallography. The compound crystallizes in the triclinic space group P1 (No. 2), with a = 7.476(2) Å,b= 11.373(3) Å, c = 16.397(4) Å, a = 87.27(2)·, β = 85.32(2)·, γ = 88.74(2)·, and Z = 2. The structure was refined to R = 0.034 and Z_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) Å. A theoretical analysis, based on EHMO calculations, highlights the nature of the single M-M′ bond in d⁸-d⁸ dimers of this type (4:3) and points out the correlations with the better known d⁸-d⁸ 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 d⁸-d⁹ and d⁸-d¹⁰ 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.