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

Abstract

The molecular structure of trans-[(NH&Pt@l-MeC-)2Pd(NH3)] [Pd(NH3)4]0.5(N03)3-3H20 (1) has been determined by X-ray crystallography. The compound crystallizes in the triclinic space group P i (No. 2), with a = 7.476(2) A, b = 11.373(3) A, c = 16.397(4) A, a = 87.27(2), ,8 = 85.32(2), y = 88.74(2), and Z = 2. The structure was refined to R = 0.034 and R, = 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) A. 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 ds-ds 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 ds-d9 and ds-dIo electron counts [e.g. Pt(I1)-Cu(1I) and Pt(I1)-Hg(I1) 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.