CRYSTAL AND MOLECULAR-STRUCTURES OF [RU(BPY)3](PF6)3 AND [RU(BPY)3](PF6)2 AT 105-K

Dark green-black crystals of [Ru(bpy)3](PF6)3 (III) were obtained from [Ru(bpy)3](PF6)2 (II) by oxidation with MnO2 in 7 M H2SO4. The X-ray crystal structures of III (trigonal R3cBAR, a = 17.846 (7) angstrom, c = 19.95 (7) angstrom, Z = 6) and II (trigonal P31c, a = 18.338 (7) angstrom, c= 16.216 (13) angstrom, Z = 6) were determined at 105 K. The low temperature structure of II shows the a- and b-axis increased by a factor of square-root 3 relative to the room temperature structure and has three distinct Ru sites (phase transition at 190 (5) K). This explains many recent spectroscopic observations. The Ru-N distances of II (2.053 (2) angstrom) are virtually indistinguishable from those of III (2.057 (3) angstrom). This is compatible with the high electron self-exchange rate between II and III. The pattern of differences in intra-bpy bond lengths seems in contradiction to the conventional pi-backbonding model. Changes of bpy distances in [M(bpy)3], bpy, Hbpy+, and H2bpy2+ are examined by a principal component analysis, which yields two main factors: The first factor is related to the temperature of measurement, the crystal packing pattern or the model used to interpret the diffraction data. The second factor can be characterized as electronic in nature and confirms that there are as vet unresolved contradictions in the model of pi-backbonding as applied to the ligand geometry of [M(bpy)3]n+ complexes.