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Dark green-black crystals of [Ru(bpy),](PF,), (111) were obtained from [Ru(bp )3](PF6)2 (11) by oxidation with Mn02 in 7 M H2S04. The X-ray crystal structures of I11 (trigonal R k , a = 17.846 (7) 1, c = 19.915 (7) A, Z = 6) and I1 (trigonal P31c, u = 18.338 (7) A, c = 16.216 (13) A, Z = 6) were determined at 105 K. The low temperature structure of I1 shows the a-and b-axis increased by a factor of 47 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 I1(2.053 (2) A) are virtually indistinguishable from those of I11 (2.057 (3) A). This is compatible with the high electron self-exchange rate between I1 and 111. The pattern of differences in intra-bpy bond lengths seems in contradiction to the conventional a-backbonding model. Changes of bpy distances in [M(bpy),], 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 yet unresolved contradictions in the model of a-backbonding as applied to the ligand geometry of [M(bpy),]"+ complexes.

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