Comparative study on crystal structures of ruthenium bipyridine carbonyl complexes [Ru(bpy)2(CO)2](PF6)2, [Ru(bpy)2(CO)(C(O)OCH3)]B(C6H5)4.cntdot.CH3CN, and [Ru(bpy)2(CO)(.eta.1-CO2)].cntdot.3H2O (bpy = 2,2'-bipyridyl)