Three new nitro-nitrito isomeric pairs are prepared and characterized as ci's-[Ru(NO)X(2,2'-bpy)2]2+, ds-[Ru-(NO)X(pyca)2], and cri-[Ru(NO)X(2,2'-bpy)(py)2]2+ (X =

The reaction between trans-[RuCl2(dmso-S)4] (dmso = dimethyl sulfoxide) and 2,2′-bipyridine (bpy) in EtOH–H2O afforded trans(Cl),cis(S)-[RuCl2(bpy)(dmso-S)2] (1) in good yield. When dissolved in DMSO, 1 rearranged to the thermodynamically more stable cis(Cl),cis(S)-[RuCl2(bpy)(dmso-S)2] (2) along with a small amount of cis(Cl),trans(S)-[RuCl2(bpy)(dmso-S)2] (3). Isomer 2 was selectively synthesized by heating 1 in EtOH–DMSO (9:1) and, more conveniently, by the reaction of cis(Cl),fac(S)-[RuCl2(dmso-S)3(dmso-O)] with bpy in EtOH–DMSO (9:1). The structures of 1, 2, and 3 were determined using single-crystal X-ray crystallography. The crystal structure of 1 revealed that the 6- and 6′-protons of the bpy ligand repulsed the two dmso ligands within the equatorial plane, resulting in the lower thermodynamic stability of 1. In contrast, the crystal structure of 2 showed that its bpy ligand is involved in hydrogen bonding with the monodentate dmso and Cl− ligands within the equatorial plane. Our studies show that hydrogen bonds within the equatorial plane of 2 restrict the conformation of the dmso ligands in the crystal and in DMSO.

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