Ligand Substitution, pH Dependent Deoxygenation, and Linkage Isomerization Reactions of the 2,2‘-Bipyridinetetranitroruthenate Dianion

The reaction of the [Ru(bpy)(NO2)4]2- (bpy = 2,2‘-bipyridine) ion in aqueous solutions produces two different
nitrosyl complexes, depending on the pH of the solution. At acidic pH, complex cis,cis-Ru(bpy)(NO2)2(ONO)(NO)
was isolated. At neutral or basic pH, [Ru(bpy)(NO2)4]2- reacts to give cis,trans-Ru(bpy)(NO2)2(NO)(OH). Both new
complexes were fully characterized by elemental analysis and UV−vis, IR, 1H NMR, and 15N NMR spectroscopy.
A single-crystal X-ray structure of cis,trans-Ru(bpy)(NO2)2(NO)(OH) was also obtained. cis,cis-Ru(bpy)(NO2)2(ONO)(NO) isomerizes in acetone or water solution to give a mixture of the trans,cis-Ru(bpy)(NO2)2(ONO)(NO) and cis,cis-Ru(bpy)(ONO)2(NO2)(NO) linkage isomers as determined by 1H and 15N NMR spectroscopy. A single-crystal
X-ray structure of a solid solution of cis,cis-Ru(bpy)(ONO)2(NO2)(NO)/trans,cis-Ru(bpy)(NO2)2(ONO)(NO) was also
obtained. This pair of isomers is the first crystallographically characterized compound with nitro, nitrito, and nitrosyl
ligands. The kinetic studies of the Ru−NO2 → Ru−NO conversion reactions of [Ru(bpy)(NO2)4]2- in buffered solutions
from pH 3 to pH 9 complement previous studies of the reverse reaction. The reactions are first order in
[Ru(bpy)(NO2)4]2-. At high pH, the reaction is independent of the concentration of H+ while, at low pH, the reaction
is first order in the concentration of H+. The rate determining step of the high pH reaction involves breakage of the
Ru−NO2 bond while, at low pH, the mechanism involves a rapid reversible protonation of a NO2 ligand followed by
the rate determining loss of hydroxide to produce a nitrosyl ligand.