Reaction of the scorpionate salt Li[HB(mt)2(pz)] (mt = N-methyl-2-mercaptoimidazol-1-yl, pz = pyrazolyl) with the organometallic complexes [Cp*RuOMe]2 (Cp* = η5-C5Me5) (1) and [(HMB)RuCl2]2 (HMB = η6-C6Me6) (2) gave the 18-electron Ru(II) complexes [Cp*Ru(κ3-H,S,S′)-{HB(mt)2(pz)}] (3) and [(HMB)Ru(κ3-H,S,S′)-{HB(mt)2(pz)}](4B)PF6 in moderate yields. In the absence of the PF6– anion, [(HMB)Ru(κ2-S,S′-{HB(mt)2(pz)})(Cl)] [4C] was isolated as a coproduct with (4B)Cl. These complexes are the first examples of organoruthenium(II) complexes containing bis(methimazolyl)(pyrazolyl)borate ligands. Isomers of 4B were observed in solution, and the isomerization process was studied using variable-temperature 1H NMR spectroscopy. The reactivity of 3 toward O2 and CO was investigated, and in the process we isolated the first Ru(IV) peroxo complex containing a poly(methimazolyl)borate ligand, [Cp*Ru(κ2-S,S′-{HB(mt)2(pz)})(η2-O2)] (5), and a CO adduct, [Cp*Ru(κ2-S,S′-{HB(mt)2(pz)})(CO)] (6), respectively. The oxidation process was reversible, but treatment of 5 with CO converted it irreversibly to 6. All the new compounds were fully characterized, including by X-ray diffraction analyses. Cyclic voltammetric studies were also conducted for complexes 3, 5, and 6.