The reactions of the anionic cluster [Ru3(CO)10(NO)]~ with acids and CF3SO3CH3are reported. O-Methylation of the µ2-NO ligand gives high yields of Ru3(NOCH3)(CO)10. A single-crystal X-ray crystallographic analysis of this new cluster [orthorhombic crystal system, Pnma space group, a = 14.775 (4) Å, b = 12.128 (2) Å, c = 9.987 (2) Å, Z = 4] revealed the presence of a triply bridging methoxyimido (NOCH3) ligand and a triply bridging carbonyl ligand on opposite faces of an equilateral ruthenium triangle. O-Protonation with CF3SO3H gives an analogous structure; however, weaker acids, such as CF3CO2H, give only HRu3(CO)10(NO), where a Ru-Ru bond has been protonated. Addition of PPN(CF3CO2) to Ru3(NOH)(CO)10results in immediate O-H to M-H tautomerization. The PPN+ salts of 12 anions having differing basicities provide insight into the pKaof Ru3(NOH)(CO)10and the relative kinetic acidity of the O-H vs the M-H group. Reactions of the substituted anions [Ru3(CO)9(L)(NO)]−, where L = PPh3and P(OCH3)3, with acids reveal similar behavior.