Rational molecular design of catalytic systems capable of smooth O À Ob ond formation is critical to the development of efficient catalysts for water oxidation.Anew ruthenium complex was developed, whichbears pendant SO 3 À groups in the secondary coordination sphere:[ Ru(terpy)-(bpyms)(OH 2 )] (terpy = 2,2':6',2''-terpyridine,b pyms = 2,2'-bipyridine-5,5'-bis(methanesulfonate)).Water oxidation driven by aC e 4 + oxidant is distinctly accelerated upon introduction of the pendant SO 3 À groups in comparisons to the parent catalyst, [Ru(terpy)(bpy)(OH 2 )] 2+ (bpy = 2,2'-bipyridine). Spectroscopic,e lectrochemical, and crystallographic investigations concluded that the pendant SO 3 À groups promote the formation of an O À Ob ond via the secondary coordination sphere on the catalyst, whereas the influence of the pendant SO 3 À groups on the electronic structure of the [Ru(terpy)(bpy)(OH 2 )] 2+ core is negligible. The results of this work indicate that modification of the secondary coordination sphere is av aluable strategy for the design of water oxidation catalysts.