Water exchange rates in the diruthenium μ-oxo ion cis,cis-[(bpy)2Ru(OH2)]2O4+

Addition of 2 equiv of Ce4+ to the dimeric ruthenium mu -oxo ion cis, cis- [(bpy)(2)Ru(OH2)](2)O4+ (formal oxidation state M-HI, subsequently denoted (3,3)) or addition of 1 equiv of Ce4+ to the corresponding (3,4) ion gave near-quantitative conversion to the (4,4) ion, confirming our recent assignment of this oxidation state as an accumulating intermediate during water oxidation by the cis, cis- [(bpy)(2)Ru(0)](2)O4+ ((5,5)) ion. The rates of water exchange at the cis-aqua positions in the (3,3) and (3,4) ions were investigated by incubating (H2O)-O-18-enriched samples in normal water for predetermined times, then oxidizing them to the (5,5) state and measuring by resonance Raman (RR) spectroscopy changes in the magnitudes of the O-isotope sensitive bands at 780 and 818 cm(-1). These bands have been assigned to Ru=O-18 and Ru=O-16 stretching modes, respectively, for ruthenyl bonds formed by deprotonation of the aqua ligands upon oxidation to the (5,5) state. An intermediate accumulated during the course of the isotope exchange reaction that gave a (5,5) ion possessing both similar to 782 and similar to 812 cm(-1) bands; this spectrum was assigned to the mixed-isotope species, (bpy)(2)Ru(O-16)(ORu)-O-16(O-18)(bpy)(2)(4+). Kinetic analysis of solutions at various levels of oxidation indicated that only the (3,3) ion underwent substitution; the exchange rate constant obtained in 0.5 M trifluoromethanesulfonic acid, 23 degreesC, was 7 x 10(-3) s(-1), which is (10(3)-10(5))-fold larger than rate constants measured for anation of monomeric (bpy)(2)Ru(III)X(H2O)(3+) ions bearing simple sigma -donor ligands (X).