cis-Dioxoruthenium(VI) complexes, [Cn*(CF3CO2)(RuO2)-O-VI]ClO4 (1) (Cn* = 1,4,7-trimethyl-1,4,7-triazacyclononane) and cis-[(Tet-Me-6)(RuO2)-O-VI](ClO4)(2) (2) (Tet-Me-6 = N,N,N',N-tetramethy1-3,6-dimrthyl-3,6-diazaoctane-1,8-diamine), oxidize disubstituted alkynes to 1,2-diketones selectively in good to excellent yields under ambient conditions. The reactions proceed via the formation of dark blue [(Cn*)(CF3CO2)Ru-IV-(OC2RRO)-R-1-O-2](+) intermediates, which display a characteristic UV-visible absorption band at 550-680 nm. With bis(trimethylsilyl)acetylene as substrate and 1 as the oxidant, the intermediate was isolated and structurally characterized by X-ray crystallography as a [3 + 2] cycloadduct. The kinetics of the cycloaddition of 1 with various substituted trimethylsilylacetylenes has been studied by stopped-flow spectrophotometry. With the exception of bis(trimethylsilyl) acetylene, the second-order rate constants were found to vary over a range of less than an order of magnitude irrespective of a 2.3 eV change of the calculated I, of the alkynes; therefore, a rate-limiting single electron-transfer mechanism is unlikely. The participation of oxirene (oxene insertion) and metallaoxetene ([2 + 2] cycloaddition) intermediates appears to be implausible based on product analysis. A. linear Hammett correlation was established using sigma (+) and sigma (.)(JJ) parameters for the cycloaddition of 1 with para-substituted aryl trimethylsilylacetylenes, and the rate-limiting vinyl radical intermediate formation is proposed.