Reaction of (HBpz(3))ReO(Ph)(OTf) with oxygen atom donors leads to oxidation of the phenyl group [HBpz(3) = hydrotris(1-pyrazolyl)borate, OTf = triflate, OSO2CF3]. Reaction with Me(2)SO gives the adduct [(HBpz(3))ReO(Ph)(OSMe(2))]OTf, which undergoes phenyl-to-oxo migration at 25 degrees C to give the phenoxide complex [(HBpz(3))ReO(OPh)(OSMe(2))]OTf and Me(2)S. The Me(2)SO adduct readily and reversibly loses Me(2)S (k = 2.9(4) s(-1) at 25 degrees C) as indicated by isotope exchange reactions and magnetization transfer. The Me(2)SO adduct also slowly oxidizes Me(2)SO to Me(2)SO(2). These reactions all proceed via an intermediate rhenium(VII) dioxo complex, [(HBpz(3))ReO2(Ph)]OTf. This dioxo complex can be observed at low temperature on reaction of (HBpz(3))ReO(Ph)(OTf) with pyridine N-oxide. It rearranges at 0 degrees C by phenyl-to-oxo migration to give phenoxide products and the catecholate complex (HBpz(3))ReO(O2C6H4). The kinetics of this migration have been measured (Delta H-double dagger = 14.8(7) kcal/mol, Delta S-double dagger = -20.5(25) eu). From these data and the activation parameters for reactions of [(HBpz(3))ReO(Ph)(OSMe(2))]OTf, a detailed free energy surface for the reactions of [(HBpz(3))ReO2(Ph)]OTf is constructed. The dioxo complex reacts very rapidly with Me(2)S (1.7 x 10(5) M(-1) s(-1)) with essentially no enthalpic barrier, consistent with the action of a highly electrophilic oxo ligand. Electrophilicity of the oxo groups is suggested to be a critical factor in facilitating the phenyl-to-oxo migration. A general explanation for the relative ease of various organometallic migration reactions, based on analogies with organic [1,2]-shifts, is presented.