Treatment of Cr(NRAr)(3) (R = C(CD3)(2)CH3 or tert-butyl, Ar = 2,5-C6H3FMe) with oxygen gives the purple chromium(VI) complex O2Cr(NRAr)2 in 83% purified yield. Upon one-electron reduction with cobaltocene, the d(1) dioxo cobaltocenium salt [CoCp2][O2Cr(NRAr)(2)] was isolated in 89% yield. The dioxo anion displays solid-state magnetic behavior consistent with a Curie-Weiss paramagnet with one unpaired electron. The chromium-(V) dioxo [CoCp2][O2Cr(NRAr)(2)] acts as a nucleophile toward ClSiPh3, ClC(O)Ph, and MeOTf to afford the monooxo complexes OCr(OSiPh3)(NRAr)(2) (86% yield), OCr[OC(O)Ph](NRAr)(2) (34% yield), and OCr(OMe)(NRAr)(2) (76% yield), respectively. Two of these rare examples of monooxo chromium(V) derivatives, OCr(OSiPh3)(NRAr)(2) and OCr[OC(O)Ph](NRAr)2, were structurally characterized and display Cr-O-oxo distances of 1.611(6) and 1.588(3) Angstrom, respectively. The benzoate complex features an eta(1)-carboxylate ligand. All of the monooxo complexes show three peaks in their H-2 NMR spectra consistent with isomers arising from restricted rotation about the Cr-N bonds. Two of the complexes, O2Cr(NRAr)(2) and OCr(OSiPh3)(NRAr)(2), were examined by cyclic voltammety..