The reaction of [Cr(NCCH3)(6)](2+) with dioxygen in acetonitrile (MeCN) solution acidified with HBF4 gave red crystals of the binuclear complex [(CH3CN)(5)Cr(OH)Cr(NCCH3)(5)](BF4)(5) (1). From the X-ray crystal structure of 1, the Cr-O-Cr angle was found to be 147.5(2)degrees. Magnetic susceptibility measurements of 1 showed an antiferromagnetic coupling between the two chromium(III) centers with a triplet energy J = 35.9(1) cm(-1). On redissolution of 1 in MeCN, the hydroxo bridge was deprotonated, and a green solution of the complex [(CH3CN)(5)CrOCr(NCCH3)(5)](4+) formed. The electronic absorption spectrum of this solution is very similar to the spectrum of the classical complex [(H3N)(5)CrOCr(NH3)(5)](4+) with intense bands in the UV and near-UV region. From the temperature dependence of the absorption spectrum near 12900 cm(-1), the triplet energy J was found to be 1067(19) cm-1. The acidity of the hydroxo bridge in 1 is very high with an acid dissociation constant Ka much greater than 1 M.