title: Synthesis, crystal structure, and magnetic properties of mu-hydroxo-bis[pentakis(acetonitrile)chromium(III)] tetrafluoroborate: an acetonitrile analogue to "acid rhodo".
abstract: The reaction of [Cr(NCCH(3))(6)](2+) with dioxygen in acetonitrile (MeCN) solution acidified with HBF(4) gave red crystals of the binuclear complex [(CH(3)CN)(5)Cr(OH)Cr(NCCH(3))(5)](BF(4))(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 [(CH(3)CN)(5)CrOCr(NCCH(3))(5)](4+) formed. The electronic absorption spectrum of this solution is very similar to the spectrum of the classical complex [(H(3)N)(5)CrOCr(NH(3))(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 K(a) >> 1 M.
authors.affiliation: Department of Chemistry and Centre for Crystallographic Studies, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen, Denmark.
authors.forename: Niels H
authors.initials: NH
authors.lastname: Andersen
authors.forename: Anders
authors.initials: A
authors.lastname: Døssing
authors.forename: Anne
authors.initials: A
authors.lastname: Mølgaard