Oxidation of [M(η-C5H5)2], M = Cr, Fe or Co, by the new Bronsted acid H2O•B(C6F5)3 yielding the salts [M(η-C5H5)2]+A-, where A- = [(C6F5)3B(μ-OH)B(C6F5)3]- or [(C6F5)3BOH•••H2OB(C6F5)3]-

The Bronsted acids H2O . B(C6F5)(3) and D2O . B(C6F5)(3) have been synthesized. Reaction of neutral divalent metallocenes [M(eta-C5H5)(2)], M=Cr, Fe or Co, with two equivalents of H2O . B(C6F5)(3) 2a resulted in metallocene oxidation and formation of salts containing [M(eta-C5H5)(2)](+) cations together with the hydroxoborate anion [HOB(C6F5)(3)](-) which is hydrogen bonded to the second acid equivalent, namely [M(eta-C5H5)(2)][(F5C6)(3)BOH ... H2OB(C6F5)(3)], M=Cr 3a, Fe 4a or Co 5a. Treatment of one equivalent of 2a and one equivalent of B(C6F5)(3) 1 with [M(eta-C5H5)(2)] yielded salts containing the same metallocene cations but now with mu-OH bridged anions, as in [M(eta-C5H5)(2)][(F5C6)(3)B(mu-OH)B(C6F5)(3)], where M=Cr 3b or Co 5b. All products have been characterised by NMR spectroscopy, elemental analysis, and the single-crystal structures of 2a, 3a, 4a, and 5a have been determined.