Oxidation of [M(eta-C5H5)(2)], M = Cr, Fe or Co, by the new Bronsted acid H2O center dot B(C6F5)(3) yielding the salts [M(eta-C5H5)(2)](+)A(-), whereA(-) = [(C6F5)(3)B(mu-OH)B(C6F5)(3)](-) or [(C6F5)(3)BOH center dot centerdot center dot H2OB(C6F5)(3)](-)

The Bronsted acids H2O . B(C6F5)(3) and D2O . B(C6F5)(3) have been synthesi zed. Reaction of neutral divalent metallocenes [M(eta-C5H5)(2)], M=Cr, Fe o r Co, with two equivalents of H2O . B(C6F5)(3) 2a resulted in metallocene o xidation and formation of salts containing [M(eta-C5H5)(2)](+) cations toge ther with the hydroxoborate anion [HOB(C6F5)(3)](-) which is hydrogen bonde d to the second acid equivalent, namely [M(eta-C5H5)(2)][(F5C6)(3)BOH ... H 2OB(C6F5)(3)], M=Cr 3a, Fe 4a or Co 5a. Treatment of one equivalent of 2a a nd one equivalent of B(C6F5)(3) 1 with [M(eta-C5H5)(2)] yielded salts conta ining 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 determine d.