Weakly-coordinating anions stabilise the unprecedented monovalent and divalent eta-benzene nickel cations [(eta-C5H5) Ni(eta-C6H6) Ni(eta-C5H5)](2+) and [Ni(eta-C6H6)2](2+)

Nickelocene in benzene reacts with the Bronsted acid H2O-B( C6F5)(3) to giv e the salt [(eta-C5H5) Ni(eta-C6H6) Ni(eta-C5H5)][ B-3(mu-O)(3)(C6F5)(5)] w hich is the first example of a triple-decker nickel sandwich with a bridgin g eta-benzene ligand; the borate anion is also unprecedented; treatment of Ni(eta-C5H5)(2) with Brookhart's acid [H(OEt2)(2)][B(3,5(CF3) 2C6H3)(4)] in benzene gives the paramagnetic bis(eta-benzene) nickel derivative {[Ni(eta -C6H6)(2)][B(3,5(CF3) 2C6H3)(4)]2 . Ni(eta-C5H5)(2)} in which nickelocene i s present as a molecule of crystallisation.