Complexes of the form [Os(NH3)(5)(L)](OTf)(2) (where L an unactivated arene or polyaromatic hydrocarbon) are readily protonated by triflic acid (HOTf) to generate stable arenium, naphthalenium, and anthracenium cations. A series of substituted anisole complexes were also investigated. The metal stabilizes the hydrocarbon arenium system, in most cases, by coordinating the organic ligand in an eta(3) fashion. Where L = m-xylene, however, NMR data strongly suggest that the arenium ion is essentially dihapto-coordinated, where an allyl cation fragment remains uncoordinated. For the corresponding anisolium systems, NMR data indicate eta(2)-coordination. It is likely that eta(2) and eta(3) geometries represent limiting cases for a continuum of distorted allyl (pseudo-allyl) complexes. The pK(a) values determined for these complexes are dramatically higher than those of the free arenium cations.