A series of eta(2)-[Os(NH3)(5)(vinyl ether)](2+) complexes have been prepared by three independent methods that involve direct coordination of a vinyl ether, alcohol addition to an eta(2)-alkyne complex, or nucleophilic substitution of an eta(2)-vinyl ether species. In the presence of an acid catalyst, the vinyl ether ligand undergoes a novel acid-catalyzed substitution reaction at the alpha-carbon with a broad range of nucleophiles that includes alcohols, amines, carboxylates, hydrides, silylated enols, nitriles, phosphines, and dialkyl sulfides. These reactions appear to proceed through an elimination-addition process where the first step is loss of an alcohol to form an eta(2)-vinyl cation intermediate. In cases where the alpha-carbon bears an alkyl group, an eta(2)-vinyl cation species can be isolated and characterized. For example, protonation of [Os(NH3)(5)(eta(2)-2-methoxypropene)](2+) (3) in neat HOTf allows the characterization of the substitution reaction intermediate eta(2)-[Os(NH3)(5)(C3H5)](3+) (32), formally a metallocyclopropene that behaves chemically like a vinyl cation. In contrast, when the alpha-carbon of the vinyl ether bears a hydrogen such as with [Os(NH3)(5)(eta(2)-ethoxyethene)](2+) (1), the hypothetical vinyl cation intermediate, in absence of a suitable nucleophile, undergoes an intramolecular 1,2-hydrogen shift to yield the Fischer carbyne [(NH3)(5)Os=CCH3](3+) (33). Examples of nucleophilic substitution reactions for other types of eta(2)-[Os(NH3)(5)(olefin)](n+) complexes are also demonstrated.