A series of complexes has been prepared of the form [Os(NH3)(5)(4,5-eta(2)-L)](2+) where L = furan and various alkylated furans. These heterocycle complexes were surveyed for their reactivity with various electrophiles (e.g., acids, silyl triflate, N-methylacetonitrilium triflate, acetals, aldehydes, and Michael accepters). Electrophilic addition to C(3) results in an unstable reaction intermediate, a 4,5-eta(2)-3H-furanium species, that leads to complexes of 4,5-eta(2)-3-substituted furans, 2,3-dihydrofurans, C(5)-substituted 2-oxoalkenes, cyclic vinyl ethers, or osmium carbyne complexes. The specific reactivity displayed by the eta(2)-furan complexes strongly depends on the presence and location of alkyl substituents and their ability to stabilize eta(2)-vinyl cation intermediates resulting from C(5)-O bond cleavage. In all cases investigated, the addition of an electrophile at C3 occurs anti to osmium coordination.