We have synthesized a new type of ten-membered osmacycles by reaction of osmapyridinium with HCCCH(OH)R (R = Ph, Et). We propose that these reactions take place initially by coordination of the alkynes, [2 + 2] cycloaddition, subsequent 1,2-hydrogen migration and a final reductive elimination. The reactions with phenylacetylenes do not afford the corresponding derivatives but rather give η⁴-coordinated cyclopentadiene complexes, which are proposed to derive from a [4 + 2] cycloaddition process. Related reactions of the η⁴-coordinated cyclopentadiene complexes are also discussed.