Reactions of [H2Os3(CO)10] with the substituted diynes R-C2-C2-R′ (1, R = Ph, R′ = CH2NHPh; 2, R = Ph, R′ = CH2NHCH2Ph; 3, R = R′ = CH2NHPh) afford the clusters [HOs3(CO)10(L)] (L = {μ-η1:η2-PhCH2C(H)=C-C(H)=C-NPh} (4), {μ-η1:η2-PhCH2C(H)=C-C(H)= C-NCH2Ph} (5), and {μ-η1:η1-CH3CC=C-C(H)=C(H)-NPh} (6), which have been characterized by single-crystal X-ray diffraction analyses as well as by various spectroscopic methods. In clusters 4-6, the starting diynes 1-3 have been rearranged to form substituted pyrrolyl rings which bridge two osmium atoms in η1:η2- (4, 5) or η1:η1- (6) coordination modes depending on the nature of substituents in 1-3. A possible reaction pathway for the diyne cyclization reactions that yield 4-6 involves initial transfer of a hydride onto a coordinated diyne followed by a series of 1,3-shifts and subsequent nucleophilic attack of the nitrogen on the third carbon atom of the diyne system to afford the five-membered pyrrolyl ring. Cluster 6 and its previously characterized furanyl analogue