Both [Ru₃(CO)₁₂] and [Os₃(CO)₁₂] react with 3-dimethylaminoprop-1-yne (HCCCH₂NMe₂) to give fairly low yields of the compounds [M₃H(CO)₉(Me₂NCCCH₂)](M = Ru or Os), apparently derived by oxidative addition with cleavage of the terminal C–H bond and a 1,3 shift of the NMe₂ group. Crystals of [Ru₃H(CO)₉(Me₂NCCCH₂)] are monoclinic, space group P2₁/n with Z= 4 and cell dimensions a= 13.067(2), b= 10.398(2), c= 15.544(2)Å, and β= 112.78(2)° the X-ray structure was refined to R= 0.019 8 for 3 138 observed reflections. The µ₃ ligand is not bound like the µ₃-allenyl ligand in [Ru₃H(CO)₉(µ₃-EtCCCHMe)] but extensive π donation from the NMe₂ group modifies the ligand-to-metal bonding and makes a zwitterionic description [Ru₃^–H(CO)₉(Me₂N⁺C–CCH₂)] more realistic. In spite of the different bonding, the terminal CH₂ protons of the triruthenium compound exchange (n.m.r. coalescence), corresponding to the methyl exchange previously observed for [Ru₃H(CO)₉-(µ₃-MeCCCMe₂)]. The triruthenium compound much less readily undergoes a hydrogen-atom shift observed generally for compounds of type [Ru₃H(CO)₉(R¹CCCHR²)], being stable up to at least 140 °C. However, a PPh₃-catalysed shift is observed at 55 °C to give [Ru₃H(CO)₉(Me₂NCCHCH)] as well as the substitution product [Ru₃H(CO)₈(PPh₃)(Me₂NCCCH₂)].