Constraint of a ruthenium-carbon triple bond to a five-membered ring

The incorporation of a metal-carbon triple bond into a ring system is challenging because of the linear nature of triple bonds. To date, the synthesis of these complexes has been limited to those containing third-row transition metal centers, namely, osmium and rhenium. We report the synthesis and full characterization of the first cyclic metal carbyne complex with a second-row transition metal center, ruthenapentalyne. It shows a bond angle of 130.2(3)degrees around the sp-hybridized carbyne carbon, which represents the recorded smallest angle of second-row transition metal carbyne complexes, as it deviates nearly 50 from the original angle (180 degrees). Density functional theory calculations suggest that the inherent aromatic nature of these metallacycles with bent Ru C-C moieties enhances their stability. Reactivity studies showed striking observations, such as ambiphilic reactivity, a metal-carbon triple bond shift, and a [2 + 2] cycloaddition reaction with alkyne and cascade cyclization reactions with ambident nucleophiles.