An air-stable half-sandwich RuII complex as an efficient catalyst for [3+2] annulation of 2-arylcyclo-2-enones with alkynesElectronic supplementary information (ESI) available: Experimental procedures, analytical data for products, and NMR spectra of products. CCDC 1423239 (II) and 1423240 (3ah). For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5cc10169g

An air-stable half-sandwich ruthenium II complex, [Ru( p -cymene)(MeCN) 3 ](SbF 6 ) 2 , was synthesized and characterized. It exhibited unique catalytic ability in a [3+2] annulation reaction of 2-arylcyclo-2-enones with internal alkynes via C(sp 2 )-H/alkene functionalization, which offered a rapid and atom-economical methodology to construct various synthetically challenging spiro carbocyclic indenes bearing a quaternary carbon. An air-stable half-sandwich Ru II complex, [Ru( p -cymene)(MeCN) 3 ](SbF6) 2 , catalyzed a [3+2] annulation reaction of 2-arylcyclo-2-enones with alkynes via C(sp 2 )-H/alkene functionalization to construct synthetically challenging spiro carbocyclic indenes bearing a quaternary carbon.