Isotopic probes for ruthenium-catalyzed olefin metathesisElectronic supplementary information (ESI) available: Details for handling labelled ethylene; improved synthesis of GIIm, and spectra for all eight complexes of Fig. 1. See DOI: 10.1039/c4cy01118j

Routes are described to previously unreported first- and second-generation Grubbs metathesis catalysts bearing a 13 C label at the key benzylidene or methylidene site. Improved syntheses of the 2 H-labelled isotopologues are also presented. Labelling at the alkylidene position is important because it provides unique, direct information about changes at the active site of the catalyst, and the fate of the [Ru]&z.dbd;CHR ligand during catalyst deactivation. A case study demonstrates the power of 13 C-labelling in tracking the methylidene moiety in amine-induced decomposition of the second-generation complex RuCl 2 (PCy 3 )(H 2 IMes)(&z.dbd; 13 CH 2 ). Also reported is the solubility of ethylene in C 6 D 6 and CD 2 Cl 2 , measured at 296 ± 1.5 K and 101.0 ± 0.8 kPa. 13 C-labelled Grubbs catalysts, RuCl 2 (L)(PCy 3 )(&z.dbd; 13 CHR) (R = H, Ph), pinpoint the fate of the methylidene (benzylidene) moiety during metathesis and deactivation.