Reversible CH Bond Activation Reactions of the N ‐Heterocyclic Carbene Ligands in Ru(Ph 2 PCH 2 CH 2 CH 2 PPh 2 )(IMes)(CO)H 2 and Ru(Ph 2 AsCH 2 CH 2 PPh 2 )(IMes)(CO)H 2 (IMes=1,3‐Dimesityl‐1,3‐dihydro‐2 H ‐imidazol‐2‐ylidene)

Thermolysis of Ru(IMes)(dppp)(CO)H 2 ( 1 , IMes=1,3‐dimesityl‐1,3‐dihydro‐2 H ‐imidazol‐2‐ylidene; dppp=Ph 2 PCH 2 CH 2 CH 2 PPh 2 ) in the presence of trimethylvinylsilane yields Ru(IMes')(dppp)(CO)H ( 2 ) resulting from intramolecular ArCH 2 –H CH bond activation of the N ‐heterocyclic carbene ligand. The structures of 1 and 2 have been established by X‐ray crystallography. Treatment of Ru(IMes)(arphos)(CO)H 2 ( 3 , arphos=Ph 2 AsCH 2 CH 2 PPh 2 ) with CH 2 CHSiMe 3 at 85 °C produces a mixture of three CH activated products 4a – c . The CH activated complexes 2 and 4a – c reform their dihydride precursors upon heating under hydrogen.