Reaction of [RuHClL2](2) (L = (PPr3)-Pr-i) with 2-vinylpyridine gives L2ClRu(eta(2)-CH=CHC5H4N) with liberation of H-2. Reaction of [RuHClL2](2) with a range of olefins D(H)C=CR(EWG) substituted by electron-donating (D) and -withdrawing (EWG) groups occurs by oxidative addition of a vinyl C-H bond to give the metallacycles L2ClHnRu(eta(2)-double dagger C(D)=CR(EWG)). The C-13 chemical shift of double dagger C and the fate of the "H-n" unit (decoordination, binding as H-2, or binding as two hydrides) are strongly correlated, depend on the donating and withdrawing power of D and EWG, and can be used to decide whether double dagger C binds to Ru as a carbene or as a vinyl. These results emphasize the reducing power of Ru(II) when pi-acid ligands such as CO are absent.