Cooperative B‐H bond activation reactions with thio‐ and iminophosphoryl tethered ruthenium carbene complexes are reported. The complexes show surprisingly different reactivities towards the commonly employed boranes CatBH, PinBH and BH3∙LB as a result of different modes of metal ligand cooperation. While the iminophosphoryl system allows for selective 1,2‐addition of the B‐H bond across the Ru=C double bond, the sulfur analogue only delivers the 1,2‐addition product for CatBH, while activation of BH3 and PinBH lead to further insertion reactions in one or more sides of the Ru‐C‐P‐S‐ring. The different reactivities can be explained by the differences in the electronics of the carbene complexes and the phosphoryl tether and by the Lewis acidities of the boranes. DFT calculations show that the mechanism of the reactions either proceeds via an addition across the Ru=C bond with different regioselectivities or across the Ru‐S linkage.