The reactivity of ruthenium hydride complexes supported by 2-((di-tert-butylphosphaneyl)methyl)pyridine, L1 and 2-(di-tert-butylphosphaneyl)pyridine, L2, was explored. The reaction of {Ru(COD)Cl2}x with L1 in the presence of base and 10 bar of H2 gave the expected complex [Ru(L1)2(H)Cl], 1, while the same reaction with L2 gave [Ru(L2)(P(H)tBu2)(H)Cl], 2, that results from the cleavage of a C-P bond. We were able to establish that under the reaction conditions the first species formed is [Ru(L2)2(H)Cl], 3, and that this species decomposes to give complex 2 and is in equilibrium with [Ru(L2)2Cl2], 4. The proposed mechanism obtained by DFT has the protonation of the carbon as the highest energy step (38.9 kcal/mol), consistent with a slow reaction. Preliminary studies reveal that complex 2 is a very active catalyst in the hydrogenation of benzaldehyde (TONs up to 44,000).