Our group has been studying the formation and reactivity of d 8 transition metal hydride complexes with the formula [HM(triphos)(PR3)] n+ , which contain one triphosphine ligand and one monophosphine ligand (triphos = PhP(CH2CH2PPh2); PR3 = PPh3, PEt3, PMe3) to evaluate their viability as catalytic intermediates for reactions such as H2 electrocatalysis and CO2 reduction. Palladium triphosphine complexes of this type are notably active catalysts for the electrocatalytic reduction of CO2, although the active catalyst species was found to be [Pd(triphos)(NCCH3)] 2+ , formed upon dissociation of the monophosphine and coordination of solvent. Complexes with larger chelate bite angles, such as the trimethylene analogue of the triphos ligand, i.e. RP(CH2CH2CH2PR2)2, are inactive due to the formation of an inactive hydride complex. We sought to characterize the formation, structure, and thermodynamic hydricity of the analogous palladium hydride species with the catalytically active triphos ligand. For instance, it is not clear whether the monophosphine would remain bound, forming [HPd(triphos)(PR3)] + species analogous to the Co, Rh, and Ni metal hydrides we have been studying, or whether it would dissociate to form a [HPd(triphos)] + species analogous to the catalytically inactive Pd hydride. To form these species and characterize their thermodynamic hydricities, we attempted to study their H2 heterolysis equilibria using strong bases, which is a general method for generating hydrides of this type. Instead, we observed the formation of the cyanomethyl complex, [Pd(triphos)(CD2CN)]BF4, 1, upon addition of the bases to the triphosphine complex in acetonitrile-d3. This result was unexpected, as the phosphazene and guanidine bases used in these studies are not basic enough to deprotonate free acetonitrile and have been used in other analogous studies in acetonitrile without similar side-reactions. Here we report the structures of 1 and an analogous complex, 2, that contains a tridentate PN H P ligand, HN(CH2CH2PPh2)2, along with a discussion of the thermodynamic implications for the acidity and Pd-C bond energies.