Electrophilic transition metal complexes like {MII(EtXantphos)2}2+ (MII = PdII, PtII) heterolyze H2 into a hydride-associated electrophile {H–MII(EtXantphos)2}+ and a proton, which typically associates to an added base (or basic ligand). For {H–MII(EtXantphos)2}+, the metal can be the most basic site in the system, which results in a product that is indistinguishable from oxidative addition {(H)2MIV(EtXantphos)2}2+. By considering the kinetics and thermodynamics of each elementary step – initial heterolysis, followed by a subsequent return of the heterolyzed proton – we suggest that oxidative addition products may be underrepresented tautomers in heterolytic pathways. The gained understanding was used to characterize the first (di)hydride of PdIV, generated by formal oxidative addition of H2 to PdII.

Supporting ligands limit the degree of electrophilic activation for any substrate because they also reduce the Lewis acidity of the transition metal ion. Here, we temporarily mask the Lewis acidity of...