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Secondary arsines were observed to decomplex strongly chelating phosphapalladacycles rapidly under mild conditions. As–H bond migration led to ortho-protonation of the C,P ligand. On the other hand, N–H and P–H bonds in amine and phosphine analogues were acidified under the same conditions and no ligand decomplexation of the metallacycle was observed. Subsequently, secondary arsines bearing the As–H bond were applied in deuteration by directed ortho-metalation (DoM) and decomplexation strategies. In the former, HAsPh2was a convenient deuterium shuttle which facilitated the ortho-deuteration of DoM-palladacycle intermediates under neutral conditions. High deuterium incorporation (>99%) was achieved without specialized preformed deuterating agents. In the latter, HAsPh2afforded the preligands of monomeric, dimeric, bidentate, and tridentate complexes of Pd in excellent yields of 95–98% within 30 min at room temperature. Key reaction steps for the intramolecular insertion of As–H into the Pd–C bond were outlined, and the effects of regioisomerism on the ortho-protonation pathway were evaluated.