The unexpected reactivity and stability limits of phosphorus ylide-based aluminum- and gallium-carbon ambiphiles are described. While the previously published t-butyl-substituted compound (2-{AltBu₂}-C₆H₄)Ph₂PCMe₂ (1^tBu) reacts reversibly with NH₃ at room temperature with cleavage of a N–H bond, the reaction with MeNH₂ is much less favourable and proceeds irreversibly only at 90 °C. All other title compounds 1^R with R = Me, Et, Mes, and C₆F₅ decomposed in the presence of NH₃. The decomposition of 1^Et in the presence of ammonia can be well followed by NMR spectroscopy. All title compounds remain stable in the presence of t-BuNH₂ and Et₂NH. In addition, we found an unexpected reactivity in the reaction of 1^R and the gallium analogues 2^R with isocyanates. Instead of yielding the expected ring-expansion products, the title compounds catalyse the trimerization of isocyanates. We also present the reactivity towards MeOH and H₂O. Quantum chemical calculations show that activation of the O–H bonds should be feasible at room temperature. Experimental findings, however, only show the decomposition of 1^tBu in the corresponding reactions. Nevertheless, the cleavage of the O–H bond is feasible and affords the activation products 7 and 8 starting from the ammonia activation product 3.