The phosphaaluminirenes HC[(CMe)(NDipp)](2)Al[C(R)=P] (Dipp = 2,6-i-Pr2C6H3, R = tBu or adamantyl) 2 and 3, featuring an unsaturated three-membered AICP ring, have been synthesized as crystalline solids via a [1 + 2] cycloaddition reaction of the aluminum(I) complex HC[(CMe)(NDipp)](2)Al (1) with phosphaalkynes. Computational investigations infer three-centered 2 pi-electron aromaticity of the AICP rings. Compound 3 is readily protonated by tBuOH to induce a ring-opening sigma-bond metathesis, giving an alumina-substituted P-hydrogeno phosphaalkene 4. Remarkably, the high strain of the A1CP ring of 3 allows for facile ring enlargement in reactions with CyNC, bis(diisopropylamino) cyclopropenylidene (BAC), elemental Se, Ph2CO, PhCH=CHCOPh, and PhCN at room temperature. These furnish a series of unprecedented main group heterocycles 5-10 with the C=P unsaturated bonds remaining intact. The mechanisms are considered in light of thorough density functional theory (DFT) calculations.