The mechanism of insertion reaction between phosphenium cation and phosphindene with oxirane has been investigated theoretically to better understand the reactivity for the valence isoelectronic of carbene. Phosphenium cation acts as an electrophilic reagent and accepts σ electrons of oxirane to form a complex in the first combination step. The greater the positive charge on phosphorus in phosphenium cation, the more stable the formed complex is. The introduction of substituents will decrease the positive charge on phosphorus in phosphenium cation. The order of positive charge on phosphorus is HP⁺-F > HP⁺-OH > HP⁺-NH₂, which is consistent with their Lewis acidities. The complex transforms to a four-membered ring product via a transition state in the second insertion step. The product is more stable than the complex due to the decrease of ring extension.

Phosphenium cation acts as an electrophilic reagent and accepts σ electrons of oxygen in oxirane to form a four-membered ring product. The introduction of substituents will decrease the positive charge on phosphorus in phosphenium cation, which reduced the stability of the products.