The reported chemistry and reactivity of guanidinate supported group 15 elements in the +3 oxidation state, particularly phosphorus, is limited when compared to their ubiquity in supporting metallic elements across the periodic table. We have synthesized a series of chlorophosphines utilizing homo- and heteroleptic (dianionic)guanidinates and have completed a comprehensive study of their reactivity. Most notable is the reluctancy of these four-membered rings to form the corresponding N-heterocyclic phosphenium cations, the tendency to chemically and thermally eliminate carbodiimide chloro(imino)phosphine into a P-N bond of the P-N-C-N evidence for the unwillingness of the halide abstraction reaction the target phosphenium cations and the underscoring strength, and the scarcely observed ring expansion by insertion of a framework. Computational analysis has provided corroborating by demonstrating the exceptional electron acceptor properties of of the P-X bond.