Phosphenium ions, not to be confused with phosphonium or phosphirenium, are divalent cations of phosphorus of the form [PR2]+. Phosphenium ions have long been proposed as reaction intermediates. Synthesis Legacy methods The first cyclic phosphenium compounds were reported in 1972 by Suzanne Fleming and coworkers. Acyclic phosphenium compounds were synthesized by Fleming's thesis advisor Robert Parry in 1976. Methods Several methods exist for the preparation of two-coordinate phosphorus ions. A common method involves halide abstraction from halophosphines: R2PCl + AlCl3 → [R2P+][] Protonolysis of tris(dimethylamino)phosphine affords the phosphenium salt: P(NMe2)3 + 2 HOTf → [P(NMe2)2]OTf + [HNMe2]OTf Weakly coordinating anions are desirable. Triflic acid is often used. N-heterocyclic phosphenium (NHP) have also been reported. Reaction of PI3 with the α-diimine yields the NHP cation by reduction of the diimine and oxidation of iodine. Structure and bonding According to X-ray crystallography, [(i-Pr2N)2P]+ is nearly planar consistent with sp2-hybridized phosphorus center. The planarity of the nitrogen center is consistent with the resonance of the lone pair of the nitrogen atom as a pi bond to the empty phosphorus 3p orbital perpendicular to the N−P−N plane. An idealized sp2 phosphorus center would expect an N−P−N angle of 120°. The tighter N−P−N angle observed in the crystal structure can be interpreted as the result of repulsion between the phosphorus lone pair with the bulky i-Pr2N ligands, as the and molecules have bond angles closer to 110° and 90°, respectively. Calculations also show that the analogy to carbenes is lessened by strongly π-donating substituents. With NH2 substituents, the phosphenium cation assumes allyl character. Generalized Valence Bond (GVB) calculations of the phosphenium ions as having a singlet ground state, singlet-triplet separation increases with increasing electronegativity of the ligands. The singlet-triplet separation for and were calculated to be 20.38 and 84.00 kcal/mol, respectively. Additionally, the triplet state of the phosphenium ion displays a greater bond angle at the phosphorus. For example, the calculated bond angle of the singlet state of is approximately 94° compared to 121.5° in the triplet state. Calculated bond lengths between the two states are not significantly impacted. Reactivity Phosphenium is isoelectronic with singlet (Fisher) carbenes and are therefore expected to be Lewis acidic. Adducts are produced by combining [P(NMe2)2]+ and P(NMe2)3: P(NMe2)2]+ + P(NMe2)3 → [(Me2N)3P−P(NMe2)2]+ Being electrophilic, they undergo C−H insertion reactions. Reactions with dienes Phosphenium intermediates are invoked as intermediates in the McCormack reaction, a method for the synthesis of organophosphorus