Cation Stabilities, Electrophilicities, and “Carbene Analogue” Character of Low Coordinate Phosphorus Cations

Abstract

The stabilities of low coordinated phosphorus cations can be expressed in the frame of the HSAB concept by the transferred charge density Δq(N) which a cation receives upon formation of a donor‐acceptor adduct with a Lewis base N. This concept allows to differentiate between relative stabilities towards different reaction partners, and to compare the electrophilicities of phosphenium ions to those of isoelectronic carbenes and silylenes. An analysis of substituent influences on Δq(H) in cations [P(R)2]+suggests an increasing stabilizing power of substituents in the series R = Cl < CH3 < OH, SH < NH2. The same ordering was derived from isodesmic hydride transfer reactions. Interpretation of population analyses suggests that the individual substituent contributions to cation stabilities result from a balance between π‐donation into the empty p(P) orbital and electrostatic stabilization by polar P–R σ‐bonds. A further stabilizing effect, which is of similar magnitude as in isoelectronic carbenes or silylenes, may arise from cyclic π‐conjugation between a diaminophosphenium fragment and an adjacent double bond. Substituent effects influence further the nature of the frontier orbitals of phosphenium ions, resulting in orbital sequences which resemble those of carbenes, allyl anions, or phospholides, respectively. The absence of frontier orbital related changes in reactivity patterns suggests that in all reactions, including metal complex formation, phosphenium ions behave as purely electrophilic rather than ambiphilic species.

PX4+, P2X5+, and P5X2+ (X=Br, I) Salts of the Superweak Al(OR)4 Anion [R=C(CF3)3]

“…The carbene-like behavior of PR 2 cations (R H, Cl, NH 2 , OH, SH, organo) was earlier investigated on theoretical grounds. [51] In agreement with our results, it followed from this paper that only for electronegative ligands R with little p back donation-that is, R H, Cl-should the PR 2 cation act as an electrophilic carbene analogue with a HOMO mainly residing on the phosphorus atom. With stronger p back donation, as for R OH, NH 2 , the HOMO is centered on the oxygen or nitrogen atoms.…”

Phospha-Fischer Carbenes: Synthesis, Structure, Bonding, and Reactions of Pd(0)− and Pt(0)−Phosphenium Complexes

“…For Fischer carbenes, the percentage of ∆Eorbint attributable to -bonding has been reported to lie between 25 and 50 %. 75,83,84 Therefore, the bonding in 14 is clearly of Fischer type also at the quantitative level and, as anticipated, 79 the M-NHP bond is dominated by -backbonding.…”

“…[72][73][74][75][76][77][78] The most authoritative investigations into M-NHP bonding have been published by Gudat and coworkers. 24,33,79 Together with others in the field, 80 they have analyzed the M-P bond primarily with orbital-based approaches, such as Mulliken and natural population analyses. To the best of our knowledge, there are no reports of an energy decomposition analysis (EDA) of M-NHP bonding within the framework of a fragment molecular orbital (FMO) approach.…”

Synthesis and characterization of a 2-chloro-1,3,2-diaza-phospholidine-4,5-diimine

“…Analysis of calculated bond distances, Wiberg Bond indices, and electron densities at the bond critical points (ρ) for the P-Cl-bonds of the model compounds 1b, 2b, and 7b ( Table 2) reveals further that decreasing cation stability in the order 4b, 3b, 8b correlates with increasing P-Cl bond shortening in the neutral chlorophosphine precursors 2b, 1b, 7b, and confirms the interpretation of this effect as arising from an increased covalent bond order. The decrease in the NPA charges q(Cl) on the chlorine atom with increasing P-Cl bond shortening suggests in line with the results of previous studies 17 that this trend is also associated with a lower degree of hyperconjugative electron transfer from the nitrogen lone-pairs into the σ*(P-Cl) orbital. Altogether, the computational results allow thus to conclude that the imino-substituents at the heterocyclic ring destabilize the phosphenium cation and strengthen at the same time the P-Cl bond in the neutral chlorophosphine precursor.…”

“…4,17 In contrast, ∆E 3 was calculated to be positive at all levels of theory. Comparison of the values of the differences (∆E 1 -∆E 2 ) and (∆E 3 -∆E 2 ) suggests that the destabilization imposed by formal replacement of the C 2 H 4 -backbone of 3b by two imino-groups (~ +20 kcal mol -1 ) is approximately twice as large as the extra stabilization brought by the presence of an additional double bond and ring aromatization in 4b (~ -10 kcal mol -1 ).…”