Phosphirenium cation

According to these consideration the diamino-substituted phosphenium (an alternative suggestion for its nomenclature is phosphanylium) cation, 5, and the phosphanetriylammonium (iminophosphenium) cation, 6, possess the largest intrinsic (gas phase) stabihties. Since in the X-ray structures the molecules are to a first-order isolated, this theoretical stability scale determined for the gas phase should also mimic the various trends of the stabilities of the cations and their chelation behaviour. The methylenephosphenium, 7, and the PjH cations, 8, suffer from poor stabihties. On the other hand the phosphirenium cation, 11, is considered to be fairly well stabilized. It is due to n-electron delocalisation of the positive charge in the phosphirenium cation. Intermediate cases in stabihty are the PO+ (9) and PS+ cations (10). Of further interest are the frontier orbital considerations, as shown in Fig. 2. 

Figure 1. B3LYP/6-31 lG(d) optimized structure of the Cr-(CO)s complex of the phosphirenium cation.

It is worth mentioning at this point that the formation of a a -phosphorus from a -phosphorus resulted in a-aromatic compounds in the case of the l//-phosphirenium cation as well. Disubstitution at the phosphorus in l//-phosphirenium cation (5) resulted in the preservation of aromaticity with proper substituents (fluorine) in 36, as a result of the interaction with PF2 a -orbitals,as indicated by isodesmic reactions. A similar phenomenon has been observed also for 1,1-difluorocyclopropene, 1,1-bissilylcyclopentadiene, and 1,1-bisstannylcyclopen-tadiene. In the latter case, the phenomenon has been called hyperconjugate aromaticity . The effect of... 

Scheme 8 Unsaturated three-membered ring phosphirene 5 and phosphirenium cation...

A study of the reactivity of l-chloro-lH-phosphirenes 342 with nucleophiles has shown that the chlorine is easily replaced. Ab initio calculations suggest that IH-phosphirenes invert their configuration at phosphorus by a rotation of the PX group above the C2 moiety, rather than by the more usual trigonal inversion pathway involving a C2v-transition state. Related calculations on the aromaticity of IH-phosphirenium cations 343 have shown that the a ABMO associated with the P-X bond acts like an empty p-orbital. A facile route to the phosphirenium salts 344 is afforded by the reaction of the phosphiranium salt 345 with alkynes. The P -azaphosphirene system 346 has been obtained from the reaction of an iminophosphene with terminal alkynes. ... 

Although parent phosphirenes (Table 6.21) are not known, a number of derivatives have been well characterised. Typical of these are 1,2,3 triphenylphosphirene (6.889a) and the tetraphenylphosphi-renium cation (6.889b) (Figure 6.23). Phosphirenium salts do not exhibit aromatic behaviour. 

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