Phosphorus orbital interactions with

Calculations on the hypothetical diradical (90) indicate that interaction with the if-orbitals on phosphorus could produce a singlet state. ... 

The presence of two heteroatoms with different electronic properties in one molecule (one atom possessing a lone electron pair and another one with a vacant orbital, donor and acceptor, base and acid) separated by some fragment results in an interaction between the heteroatoms. Three types of the phosphorus-boron interactions have been revealed through-bond, intramolecular trans-annular (dative P—B bond), and intermolecu-lar (dative P—B bond) (Fig. 3). 

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... 

Schier and Schmidbaur93 performed a clever experiment that addressed part of this question does the orientation of the carbanion relative to the phosphorus atom play any role Scheme 2 shows two syntheses of ylides involving cyclopropyl substituents. In the first reaction, since the pKa of cyclopropane is considerably below that of propane, the expected product is the cyclopropylide. However, the isopropylide is the only recovered product. The second reaction also demonstrates the avoidance of the cyclopropylide product. The cyclopropylide possesses a very pyramidal carbanion that is directed away from phosphorus, allowing for minimal orbital overlap. The isopropylide is much less pyramidal and phosphorus can better assist in stabilizing the carbanion. While this stabilization does not require explicit orbital overlap (the electrostatic interaction of the carbanion with the onium is expected to be smaller in the cyclopropylide since it is directed away from P), it does suggest that some orbital interactions are involved. Hence, although the ylene contribution is small, it is unlikely that the ylene contribution is nil. 

Of the two explanations advanced, one is the usual ir-bonding concept involving d orbitals of the third-row elements (34), whereas the other refers to a more electrostatic picture (60). The lone pair of electrons at the carbanion is supposed (a) to interact with the suitable empty orbitals of silicon and (b) to suffer much less repulsive interactions from bond pairs in the presence of the larger elements. Both arguments call for the planar or quasi-planar geometry of the carbanion, which is observed in phosphorus ylides. 

The nature of the frontier orbitals is also of interest and a Mulliken population analysis of the constituent orbitals establishes that the highest occupied molecular orbital (HOMO) of each phosphine consists primarily of a lone pair sp hybrid on phosphorus. The orbital energy ordering PMe3 < PH3 < PF3 also parallels the percentage phosphorus s-character of the HOMO of PMe3 (11% s and 60% p), PH3 (14% s, 67% p) and PF3 (29% s, 32% p). In each case the back lobe of the sp hybrid interacts with the substituent attached to phosphorus in a (T-bonding fashion. 

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