D orbitals of phosphorus

As is known,<1968,108) there is some evidence for the overlap of the empty d orbital of phosphorus with the methylene p orbital this stabilizes the molecule as a partly ionic species. It would be worthwhile investigating the magnitude and signs of couplings in various ylides having a range of stabilisation. 

Molecular orbital theory may provide an explanation for stereochemical differences between carboxylate-metal ion and phosphate-metal ion interactions. Detailed ab initio calculations demonstrate that the semipo-lar 1 0 double bond of RsP=0 is electronically different from the C=0 double bond, for example, as found in H2C=0 (Kutzelnigg, 1977 Wallmeier and Kutzelnigg, 1979). The P=0 double bond is best described as a partial triple bond, that is, as one full a bond and two mutually perpendicular half-7r bonds (formed by backbonding between the electrons of oxygen and the empty d orbitals of phosphorus). Given this situation, a lone electron pair should be oriented on oxygen nearly opposite the P=0 bond, and these molecular orbital considerations for P=0 may extend to the phosphinyl monoanion 0-P=0. If this extension is valid, then the electronic structure of 0-P=0 should not favor bidentate metal complexation by phosphate this is in accord with the results by Alexander et al. (1990). 

The overlap of carbon p orbitals with arsenic d orbitals is less effective than with the d orbitals of phosphorus, and so the covalent canonical structure is expected to make less of a contribution to the hybrid structure. This has been confirmed in an X-ray study of 2-acetyl-3,4,5-triphenylcyclopentadienetriphenylarsorane.6 Yamamoto and Schmidbaur7 found (13CNMR) that the bonding in arsenic ylides was probably sp3 (cf. phosphorus, which changes from sp3—>sp2), resulting in arsenic pseudotetrahedral geometry (cf. phosphorus ylides, which are planar). 

However, certain phosphonium ylides, such as those with an electron-withdrawing substituent in the alkylidene moiety, are relatively unreac-tive toward certain substrates such as ketones (22, 77, 95). This led us to consider whether arsonium ylides might be preferable to phosphonium ylides in certain reactions (48, 94). The overlap of the p orbitals of carbon with d orbitals of arsenic is less effective than with d orbitals of phosphorus. Therefore the covalent canonical form (la) should make a smaller contribution to the overall structure of arsonium ylides than to that of the corresponding phosphonium ylides. 

Suggest another canonical structure for this compound that involves the stabilisation of the negative charge by the empty d orbitals of phosphorus. 

The gas-phase protonation reactions of a number of phosphine oxides and phospho-ramides have also been studied. In these instances protonation occurs at oxygen to form stable quasiphosphonium ions. Oxygen protonation is thermodynamically favoured over nitrogen protonation for phosphoramides. It has been suggested that this is due to the fact that the quasiphosphonium ion thus formed can be stabilized via n back-donation to the d orbitals of phosphorus. 

The phosphoryl bond in symmetrical XV R3P=0 type compounds such as POCI3 is exceptionally stable. This has been attributed to the formation of two mutually perpendicular dn-pn type orbitals using two lone pairs on the oxygen atom. These overlap with two separate d orbitals of phosphorus, giving the symmetry, although not the strength, of a triple bond. In less symmetrical tetrahedral molecules such as (3.16c), however, significant dji-pji interaction is possible with more... 

More recent developments are based on the finding, that the d-orbitals of silicon, sulfur, phosphorus and certain transition metals may also stabilize a negative charge on a carbon atom. This is probably caused by a partial transfer of electron density from the carbanion into empty low-energy d-orbitals of the hetero atom ( backbonding ) or by the formation of ylides , in which a positively charged onium centre is adjacent to the carbanion and stabilization occurs by ylene formation. 

The extra electron pairs in an expanded octet are accommodated by using d orbitals. The phosphorus atom (five valence electrons) in PC15 and the sulfur atom (six valence electrons) in SF6 make use of 3d as well as 3s and 3p orbitals ... 

Semi-empirical molecular orbital calculations have been carried out on the model phosphorane HaPiCHa. Besides the expected transfer of charge, the inclusion of the phosphorus 2>d orbitals showed a significant hyperconjugative interaction between the CHg orbitals and a 2>d orbital of appropriate symmetry on phosphorus. Calculations on cyclopropylidene-phosphorane revealed a similar interaction between the Walsh orbitals of the ring and an in-plane phosphorus 2>d orbital. 

The possibility of orbital reorganization in ion-radicals should be taken into account. Thus, organic derivatives of three-valence phosphorus exist in the sp hybrid state and an electron is removed from one of these orbitals. The cation-radical formed can retain the initial orbital configuration, but can also convert into the sp d hybrid state. In the latter case, one additional orbital of the phosphorus atom becomes accessible for the reactant attack. If Y—H bond in the reactant is weak, the reaction R3P+ -f YH proceeds with the participation of an sp frontal orbital according to the radical mechanism. Thiols are typical reactants with the weak Y—H bonds. If anions (A ) react with RjP, the vacant sp d orbital of the phosphorus appears to be a target. Scheme 3.14 illustrates the orbital pictures and the reaction directions. 

Cavell and Dobbie (214-216) have suggested that halogen transfer rearrangements in trifluoromethylphosphines arise from interactions of nonbonding fluorine p orbitals with vacant d orbitals on phosphorus. Such an explanation is consistent with observations for the Groups IV and V pentafluorophenyl derivatives, exclusive of carbon and nitrogen, and similarly fits the behavior of boron with its vacant p orbital. 

Ligands with potential 7r-bonding to phosphorus (due to back-bonding of a lone pair of electrons of the ligand into an empty d- orbital of the phosphorus) prefer equatorial positions (apicophobicity NR2 > O— Cl > OH in the same molecule). Therefore an (e) OH is more acidic than an (a) OH (the O- base in the (e) position is the preferred). 

Dewar and coworkers offered an alternative view.79 In their model the d and d)Z orbitals are hybridized to give two orbitals which are directed toward the adjacent nitrogen atoms (Fig. 16.25). This allows for formation of three-center bonds about each nitrogen. 9 This scheme, sometimes called the island model, results in delocalization over selected three-atom segments of the ring, but nodes are present at each phosphorus atom since the two hybrid orbitals of phosphorus are orthogonal to each other. Evidence has been offered in support of both models, but neither theory has been confirmed to the exclusion of the other. A third viewpoint holds that rf orbital participation is relatively unimportant in the bonding in these molecules. 1... 

The first step is a Wittig reaction18 in which the ketone is converted to the terminal olefin by reaction with a phosphorus ylide (also called a phosphorane). Phosphoranes are resonance-stabilized by overlap between the carbon p-orbital and one of the d-orbitals of the phosphorus. 

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 shortness and equality of the P-N bond lengths in (PNCl2)3 arise from electron delocalization involving the d orbitals of the P atoms and the p orbitals of the N atoms. In such a system, the a bonds formed from phosphorus sp3 orbitals overlapping with the nitrogen sp2 orbitals are enhanced by n bonding between the nitrogen p- orbitals and the phosphorus d orbitals. 

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