Phosphorus hypervalent bonding

Phosphorus also bears a significant (positive) partial charge in neutral compounds. By expanding formally its valence shell, hypervalent bonding structures are formed, which can conveniently be described by the 3c-4e model [34, 35], (Note that there is no need to consider the involvement of rf-orbitals in the bonding for the hypervalent phosphorus compounds [36, 37].)... 

Hypervalent compounds with coordinative phosphorus —> tin bonds are rare. An intermolecular P —> Sn coordination in Me2ClSnCH2CH2PPh2 and its bromo-substituted analogue was suggested based on NMR and 119Sn Mossbauer spectroscopic data201,202. The X-ray crystal structure analyses showed pentacoordinate tin atoms with the phosphorus and the chlorine atoms located in axial positions203. The P —> Sn distance amounts to... 

Although there could be some phosphorus d-orbitals contributing to lO-P-4 or lO-P-5 species, these are in a high enough position to have a very little amount of an electron in the d-orbitals. This hypervalent bonding was suggested by Musher (4) and it was found by others to have a very small amount of an electron in the d-orbitals, as suggested by Musher and by others (5) with calculations. 

During last year the majority of researches in this area has been focused on the synthesis and structural determination of novel hypervalent phosphorus compounds as well as on the stereochemistry of pentacoordinated chiral spirophosphoranes. In these studies, Mironov et al. obtained tricyclic pentacoordinated spirophosphoranes containing a phosphorus-carbon bond with high regio- and stereo-selectivity whereas Kawashima presented the synthetic route to perfectly anti-apicophilic carba-phosphatranes. ... 

In 2006 Terada and Kouchi reported the investigation of phosphonium salts in catalysis [112]. A pentacoordinated phosphorus atom is a hypervalent [113] atom, which has a formal valence shell of more than eight electrons. As shown in Scheme 40, it is possible for the lower lying o orbital of a P -EWG (Electron Withdrawing Group) bond to take up a free electron pair of a Lewis base, in order to form a new bond. If the new formed bond is trans to the EWG, the formed complex is more stable. 

To explain the existence of a conventional double bond in the phosphate ion requires five electrons to be supplied by the central phosphorus atom, which may enter into MO formation with suitable orbitals from the ligand oxygen atoms. To arrange this requires the use of one of the 3d orbitals of the phosphorus atom. This cannot occur in compounds of elements of the second period, where hypervalence is not observed. The availability of accessible d-orbitals has, in the past, been used as the justification of hypervalence in compounds of the subsequent periods. An alternative method of representing the structure of the PO4 ion is shown in Figure 6.6. 

In this canonical form the phosphorus atom has a positive charge and is isoelectronic with the silicon atom. It may then form four single covalent bonds to the four single-negatively charged oxygen atoms. In this form, the phosphorus atom is not showing hypervalency. 

Recent progress on the use of hypervalent iodine reagents for the construction of heteroatom-heteroatom bonds is reviewed. Reactions of aryl-A3-iodanes with heteroatom substrates derived from third-row elements and beyond are considered first, and an unusual example of heteroatom-heteroatom bond formation with diaryliodonium salts is then discussed. Finally, the use of sulfonylimino(aryl)iodanes for imidations of phosphorus, sulfur, selenium, and arsenic compounds, including enantioselective transformations (S,Se), and alternate hypervalent iodine approaches to N-sulfonylsulfilimines and N-sulfonylarsinimines are summarized. 

The use of hypervalent iodine reagents for heteroatom-heteroatom bond forming reactions is well established in the context of classical oxidation chemistry [1-11]. For example, oxidations of anilines to azobenzenes, thiols to disulfides, and sulfides to sulfoxides with aryl-A3-iodanes were documented decades ago [1-5]. During the last ten years, particular attention has also been given to oxidative transformations of compounds derived from heavier elements, including the interception of reaction intermediates or initially formed products with external nucleophiles. A second important development is the utilization of sulfonyliminoiodanes, ArI = NS02R, for heteroatom-nitrogen bond formation, especially for imidations of sulfur, selenium, phosphorus and arsenic com-... 

The bonding in certain tricoordinate phosphorus compounds (ct , 5-P) is also termed hypervalent (DA Scheme 5). In these structures the lone pair of the planar phosphorus is effectively involved in the delocalization (IIB Scheme 5). Bis-meth-ylene-phosphorane (and its N, O, and S analogues) are typical examples for such bonding (Scheme 5). Like in the case of the ylides, no -orbital participation is needed to describe the bonding in these compounds. Rather, the delocalized 3c-4e model can again be used [33], which also accounts for the positive charge at the central phosphorus atom, in accordance with the ylidic resonance structure (IIC Scheme 5). 

Phosphole derivatives with CN = 5 and CN = 6 are hypervalent species, which are quite rare. Indeed, the phosphoranes (CN = 5) are stable only when the P-atom bears electronegative substituents such as fluorine atoms or alkoxy groups. The geometry adopted at phosphorus is distorted trigonal bipyramidal, something enforced by the small endocyclic CPC bond angle. Compounds with CN = 6 are anions with octahedral phosphorus atoms. 

A review on the chemistry of thio derivatives of trivalent phosphorus acids which covers the literature to 1986, includes a section on pentaco-ordinate phosphorus compounds derived from addition to a-diketones and unsaturated systems activated to nucleophilic attack by electron withdrawing groups. Chemical bonding in hypervalent molecules has been discussed and qualitative bonding concepts developed to supersede the dsp and d sp models. A review on the mechanism and stereochemistry of the Wittig olefination reaction inevitably includes a discussion of the equilibrium between betaine and 1,2-oxaphosphetane intermediates. A correction has been published to reference 19 of Chapter 2 in SPR14, Vol.21, concerning the Mitsunobu Reaction. ... 

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