Hypervalent 10-1-3 species

Phosphorous ylides such as triphenylphosphine-metJhylidene may either be represented as hypervalent species incorporating a phosphorous-carbon double bond, or in terms of a zwitterion, that is, a molecule with separated positive and negative charges. 

However, at first sight the ionic-resonance model would not seem applicable to I3- and related symmetric hypervalent species, because extreme I+I ionicity differences would not be expected between central and terminal atoms of intrinsically equal electronegativity. Nevertheless, we shall show that the complementary bidirectional resonance stabilization motif (3.188) can lead to effective three-center bonding even if central and terminal atoms are of equal electronegativity. 

Pimentel employed this three-center, four-electron (3c/4e) MO model to discuss the bonding in triiodide (I3-), bifluoride (FHF-), and other prototypical hypervalent species. In triiodide and other trihalides, for example, the relevant AOs are the (pa, Pb, Pc) orbitals along the bonding axis,... 

The final cu-bonded formulas (3.213), (3.214), and (3.219)-(3.221) bear an obvious resemblance to the usual VSEPR representations of these hypervalent species. Indeed, each cu-bonded structure has the same number of formal bond pairs (bp) and lone pairs (lp) as the VSEPR representation. Furthermore, the predicted angular geometries of the two models are essentially identical, with the linear (or near-linear) cu-bonded ligands occupying axial positions in the SN2-like trigonal bipyramidal motif. 

Figure 3 ORTEP diagram of the hypervalent species 44. Reproduced from Kano, N. Goto, S. Kawashima, T. Organometallics 2003, 22, 1152-1155.

Many of those examples involving ligand coupling within hypervalent species have been documented (86MI1, 86PS13 88MI1). 

Polyauration starts from the carbon atom for which the species with four, five and six gold atoms have been prepared. These are available from the reaction of polyborylmethanes with [AuCl(PR3)] or trimethylsilyl diazomethanes with [0(AuPR3)3]+. The tetranuclear derivatives are formed with bulky phosphines and less sterically demanding phosphines enable the synthesis of the hypervalent species [230]. The structures of these complexes are tetrahedral, trigonal bypiramidal and octahedral, respectively (Figure 1.36). Many complexes of the type [RC(AuPR3)4]+ [231] have also been synthesized. 

Molecules and polyatomic ions in which the octet is exceeded -sometimes called hypervalent species - present problems. Many well-known molecules such as PF5, SF6 and C1F3 cannot be represented by Lewis structures which obey the octet rule. VB descriptions of hypervalent species can be devised by postulating hybridisation schemes... 

These arguments have been debated for many years. Chemists who reject the use of nd orbitals in hybridisation schemes prefer three-centre bonds to describe hypervalent species. These are best portrayed in MO language (see Section 7.4) translated into VB terminology, they correspond to polar (or ionic) structures, e.g. ... 

Good expositions of VB theory are given by Pauling (1960), Cartmell and Fowles (1977) (see Section A.7 of the Appendix) and Lagowski (1973) (see Section A.3). McWeeny s revisions of Coulson s classic book (1979 and 1982) (see Section A.7) emphasise the three-centre bond approach to hypervalent species. See also Dasent (1965) (Section A.8) for discussion of nonexistent compounds. 

In the case of the other hypervalent element compounds, these structural differences are strictly reflected in their terminology. For instance, sulfonium salts such as Me3S+Cl are clearly differentiated from sulfuranes such as Ph2SCl2. The latter is a hypervalent species of decet structure [10-S-4] and pseudotrigonal bipyramid with a linear Cl-S-Cl hypervalent bond however, the former is not a hypervalent compound and has pseudotetrahedral geometry with octet structure [8-S-3]. 

We have proposed (57) an additional method for such fast dissociation using ions such as (peptide + H3) >+ (eq 1). Our studies have shown that neutralization of an ionized saturated heteroatom site such as a quaternary ammonium ion gives an unstable hypervalent species. 

It is also interesting that a parallel trend is observed for 33S, 31P and 77Se chemical shifts when lone pairs are used to form hypervalent species. When one lone pair is used to coordinate one oxygen atom, S, P and Se chemical shifts increase significantly, which means that the nuclei are more deshielded (as expected, since the electron density around the nucleus decreases). However, in coordinating another oxygen atom, the loss of the second lone pair produces a shielding effect. 

The kinetics of nucleophilic substitution at the silicon atom assisted by uncharged nucleophiles have been studied by Corriu et at. (248-251). Hydrolysis of triorganochlorosilanes induced with HMPA, DMSO, and DMF was used as the model. The reaction proceeded according to the third-order kinetic law, first order with respect to the nucleophile, the silane, and the silylation substrate. Very low values of activation enthalpy and high negative entropy of activation were observed (Table VI). These results were taken as evidence for the intermediacy of silicon hypervalent species (249,251) however, they are also perfectly consistent with... 

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. 

All these experiments illustrate the great reactivity of hypervalent species. They confirm the possibility of pentacoordinated intermediates in the nucleophilic activation. This possibility cannot be ruled out only on the basis of the argument of a more crowded and less electrophilic species than tetracoordinated silicon. Furthermore, after these results, it becomes interesting to understand why these hypercoordinated species react faster than the tetracoordinated species. Two possible explanations are the increase in the length of Si —X bonds which corresponds to a higher lability and the increase of the electrophilicity of the central silicon atom. 

In the context of the preparation of novel arsenic hypervalent species and the systematic stndy of mffling in porphyrins by the influence of axially coordinated ligands to a central element, complexes with arsenic and porphyrins were isolated and fully characterized. The porphyrins used are octaethylporphyrin, (OEP) and tetraphenylporphirin (TPP). Reaction of OEPH2 or TPPH2 with ASCI3 in the presence of lutidine give [(OEP)AsCl] (53) or [(TPP)AsCl] (54) according to equation (35). ... 

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