Hypervalent bonding

Dirac wrote soon after the development of quantum mechanics that the underlying physical laws necessary for the mathematical theory of a large part of physics and the whole of chemistry are thus completely known, and the difficulty is only that the exact application of these laws leads to equations much too complicated to be soluble. This statement certainly applies to chemical bonding, and models of varying degrees of accuracy 

One of the most important chemical models is the use of Lewis dot structures to predict the structure and bonding of molecules. Although this model predates modern quantum mechanics, it is still the primary means most chemists use to form a mental picture of a molecule. The tremendous success of this model, however, can blind chemists to its limitations. Studies of the thermochemistry of gas-phase ions demonstrate dramatically that this model can be misleading in some circumstances. 

Iodine trichloride or trichloro-X -iodane (Dichloroiodo)benzene 

Diphenyliodonium chloride or chloro(diphenyl)- . -iodane [/V-(4-Methylphenylsulfonyl) i m i no] pheny I- iodane Iodine pentafluoride or pentafluoro-X -iodane Iodic acid lodylbenzene 1 -Hydroxy-1 -oxo-1 H-IX -benzo [t/j [ 1,2 ] iodoxo I -3 -one 

Iodine heptafluoride or heptafluoro-X -iodane Periodic acid 

Phenyliodo bis(trifluoroacetate) Phenyliodine(lll) bis(trifluoroacetate) [ H y d roxy(to Isy loxy) iodo] benzene Koser s reagent lodosobenzene 

2-lodosylbenzoic acid o-lodosobenzoic acid 1 -Hydroxy-1,2-benziodoxol-3-(1 H)-one 

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For a discussion of hypervalent bonding see reference 98. This picture indicates that there is an accumulation of partial negative charge on the apical ligands and thus a partial positive charge on the central atom. Thus the apical ligands should be electronegative. 

G. A. Papoian, R. Hoffmann, Hypervalent bonding in one, two and three dimensions extending the Zintl-Klemm concept to nonclassical electron-rich networks. Angew. Chem. Int. Ed. 39 (2000) 2408. 

The charge densities and bond orders of the hypervalent bond of these telluranes 159, 160, and 161 were calculated by the ab initio RHF method on the basis of 3-21G. The results are shown in Figure 8. Interestingly, these ab initio calculations reveal that the positive charges at the hyperavalent bond 159 and 160 are distributed... 

As discussed in detail in Section 2.4, the pronounced tendency of tellurium to participate in both hypervalent bonding and secondary 5p2 —> 5a interactions can lead to the construction of polymeric telluride networks. 

Scheme la shows the approximate molecular orbital model for the hypervalent X-E-X 3c-4e in EX4, such as SC14. Characters of the three molecular orbitals are bonding (v /i), nonbonding (v /2), and anti-bonding (v /3). Two electrons are in and two in v 2. Electrons in v 2 localize on X of X-E-X and the hypervalent bonds are mainly characterized by v 2. Consequently,... 

The hypervalent chalcogen chemistry has been developed to higher coordinated species with various ligands,7 10 where TBP changes to square pyramidal (SP) or octahedral (Oh), etc. Additional atomic orbitals of E, such as an 5-orbital, may operate to stabilize the structures.10b The concept is also extended over linear a-bonds constructed by m ( > 4) atoms with n electrons (extended hypervalent bonds mc-ne (in > 4)).11 14 The approximate molecular orbital model for mc-ne (m > 4) is also exhibited in Scheme la, exemplified by 4c-6e. 

What is the region of bond distances for hypervalent bonds of chalcogens Bond orders of hypervalent 3c-4e are typically around 0.5. A bond distance with a bond order of 0.5 is predicted to be 0.18 A longer than that with a bond order of 1.0.16... 

Hypervalent 10—E f (C2X2) compounds are widely studied.10a,b Furukawa and co-workers studied the 10-E-4 (C2X2) of the types 30 and 31, together with the related compounds, such as 32.10b,15<3,35,55 The interactions are established by the X-ray analysis and NMR.15d,3S Adducts 33 and 34 are also detected by NMR. If the X and Te atoms in 33 and 34 align linearly, the multi-center bonds are expected to form the extended hypervalent bonds, which will be discussed later. 

Extended hypervalent bonds containing chalcogens have been substantially discussed in the last decade, however. 

A third, more extreme, conceptual model, based on a completely ionic picture of hypervalent bonding, can also be invoked to remove perceived conflicts with Lewis-structural principles. In PF5, for example, the completely ionic ( oxidation number ) P5+(F )5 representation... 

As a result of the strong tendency toward 3c/4e hypervalent bonding, each M—L coordinative bond of a normal-valent ML transition-metal complex will be susceptible to successive cu-additions by other coordinative ligands L cf. (3.212a) ... 

The NRT formalism will be used to describe the interacting species along the entire reaction coordinate. Such a continuous representation allows the TS complex to be related both to asymptotic reactant and product species and to other equilibrium bonding motifs (e.g., 3c/4e hypervalent bonding Section 3.5). A TS complex can thereby be visualized as intermediate between two distinct chemical bonding arrangements, emphasizing the relationship between supramolecular complexation and partial chemical reaction. 

The parent compound and a set of monosubstituted bis(acylamino)diarylspiro-X4-sulfanes (360 X = H, Me, MeO, Cl, NO2) undergo hydrolysis to the corresponding sulfoxides (361). The probable mechanism involves rate-determining cleavage of one of the S—N hypervalent bonds in the spiro ring with simultaneous proton transfer to the nitrogen atom. The hydroxide ion which is formed thereby then attacks the sulfur atom in a fast step to form a diaryl(acylamino)hydroxy-k4-sulfane (362), which is converted into the sulfoxide (361) (Scheme 47). ... 

Ab initio calculation of energy changes accompanying distortions of the dioxathiaazapentalene cation (8) showed structure (8) not to have the lowest energy but to be rather a transition state between the two valence bond structures (9) and (10). This process corresponds to an in-plane bend parallel to the 3c,4e-hypervalent bond at sulfur. An out-of-plane bend about the SN bond also leads to an energy minimum but this is at a much higher level and is therefore less realistic <91PS(55)35>. 

The first isolated adduct between an aminocarbene and a Lewis acid was obtained by reacting an imidazol-2-ylidene (IV) with iodopentafluorobenzene (Scheme 8.22). In contrast to classical halonium methylides (obtained from transient electrophilic carbenes and halogen centers), which feature a characteristically small C—X—C angle, this adduct has an almost linear C—I—C framework (178.9°). This hnding is consistent with a 10e-I-2c (hypervalent) bonding at the iodine center, and a reverse ylide nature. 

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