Bonding three-centre, four-electron

A common interpretation of the interaction of chalcogens with nucleophiles considers donation of electron density from a lone pair on the donor atom into the o- (E-X) orbital (Figure 15.1). As the degree of covalency increases, a hypervalent three-centre four-electron bond is formed. Real systems fall somewhere between secondary interactions and hypervalent (three centre - four electron) bonds. The two extremes can be distinguished by the correlation of X-E and E D distances.In the hypervalent case both bond distances decrease simultaneously, whereas in the secondary bond the distances are anticorrelated. This concept has been applied in a study of selenoquinones 15.17 (R = Ph, Me) with short Se 0 contacts,for... 

FIGURE 7. Molecular orbitals for a three-centre, four electron bond (a) antibonding (n) non-bonding (b) bonding... 

Recent detailed discussions229,283,284,286,289,290 of the bonding in phosphoranes has usually taken for granted the three-centre, four-electron bond model described above and have concentrated on other related issues of importance in these systems, viz. apicophilicity, pseudorotation and the oxaphosphetanes involved in the Wittig reaction. 

In a recent study286, the term equatoriphilicity was introduced. This is because it was found, in a wide-ranging study at the MP4/DZ + P level, that the stabilization of the equatorial plane part of the molecule determines the stability of the whole molecule286. The same study confirmed the three-centre, four-electron character of the bonds to the axial ligands286. 

In translating this MO description into VB language, we speak of a three-centre, four-electron bond, often abbreviated as (3c,4e) three orbitals rather than four are used to hold the three atoms together. An equivalent description in VB language involves resonance structures ... 

On the basis of an inert pair of s electrons, formation of anionic species from anti-mony(III) and bismuth(lll) halides and their related mono- and di-organo derivatives follows if a halide ion is considered to approach the central atom trans to one of the bonds already present and the X /M—X system is treated via a three centre-four electron approach. This reproduces not only the observed structures of the anions but also the observed range of metal-halogen distances. An alternative, qualitative, molecular orbital approach is that a second halogen is bonded via the antibonding orbital of an M—X bond already present. 

The bonding may be described as three centre, four electrons s -bonding. A linear combination of the atoms gives the best overlap of the orbitals. 

R = CH2ph) is rationalized as the consequence of the dominant electronic stabilization effect associated with greater delocalization and hence greater charge balance in the phosphorus orbitals involved in the three-centre, four-electron H-P-Nax bond. This idea is supported by X-ray the short P-N x bond distance in 138H is 2.078 A... 

Fig. 12 The extension of the Lewis ideas for hypo-valent and h3rper-valent molecules. In the former, the electron deficiency is relieved by the formation of dative bonds either from lone pairs or B-H bonds. In the latter, the hyper-valency is brought into line with the EAN rule by using Lewis bond structures where the central atom bears positive charges which compensate the negative charges on the fluorines. The results in Table 1 suggest that x is associated with the number of three-centre four-electron Xe-F bonds in these structures...

Initially these hyper-valent compounds were thought to result from the promotion of electrons from xenrai valence orbitals into higher-lying d orbitals and subsequent formation of two-centre two-electron bonds with O or F, i.e. they exceed the octet rule. Contemporary interpretations of the bmiding in these compounds favour the formation of three-centre four-electron bonds. This interpretatiOTi results in the ionic canonical forms illustrated in Fig. 12, which when in resonance reproduce the observed symmetric geometries. In these molecules, x represents the number of three-centre four-electron bonds in the molecule [101]. 

A new model, based on the bonding in ozone, has been proposed for the bonding of an O 2 molecule to the iron of Hb and myoglobin (Mb). It involves basically a biradical with a singlet state stabilized by a three-centre four-electron jT-bond and leads to an easy rationalization of the facile formation and dissociation of the Fe—O2 bond since the O2 would always retain its triplet ground state character. 

Rudd, M.D., Lindeman, S.V and Husebye, S., Three-centre, four-electron bonding and structural characteristics of two-coordinate iodine(I) complexes with halogen and chalcogen ligands. Synthesis, spectroscopic characterization and x-ray structural studies of (triiodo) [tris(dimethylamino)phosphaneselenide]iodine and bis (triiodo) [tri(iV-morpholyl)phosphaneselenide]iodine(I) /diiodine molecular complex, Acta Chem. Scand. 51 (6-7), 689-708 (1997). 

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