Bond model, three center

Breakdown of the Localized Bond Model Three-Center Bonds, Conjugated Molecules, and Reaction Intermediates... 

Boranes are typical species with electron-deficient bonds, where a chemical bond has more centers than electrons. The smallest molecule showing this property is diborane. Each of the two B-H-B bonds (shown in Figure 2-60a) contains only two electrons, while the molecular orbital extends over three atoms. A correct representation has to represent the delocalization of the two electrons over three atom centers as shown in Figure 2-60b. Figure 2-60c shows another type of electron-deficient bond. In boron cage compounds, boron-boron bonds share their electron pair with the unoccupied atom orbital of a third boron atom [86]. These types of bonds cannot be accommodated in a single VB model of two-electron/ two-centered bonds. 

The carbonyl clusters provide an interesting set of models for the chemisorption of CO on nickel. It is very interesting that, for the Ni (CO) assignments, a plot of Pco versus 1/n for the three-center-bonded CO moieties extrapolates to 1950 cm for = oo (the "chemi-... 

The Three-Center, Four-Electron (3c, 4e) Bond Model... 

The difference in the axial and equatorial bond lengths has been discussed in terms of the three-center, four-electron model (Box 9.2), but this model was postulated on the basis of the known difference in axial and equatorial bond lengths and so does not provide an explanation of this difference in bond lengths,... 

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 equilibrium geometry and three-center NBOs of the protonated ethylene model are displayed in Fig 3.97. The qualitative similarities to the diborane bridge bond (Fig. 3.93) are evident. Thus, the 7t—s model (cf. Fig. 3.91) may be considered a useful descriptive picture of three-center T-bond formation in diborane. 

Figure 1-17 shows three ways to illustrate the stereochemical structures of simple molecules. The perspective diagram specifies stereochemistry unambiguously, but bond angles and center-to-center bond lengths are better represented with ball-and-stick models. In space-... 

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... 

Spherical-domain models of three-center bonds in localized-molecular-orbital models of a nonclassical carbonium ion, B4CI4, and TaeClfJ have been described 49,52) a drawing of a spherical-domain model of the methyl lithium tetramer, (LiCH, is shown in Fig. 31. Large, outer circles represent domains of electron-pairs of C—H bonds. Solid circles represent domains of Li+ ions. Shaded circles represent 4-center lithium-lithium-lithium-carbon bonds — i.e., electron-pair domains that touch, simultaneously, three lithium ions and the kernel of a carbon atom. The... 

Although complexes with C—H—metal three-center, two-electron bonds were first observed several years ago (40-42), they have received increasing attention recently as model systems for C—H activation by transition metal complexes (43). A general route to such compounds involves the protonation of diene (35,44-51) or olefin complexes (52-56). The resulting 16-electron species are stabilized by the formation of C—H—metal bridges. Irradiation of the complexes [Cr(CO)s L] [L = CO, P(CH3)3, P(OCH 3)3 jin presence of conjugated dienes having certain substituents provides a photochemical route to electron-deficient >/4 CH-diene complexes. 

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