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Expanded Octet Models

Comparing the 3c-4e and Expanded Octet Models Condensed-Phase Experiments [Pg.55]

Early computational work on hypervalent eompounds indicated that inclusion of d functions in the basis set is important in these systems, suggesting that d orbitals were occupied. It is now generally believed that the d functions in ab initio calculations act as polarization functions. Indeed, similar d function occupancy is seen in both hypervalent and nonhypervalent systems. [Pg.56]

Surprisingly, these developments have not been reflected in chemistry pedagogy. Although some advaneed texts give a careful account of hypervalent bonding, ehemistry textbooks at the introductory level still follow the expanded oetet model. Thus, a strange situation has arisen where nearly all ehemistry students are taught a model that is rejected by most speeialists aetive in the field. Lewis said in 1923 that ... the chemieal bond is always a pair of eleetrons which lies between two atomic centers and is held jointly in the shells of the two atoms, and most textbooks leave it at that. [Pg.56]

This laek of diseussion does not mean that hypervalent bonding is uneommon. It oeeurs for nearly all of the p-block elements. Noble gas compounds, of course, have 3e-4e bonds. The central atom in trihalide anions sueh as li is hypervalent. Similar 3c-4e interactions occur for halides of group 13-17 elements examples include anions such as AIF,  [Pg.56]

SiFe 2, PFg, as well as neutral compounds such as PF5, Sp6, and 5/ Silicon forms a wide variety of hypervalent compounds and the gas-phase chemistry of these has been explored in some detail.Hypervalent compounds of such unlikely elements as and He have been detected, or predicted using high-level ab initio calculations. Hydrogen bonding is also 3c-4e bonding for hydrogen or helium as the central atom, the Is orbital is substituted as the middle orbital in the schemes discussed above. [Pg.57]


The expanded octet model includes one or more d orbitals in the valence shell, allowing an electron count greater than eight. The energy cost of promoting electrons from an p orbital io d orbital (with the same principle quantum number) is presumed to be made up by the additional covalent bonding. The electron hybridization of a ten-electron system, for example, is sp d, which can be broken down into a pd pair of axial orbitals and a sp trio of equatorial orbitals. Expansion of the octet is not feasible for the second-row elements, such as C, N, O, and F, because there are no 2d orbitals. [Pg.51]

One more conceptual framework for understanding hypervalent bonding is to consider a molecule ABC to be held together by the interaction between an electron-pair donor A and an electron-pair acceptor BC, as seen in Figure 2. If the acceptor orbital is the dacceptor orbital is a a orbital of BC, then this is the 3c-4e model. This model is useful for association/ dissociation processes, and it also can be effective for considering... [Pg.54]

There is an obvious discrepancy between the experimental solution thermochemistry, which seems to support the expanded octet model, and recent computational results, which are more consistent with the 3c-4e... [Pg.57]

The electronic repulsion is greater for smaller atoms, and also manifests itself in nonhypervalent systems. For example, F has a substantially lower diatom bond energy than Cl D(F-F)=155 kJ/mol, D(C1-C1) = 239 kJ/mol). The electronegativity effect will be discussed in the next section. Overall, these results are a strong argument in favor of the 3c-4e model being more accurate than the expanded octet model. [Pg.76]

For phosphorus, sulphur and chlorine (and other main-group element) atoms, it has been shown by many researchers that d orbitals do not act primarily as valence-shell orbitals but instead as polarization functions or as acceptor orbitals for back-donation from the ligands, dius disproving the expanded octet model i.e. for these atoms, one should speak of d functions, not d oibitals . ... [Pg.230]

What is the evidence for this model rather than the sp d expanded octet one First we should ask, in what sense is it meaningftrl to talk about the participation... [Pg.57]

Hybridization models involving d orbitals, such as dsp and d sp, have been used to explain molecules with an expanded octet of electrons, for example PCI5 and SFg. This is one model used to explain the observed shapes of certain molecules. [Pg.502]

The octet rule in the Lewis model has some exceptions, which we examine in this section of the chapter. They include (1) odd-electron species, molecules or ions with an odd number of electrons (2) incomplete octets, molecnles or ions with fewer than eight electrons around an atom and (3) expanded octets, molecnles or ions with more than eight electrons aronnd an atom. [Pg.406]

Exception 3. Beyond the second row, the simple Lewis model is not strictly apphed, and elements may be surrounded by more than eight valence electrons, a feature referred to as valence-shell expansion. For example, phosphorus and sulfur (as relatives of nitrogen and oxygen) are trivalent and divalent, respectively, and we can readily formulate Lewis octet structures for their derivatives. But they also form stable compounds of higher valency, among them the familiar phosphoric and sulfuric acids. Some examples of octet and expanded-octet molecules containing these elements are shown below. [Pg.16]

A drawing of a two-dimensional, electron-domain model of a conventional Lewis lone pair is shown in Fig. 23. The lone pair and bonding pairs are structurally equivalent they have identical van der Waals envelopes. Such seems to be nearly the case for lone pairs in the valence-shells of small-core, non-octet-expanding atoms (carbon, nitrogen, oxygen and fluorine). [Pg.26]

In spite of all the theoretical evidence, accumulated over many years, it is still commonplace for students to be taught that the existence of hypercoordinate molecules such as SF and PFri relies on the utilization of d orbitals to expand the octet . Indeed, even models based on d sp , dsp2 and dsp3 hybrid orbitals or pn-dn back-bonding are still in use to describe hypercoordinate bonding to second-row elements. Of course, the consensus view that has emerged from most of the... [Pg.537]

It is significant to note that this model places the extra electrons on the ligands attached to the central atom and not on the central atom. Thus the octet may only be expanded in a formal sense. [Pg.1659]


See other pages where Expanded Octet Models is mentioned: [Pg.50]    [Pg.55]    [Pg.56]    [Pg.50]    [Pg.55]    [Pg.56]    [Pg.54]    [Pg.58]    [Pg.454]    [Pg.31]    [Pg.177]    [Pg.126]    [Pg.113]    [Pg.5254]    [Pg.5253]    [Pg.319]   


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