Hypervalent bonding importance

Another important example of hypervalent bonding is the transition state in the Sn2 nucleophilic substitution reaction, which has the same orbital configuration and electron count as a 10-electron hypervalent system. For nucleophilic substitution, the hypervalent species is a transition state rather than a stable species, as shown schematically in Figure 5. Experimental and computational studies on transition states are generally difficult. Stable hypervalent systems can serve as more tractable benchmarks to test the accuracy of computational techniques used on 10-electron systems. 

Owing to a highly polarized character of hypervalent bond, noncovalent attractive interactions of a predominantly electrostatic nature are exhemely important in the stmctural chemistry of hypervalent iodine... 

It s time now to think about hypervalent compounds. You have encountered a few of them already, as products of A reactions and as intermediates in SN2-Si mechanisms. But what is special about such compounds Is the term hypervalent synonymous with higher-valent (No.) To better understand these issues, we ll take a step back in Section 1.24 and remind ourselves what the term valence exactly means and how it differs from related concepts such as coordination number (CN), FC, and oxidation state (OS). Confusion between these terms and incorrect usage are widespread in both textbooks and the research literature. From there we ll proceed on to some related topics such as an elementary molecular orbital description of hypervalent bonding (Section 1.25). We ll conclude this chapter with a brief discussion of the inert pair effect, an important aspect of the variable valence of the heaviest (sixth-period) p-block elements. 

As an illustration of this point, Table 3 reports the first and second ionization potentials (IPs) for the central atom of some selected neutral hypercoordinated species of the type AF , together with the stability of these species with respect to dissociation of the hypervalent bonds. It can be seen that even if the first IP is an important parameter for the stability of AF , the second IP is at least as much important and indeed marks the limit between stable and unstable systems and in particular the first-row systems (NF5, OF4, F4, NeF2). The reason for the first-row exception is therefore quite clear within the present VB model. 

Three basis sets (minimal s-p, extended s-p and minimal s-p with d functions on the second row atoms) are used to calculate geometries and binding energies of 24 molecules containing second row atoms, d functions are found to be essential in the description of both properties for hypervalent molecules and to be important in the calculations of two-heavy-atom bond lengths even for molecules of normal valence. 

The best Lewis-type representation of the bonding in OCF3 would therefore appear to be as in 4, even though the carbon atom does not obey the octet rule. This molecule can be considered to be a hypervalent molecule of carbon just like the hypervalent molecules of the period 3 elements, such as SFfi. We introduced the atom hypervalent in Chapter 2 and we discuss it in more detail in Chapter 9. But it is important to emphasize that the bonds are very polar. In short, OCF3 has one very polar CO double bond and three very polar CF single bonds. A serious limitation of Lewis structures is that they do not give any indication of the polarity of the bonds, and much of the discussion about the nature of the bonding in this molecule has resulted from a lack of appreciation of this limitation. 

Crystalline trithia dication 151 contains a 3-ccntcrA-clcctron bond with a central hypervalent sulfur atom. It was shown to be sufficiently stable to be isolated and studied spectroscopically. One important feature is its boat-boat conformation resulting from the formation of the transannular bond between the three sulfur atoms <1988JA1280>. 

The increasing importance of hypervalent 3-centre 4-electron bonding on going down Group 16 is reflected in the structures of Sej anions with n = 9-11, which are without parallel for the polysulfides. Although a chain structure is still apparent for the nonaselenide anion of [Sr(15-crown-5)2]Se936 (Figure 5a),... 

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