Period 3-6 molecules hypervalent

It is difficult to give a localized orbital description of the bonding in a period 3 hypervalent molecule that is based only on the central atom 3s and 3p orbitals and the ligand orbitals, that is, a description that is consistent with the octet rule. One attempt to do this postulated a new type of bond called a three-center, four-electron (3c,4e) bond. We discuss this type of bond in Box 9.2, where we show that it is not a particularly useful concept. Pauling introduced another way to describe the bonding in these molecules, namely, in terms of resonance structures such as 3 and 4 in which there are only four covalent bonds. The implication of this description is that since there are only four cova-... 

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. 

Because they do not obey the octet rule, hypervalent molecules have often been thought to involve some type of bonding that is not found in period 2 molecules. Ideas concerning the nature of this bonding have developed along a somewhat tortuous path that it is interesting and instructive to follow. We will in the end conclude that the nature of the bonding in these molecules is not different in type from that in related period 2 molecules and that there is therefore little justification for the continued use of this concept. 

Not only molecules with LLPCN > 4, but all molecules of the elements in period 3 and beyond in their higher valence states, including most of their numerous oxides, oxoacids, and related molecules such as SO3 and (H0)2S04 should be regarded as hypervalent if AO bonds are described as double bonds (1). However, Lewis did not regard these molecules as exceptions to the octet rule because he wrote the Lewis structures of these molecules with single bonds and the appropriate formal charges (2). 

For this reason the term hypervalent has often been restricted to the molecules of the elements of period 3 and beyond with LLCPN > 4. We have discussed the nature of AO bonds with A a period 2 element in Section 8.6, where we concluded that they are best represented as double bonds. We will later come to a similar conclusion with regard to AO bonds in which A is an atom of an element from period 3 and beyond. On this basis molecules such as S02(0H)2 would be classified as hypervalent, as would the period 2 molecules OCF3 and ONF3 as discussed in Chapter 8. 

Molecules of the elements of period 3 and beyond may have higher LLP coordination numbers than four, and therefore considered to be hypervalent, because their atoms are larger than those of the period 2 elements. In other words, more than a total of four ligands and lone pairs can pack around a central atom if it is from period 3 and beyond. 

For example, atoms of both the alkaline-earth family (ZAval = 2) and the chalcogen family (ZAval = 6) correspond to FAemp = 2, and their stoichiometric proportionality (or coordination number) to monovalent atoms is therefore commonly two (AH2, ALi2, AF2, etc.). It is a remarkable and characteristic feature of chemical periodicity that the empirical valency FAemp applies both to covalent and to ionic limits of bonding, so that, e.g., the monovalency of lithium (Vuemp = 1) correctly predicts the stoichiometry and coordination number of covalent (e.g., Li2), polar covalent (e.g., LiH), and extreme ionic (e.g., LiF) molecules. Following Musher,132 we can therefore describe hypervalency as referring to cases in which the apparent valency FA exceeds the normal empirical valency (3.184),... 

There are few periodic generalizations that can be made concerning hypervalent molecules, since the factors contributing to stability, such as oxidation potential, central atom size, type of ligands, and ionic versus covalent bonding type, all interact in a complex manner. However, some general trends are evident. 

Molecules and ions with more than an octet of electrons around the central atom are often called hypervalent. Other examples of hypervalent species are SF4, AsFg, and ICl4. The corresponding molecules with a second-period atom as the central atom, such as NCI5 and OF4, do not exist. 

Hypervalent molecules are formed only for central atoms from period 3 and below in the periodic table. The principal reason for their formation is the relatively larger size of the central atom. For example, a P atom is large enough that five F (or even five Cl) atoms can be bonded to it without being too crowded. By contrast, an N atom is too small to accommodate five atoms bonded to it. Because size is a factor, hypervalent molecules occur most often when the central atom is bonded to the smallest and most electronegative atoms— F, Cl, and O. 

The notion of a valence shell containing more than an octet of electrons is also consistent with the presence of unfilled nd orbitals in atoms from period 3 and below. By comparison, elements of the second period have only the 2s and 2p valence orbitals available for bonding. Detailed analyses of the bonding in molecules such as PF5 and SF, suggest that the presence of unfilled 3d orbitals in P and S has a relatively minor impact on the formation of hypervalent molecules, and the general current belief is that the increased size of third-period atoms is the more important factor. 

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