Hypervalent, description

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. 

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-... 

Hybrid Orbital Descriptions of the Bonding in Hypervalent Molecules... 

A satisfactory description of the bonding in hypervalent molecules can also be given in terms of molecular orbitals but this does not directly correspond to the very useful picture of five or more localized bonds (see, for example, Mingos, 1998, p. 250). 

The NBO donor acceptor picture of hypervalency hyperbonds Let us re-examine the 3c/4e MO description from the localized NBO perspective. NBO analysis of the MO wavefunction (3.193) may lead to the Lewis structure A + B—C, with NBOs... 

The cu-bonding model provides a more complete and fundamental description of hypervalent molecules that are often interpreted in terms of the VSEPR model.144 In the present section we examine some MX species that are commonly used to illustrate VSEPR principles, comparing and contrasting the VSEPR mnemonic with general Bent s rule, hybridization, and donor-acceptor concepts for rationalizing molecular geometry. Tables 3.32 and 3.33 summarize geometrical and NBO/NRT descriptors for a variety of normal-valent and hypervalent second-row fluorides to be discussed below, and Fig. 3.87 shows optimized structures of the hypervalent MF species (M = P, S, Cl n = 3-6). 

Another type of hypervalency is encountered in textbook descriptions of the oxyanions of common laboratory acids. Generations of chemistry students have been taught that the correct representations of these species are in terms of resonance-delocalized hypervalent Lewis-structure diagrams, such as sulfate (S042-),... 

The deviations between theory and experiment normally tend to be somewhat smaller for first-row than for second-row compounds. Among the established semiempirical methods, PM3 seems to be the best for the first-row compounds, but the OM1 and OM2 approaches with orthogo-nalization corrections [23-25] perform even better, with mean absolute deviations being around 3-5 kcal/mol. For the second-row compounds, MNDO/d is currently the most accurate among the semiempirical methods considered. This performance has been attributed [16-18] to the use of an spd basis which allows a balanced description of normalvalent and hypervalent molecules. OM1 and OM2 have not yet been parameterized for second-row elements. 

Dipole moments for hypervalent molecules calculated from semi-empirical models are generally larger than experimental values (sometimes by a factor of two or more), suggesting descriptions which are too ionic. Figure 10-11 provides an overview for the PM3 model. Semi-empirical models should not be used. 

Molecules and polyatomic ions in which the octet is exceeded -sometimes called hypervalent species - present problems. Many well-known molecules such as PF5, SF6 and C1F3 cannot be represented by Lewis structures which obey the octet rule. VB descriptions of hypervalent species can be devised by postulating hybridisation schemes... 

Three-centre bonding is invoked in situations where the o framework cannot be described in terms of two-centre, electron-pair bonds, although it can often be accommodated by postulating resonance of a different type from that usually encountered. Two types of three-centre bond can be distinguished. The first is often postulated in hypervalent molecules/polyatomic ions AB where the central atom exceeds the octet in its Lewis formulation, as an alternative to the use of d orbitals which many chemists find objectionable. The second type occurs where there appear to be insufficient electrons - regardless of the supply of orbitals -to form the requisite number of bonds in a Lewis/VB description. In other words, the first type is postulated where we have an insufficiency of orbitals, and the second where there is a deficiency of electrons compounds containing the latter type are often described as electron-deficient . 

Scheme 27 Hypervalent a. / -phospholes 41 (E O, S, NH, CH2) with ylenic and ylidic description...

Initially, hypervalent molecules like PCI5 and SFe were described in terms of sp d" hybrid orbitals, in an extension of the sp" orbital description used to characterize molecules that obey the octet rule. However, ab initio calculations have shown that d orbitals play only a minor role in the bonding of hypervalent molecules. An alternative approach proposed by Pauling invoked combinations of resonance stmctures involving four covalent bonds and one or more additional ionic bonds, as shown in Scheme 1 for PF5. 

An alternative explanation, one that employs a highly ionic model neglecting d orbital participation, has been proposed. Described originally by Musher in VB terms, it has an equivalent MO description, which is given here. It is in this model that the hypervalent bond is clearly seen. The essential features of the MO model in its most rudimentary form are (1) the central atom uses only valence s and p orbitals ... 

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