Molecules with Expanded Valence Shells

An atom in a molecule is most stable if it can achieve the electronic configuration of the nearest noble gas, thus having a completely filled valence shell. Hydrogen with two electrons around it, a duet, achieves the configuration of helium. Second-row elements achieve the configuration of neon with an octet of valence electrons. Third-row elements achieve an octet but may also expand their valence shell for example SFg is a stable molecule with six single bonds to sulfur (12 bonding electrons total). 

Molecular Geometries of Molecules with Expanded Valence Shells... 

Molecules with expanded valence shells typically involve nonmetal atoms of the third period and beyond that are bonded to highly electronegative atoms. For example, phosphorus forms two chlorides, PCI3 and PCI5. We can write a Lewis structure for PCI3 with the octet rule. In PCI5, with five Cl atoms bonded directly to the central P atom, the outer shell of the P atom appears to have ten electrons. We might say that the valence shell has expanded to ten electrons. In the SFg molecule, the valence shell appears to expand to 12. 

Figure 10-13 extends the balloon analogy to these cases. The cases for five- and six-electron groups are typified by PCI5 and SF, molecules with expanded valence shells. 

We extended the concept of hybridization of orbitals to molecules with expanded valence shells to include d orbitals, that is, sp d and sp d hybrid orbitals to accommodate five- and six-electron pairs, respectively. Although this is an appealing idea, it has come into question because quantum-mechanical calculations have shown that the wave functions contain very little contribution from d orbitals. Thus, it appears that to describe bonding in SF we should avoid using d orbitals in hybridization schemes. 

Sulfur, unlike oxygen, has the capacity to expand its valence shell beyond the normal octet of electrons to form hypervalent compounds such as sulfur tetrafluoride (SF4) with 10 electrons in the outermost shell and sulfur hexafluoride (SF6) containing 12 electrons in the valence shell.63,7 The chemistry of hypervalent sulfur started in 1873 with the discovery of the unstable compound sulfur tetrachloride (SCI4). The existence of hypervalent sulfur compounds is an important feature of the chemistry of sulfur and the precise nature of the bonding in these molecules has remained a puzzling problem. 

Sulfuranes with hypervalent sulfur atoms possess expanded valence shells, and consequently the molecules are relatively unstable. The central sulfur atom can, however, attain the normal stable octet of electrons by extruding a ligand or a pair of ligands in an elimination process. The former results in ligand coupling (Figure 8 and Schemes 14a and... 

Other ions and molecules behave as Lewis acids by expansion of the valence shell of the central element. Anhydrous tin(IV) chloride is a colorless liquid that also is frequently used as a Lewis acid catalyst. The tin atom (Group IVA) can expand its valence shell by utilizing vacant d orbitals. It can accept shares in two additional electron pairs, as its reaction with hydrochloric acid illustrates. 

S2.3 The Lewis structures and molecular shapes for XcFi and ICb are shown below. The XeFi Lewis structure has an octet for the 4 F atoms and an expanded valence shell of 10 electrons for the Xe atom, with the 8 + (2 x 7) = 22 valence electrons provided by the three atoms. The five electron pairs around the central Xe atom will anange themselves at the comers of a trigonal bipyramid (as in PF5). The three lone pairs will be in the equatorial plane, to minimize lone pair-lone pair repulsions. The resulting shape of the molecule, shown at the right, is linear (i.e., the F-Xe-F bond angle is 180°). 

Relativistic effects play an important role in the spectroscopy of atoms and molecules whenever heavy atoms are involved or electronic or nuclear spins become significant, as in ESR and NMR spectroscopy or for the fine and hyperfine structure of electronic states. Also the chemical behaviour of the heavy elements, beyond Z 50, is strongly influenced by relativistic effects. As the chemical interactions are affected by the slow valence electrons, relativistic effects were thought to be of minor importance. However, electrons of shells with low / values, especially s electrons, do penetrate the atomic core and experience relativistic retardation effects near highly charged nuclei these shells therefore contract. On the other hand, electrons in shells with higher / values, / 2, are screened better, due to the contracted s shells, from the nuclear charge and therefore become destabilized thus these shells are more expanded than expected. 

The octet rule is a useful guide for most molecules with Period 2 central atoms, but not for every one. Also, many molecules have central atoms from higher periods. As you ll see, some central atoms have fewer than eight electrons around them, and others have more. The most significant octet rule exceptions are for molecules containing electron-deficient atoms, odd-electron atoms, and especially atoms with expanded valence shells. 

A stepwise process is used to convert a molecular formula into a Lewis structure, a two-dimensional representation of a molecule (or ion) that shows the relative placement of atoms and distribution of valence electrons among bonding and lone pairs. When two or more Lewis structures can be drawn for the same relative placement of atoms, the actual structure is a hybrid of those resonance forms. Formal charges are often useful for determining the most important contributor to the hybrid. Electron-deficient molecules (central Be or B) and odd-electron species (free radicals) have less than an octet around the central atom but often attain an octet in reactions. In a molecule (or ion) with a central atom from Period 3 or higher, the atom can hold more than eight electrons by using d orbitals to expand its valence shell. 

All molecules with five or six electron groups have a central atom from Period 3 or higher because only these atoms have the d orbitals available to expand the valence shell beyond eight electrons. 

Many molecules with central atoms from Period 3 or higher take part in Lewis acid-base reactions in which the central atom expands its valence shell. SnCl4 reacts with (CH3)3N as follows ... 

Other examples of molecules and ions wifh "expanded" valence shells are SF4, AsFg , and 104 . The corresponding molecules with a second-period atom, such as NCI5 and OF4, do not exist. Let s take a look at why expanded valence shells are observed only for elements in period 3 and beyond in the periodic table. 

On the basis of recent theoretical calculations, some chemists have questioned whether valence d orbitals are actually used in the bonding of molecules and ions with expanded valence shells. Nevertheless, the presence of valence d orbitals in period 3 and beyond provides the simplest explanation of this phenomenon, especially within the scope of a general chemistry textbook. 

The octet rule applies to most molecules (and ions) with Period 2 central atoms, but not every one, and not to many with central atoms from Period 3 and higher. Three important exceptions occur in molecules with (1) electron-deficient atoms, (2) odd-electron atoms, and (3) atoms with expanded valence shells. In this discussion, you ll also see that formal charge has limitations for selecting the best resonance form. 

Expanded Valence Shells Many molecules (and ions) have more than eight valence electrons around the central atom. An atom expands its valence shell to form more bonds, which releases energy. The central atom must be large and have empty orbitals that can hold the additional pairs. Therefore, expanded valence shells (levels) occur only with nonmetals from Period 3 or higher because they have d orbitals available. Such a central atom may be bonded to more than four atoms or to four or fewer. 

In a molecule (or ion) with a central atom from Period 3 or higher, that atom can have more than eight valence electrons because it is larger and has empty d orbitals for expanding its valence shell. 

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