Expanded valence shells

S. Oae Ligand coupling reactions through hypervalent and similar valence-shell expanded intermediates [11]... 

The effect of resonance is even more pronounced in sulfuric acid. The availability of d orbitals on sulfur enables us to write valence-shell-expanded Lewis structures containing as many as 12 electrons (Sections 1-4 and 1-5). Alternatively, charge-separated structures with one or two positive charges on sulfur can be used. Both representations indicate that the pATa of H2SO4 should be low. 

It also forms compounds known as carbonyls with many metals. The best known is nickel tetracarbonyl, Ni(CO)4, a volatile liquid, clearly covalent. Here, donation of two electrons by each carbon atom brings the nickel valency shell up to that of krypton (28 -E 4 x 2) the structure may be written Ni( <- 0=0)4. (The actual structure is more accurately represented as a resonance hybrid of Ni( <- 0=0)4 and Ni(=C=0)4 with the valency shell of nickel further expanded.) Nickel tetracarbonyl has a tetrahedral configuration,... 

Third-row elements such as sulfur can expand their valence shell beyond eight electrons, and so sulfur-oxygen bonds in sulfoxides and sulfones are sometimes represented as double bonds. 

The thiophene sulfur atom shows very few of the reactions expected of a sulfide. The oxidation to a sulfone is difficult to achieve, but is of special interest, as knowledge of its aromatic character or lack of it would give information about the ability of sulfur to expand its valence shell beyond eight electrons. 

The major features of molecular geometry can be predicted on the basis of a quite simple principle—electron-pair repulsion. This principle is the essence of the valence-shell electron-pair repulsion (VSEPR) model, first suggested by N. V. Sidgwick and H. M. Powell in 1940. It was developed and expanded later by R. J. Gillespie and R. S. Nyholm. According to the VSEPR model, the valence electron pairs surrounding an atom repel one another. Consequently, the orbitals containing those electron pairs are oriented to be as far apart as possible. 

The octet rule tells us that eight electrons fill the outer shell of an atom to give a noble-gas ns1ns(l valence-shell configuration. However, when the central atom in a molecule has empty d-orbitals, it may be able to accommodate 10, 12, or even more electrons. The electrons in such an expanded valence shell may be present as lone pairs or may be used by the central atom to form additional bonds. 

A note on good practice Although expanded valence shell is the logically precise term, most chemists still use the term expanded octet. 

Because the additional electrons must be accommodated in valence orbitals, only nonmetal atoms in Period 3 or later periods can expand their valence shells. 

Elements that can expand their valence shells commonly show variable covalence, the ability to form different numbers of covalent bonds. Elements that have variable covalence can form one number of bonds in some compounds and a different number in others. Phosphorus is an example. It reacts directly with a limited supply of chlorine to form the toxic, colorless liquid phosphorus trichloride ... 

EXAMPLE 2.7 Writing a Lewis structure with an expanded valence shell... 

The fluoride SF4 forms when a mixture of fluorine and nitrogen gases is passed over a film of sulfur at 275°C in the absence of oxygen and moisture. Write the Lewis structure of sulfur retrafluoride and give the number of electrons in the expanded valence shell. 

STRATEGY Because sulfur is in Period 3 and has empty 3d-orbitals available, it can expand its valence shell to accept additional electrons. After assigning all the valence electrons to bonds and lone pairs to give each atom an octet, assign any remaining electrons to the sulfur atom. 

In this structure sulfur has 10 electrons in its expanded valence shell. 

When different resonance structures are possible, some giving the central atom in a compound an octet and some an expanded valence shell, the dominant resonance structure is likely to be the one with the lowest formal charges. However, there are many exceptions and the selection of the best structure often depends on a careful analysis of experimental data. 

Silicon compounds can also act as Lewis acids, whereas carbon compounds typically cannot. Because a silicon atom is bigger than a carbon atom and can expand its valence shell by using its d-orbitals, it can accommodate the lone pair of an attacking Lewis base. A carbon atom is smaller and has no available d-orbitals so in general it cannot act as a Lewis acid. An exception to this behavior is when the carbon atom has multiple bonds, because then a Tt-bond can give... 

Carbon is the only member of Group 14/IV that commonly forms multiple bonds with itself singly bonded silicon atoms can act as Lewis acids because a silicon atom can expand its valence shell. 

Examples freezing N2(g) + 3 H2(g) - 2 NH,(g). expanded valence shell A valence shell containing more than eight electrons. Also called an expanded octet. Examples the valence shells of P and S in PC1S and SFh. expansion work See work. experiment A test carried out under carefully controlled conditions. 

Experimentally, spin-allowed d-d bands (we use the quotation marks again) are observed with intensities perhaps 100 times larger than spin-forbidden ones but still a few orders of magnitude (say, two) less intense than fully allowed transitions. This weakness of the d-d bands, alluded to in Chapter 2, is a most important pointer to the character of the d orbitals in transition-metal complexes. It directly implies that the admixture between d and p metal functions is small. Now a ligand function can be expressed as a sum of metal-centred orbitals also (see Box 4-1). The weakness of the d-d bands also implies that that portion of any ligand function which looks like a p orbital when expanded onto the metal is small also. Overall, therefore, the great extent to which d-d bands do satisfy Laporte s rule entirely supports our proposition in Chapter 2 that the d orbitals in Werner-type complexes are relatively well isolated (or decoupled or unmixed) from the valence shell of s and/or p functions. 

The list of molecules whose PECD has been experimentally studied is quickly expanding, and in the VUV valence shell region now includes the prototypical chiral species camphor [36, 64, 65], bromocamphor [65, 80], fenchone and carvone [38], methyl oxirane [62, 63], glycidol [37, 38], and 3-hydroxytetrahy-drofuran [61]. Studies of camphor [56], fenchone [38], and carvone [55] have all been extended to cover the SXR C li core region. 

SF4 can also react as a Lewis base, but in some cases a fluorine atom is the electron pair donor. With the strong Lewis acids such as BF3 and SbF5, the reactions can lead to the formation of the SF3+ cation. It is also possible for SF4 to react with Lewis bases and thereby expand the number of electrons in the valence shell. When reacting with l , the Sl s species results. 

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