Octet rule Main-group elements

Many transition-metal complexes, including Ni(CO)4, obey the 18-electron rule, which is to transition-metal complexes as the octet rule is to main-group elements like carbon and oxygen. It states that... 

This mode of calculation has been called the EAN rule (effective atomic number rule). It is valid for arbitrary metal clusters (closo and others) if the number of electrons is sufficient to assign one electron pair for every M-M connecting line between adjacent atoms, and if the octet rule or the 18-electron rule is fulfilled for main group elements or for transition group elements, respectively. The number of bonds b calculated in this way is a limiting value the number of polyhedron edges in the cluster can be greater than or equal to b, but never smaller. If it is equal, the cluster is electron precise. 

In Chap. 3 the elementary structure of the atom was introduced. The facts that protons, neutrons, and electrons are present in the atom and that electrons are arranged in shells allowed us to explain isotopes (Chap. 3), the octet rule for main group elements (Chap. 5), ionic and covalent bonding (Chap. 5), and much more. However, we still have not been able to deduce why the transition metal groups and inner transition metal groups arise, why many of the transition metals have ions of different charges, how the shapes of molecules are determined, and much more. In this chapter we introduce a more detailed description of the electronic structure of the atom which begins to answer some of these more difficult questions. 

The octet rule is one of the cornerstones of chemical bonding theory. While the vast majority of molecules conform, apparent exceptions occur for molecules incorporating second-row (and heavier) main-group elements. Apparent refers to the fact that molecules such as dimethylsulfoxide and dimethylsulfone may either be represented in terms of structures with ten and twelve valence electrons, respectively, surrounding sulfur, or as zwitterions with the normal complement of eight valence electrons (see also discussions in Chapters 5 and 16). 

Octet Rule. The notion that main-group elements prefer to be surrounded by eight electrons (going into s, px, Py, Pz orbitals). 

FIGURE 6.10 The octet rule occasionally fails for the shaded main-group elements. These elements, all of which are in the third row or lower, can use low-energy unfilled d orbitals to expand their valence shell beyond the normal octet. 

Which main-group elements occasionally break the octet rule ... 

What is the octet rule, and why does it apply primarily to main-group elements, not to transition metals ... 

Electron dot diagrams are most useful for main group elements, and the systematic procedure for drawing electron dot diagrams works only for species in which all atoms obey the octet rule. 

No first-row atom (B, C, N, O) can have more than eight electrons in its valence shell. (The octet rule is less sacred for heavier main group elements such as P and S, and it does not hold at all for transition metals.)... 

Main group elements like C and S have 4 valence AOs, one s and three p, and they follow the octet rule (although heavier main group elements can extend their octet). Transition metals, by contrast, have 10 valence AOs—one s, five d, and three p, in that order—and they follow the 18-electron rule. The 18-electron rule is much less rigorous for transition metals than the octet rule is for main-group elements. First, it can be difficult to surround a metal, especially an early metal, with sufficient numbers of substituents to provide 18 electrons to the metal. Second, the valence orbitals of metals are sufficiently extended from the nucleus that the nucleus doesn t care much about what s going on in its valence shell. 

The observation that the main-group elements tend to form ions with eight electrons in their valence shells leads to a statement called the octet rule ... 

The octet rule In reactions involving main-group elements, atoms tend to gain, lose, or share the necessary number of electrons needed to achieve an octet of electrons in their valence shells. 

The previous examples have involved only main-group elements since they form monatomic ions with noble gas configurations in agreement with the octet rule. This predictable behavior lets you figure out formulas of ionic products if you know the electronic configurations of the reactants. 

So, does this mean the octet rule is useless The answer is no, because it works as well for many main-group elements, especially the critically important elements C, N, and O, which are central atoms in thousands of compounds. As you learn more chemistry, you will learn when the octet rule is exceeded. 

The central idea of the ionic bonding model is the transfer of electrons from meted atoms to nonmetal atoms to form ions that come together in a solid ionic compound. For nearly every monatomic ion of a main-group element, the electron configuration has a filled outer level either two or eight electrons, the same number as in the nearest noble gas (octet rule). 

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