Less than an Octet of Valence Electrons

A second type of exception occurs when there are fewer than eight valence electrons around an atom in a molecule or polyatomic ion. This situation is also relatively rare (with the exception of hydrogen and helium as we have aheady discussed), most often encountered in compounds of boron and beryllium. As an example, let s consider boron trifluoride, BF3. If we follow the first four steps of our procedure for drawing Lewis structures, we obtain the structure 

Each of these structures forces a fluorine atom to share additional electrons with the boron atom, which is inconsistent with the high electronegativity of fluorine. In fact, the formal charges tell us that this is an unfavorable situation. In each structure, the F atom involved in the B = F double bond has a formal charge of +1, while the less electronegative B atom has a formal charge of —1. Thus, the resonance structures containing a B = F double bond are less important than the one in which there are fewer than an octet of Valence electrons around boron  

We usually represent BF3 solely by the dominant resonance structure, in which there are only six valence electrons around boron. The chemical behavior of BF3 is consistent with this representation. In particular, BF3 reacts energetically with molecules having an unshared pair of electrons that can be used to form a bond with boron, as, for example, in the reaction 

In the stable compound NH3BF3, boron has an octet of valence electrons. 

---

Compounds (a), (b) and (d) have a central atom that disobeys the octet rule with a share in less than an octet of valence electrons. 

A Write the Lewis formula for each of the following covalent compounds. Which ones contain at least one atom with a share in less than an octet of valence electrons (a) BeBr2 (b) BBf3 (c) NCI3 (d) AICI3. A Which of the following species contain at least one atom that is an exception to the octet rule ... 

No. The elements in Groups 4 through 7 do attain the octet, but the elements in Groups 2 and 3 have less than an octet. (The elements in the third and higher periods, such as Si, S, and P, may achieve more than an octet of valence electrons.)... 

For molecules with more than an octet of valence-shell electrons on the central atom, the employment of correlation diagrams has been less systematic. Instead, some individual cases have been treated to see what distortions from assumed idealized geometries might be expected. For example, the T-shaped C1F3 molecule has been treated as indicated in Fig. 4-6, where the results suggest that the angle should be 10° less than 90°, in semi-quantitative agreement with observation. 

As the next examples show, the provisional stmcture may contain one or more inner atoms with less than octets of valence electrons. These provisional stmctures must be optimized in order to reach the most stable molecular configuration. To optimize the electron distribution about an inner atom, move electrons from adjacent outer atoms to make double or triple bonds until the octet is complete. Examples and illustrate this procedure. 

Notice that the zinc atom is associated with only four valence electrons. Although this is less than an octet, the adjacent carbon atoms have no lone pairs available to form multiple bonds. In addition, the formal charge on the zinc atom is zero. Thus, Zn has only four electrons in the optimal Lewis structure of dimethyizinc. This Lewis stmcture shows two pairs of bonding electrons and no lone pairs on the inner atom, so Zn has a steric number of 2. Two pairs of electrons are kept farthest apart when they are arranged along a line. Thus, the C—Zn—C bond angle is 180°, and linear geometry exists around the zinc atom. 

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. 

Summarize exceptions to the octet rule by correctly pairing these molecules and phrases odd number of valence electrons, PCI5, CIO2, BFI3, expanded octet, less than an octet. 

At the so-called radical center an organic radical R- has an electron septet, which is an electron deficiency in comparison to the electron octet of valence-saturated compounds. Carbon atoms are the most frequently found radical centers and most often have three neighbors (see below). Carbon-centered radicals with their electron septet occupy an intermediate position between the carbenium ions, which have one electron less (electron sextet at the valence-unsaturated C atom), and the carbanions, which have one electron more (electron octet at the valence-unsaturated C atom). Since there is an electron deficiency present both in C radicals and in carbenium ions, the latter are more closely related to each other than C radicals are related to carbanions. Because of this, C radicals and carbenium ions are also stabilized or destabilized by the same substituents. 

Calculate the total number of valence electrons and put them into your skeleton observing, if possible, the octet rule. If, on the first attempt at dot distribution, an atom has less than a full octet (either held or shared), electrons may be moved in the necessary direction by forming an additional double or triple bond to the deficient atom using a pair of unshared electrons from an adjacent atom. 

The nucleophilic attack upon the carbon in a carboxylic amide passes through a transition state that has a stable octet of electrons in a tetrahedral arrangement. The transition state of the nucleophilic attack upon a sulfonamide has a pentavalent sulfur, which is a relatively unstable species. This is because sulfur has a decet of electrons (remember that sulfur can accommodate more than eight valence electrons) which is less stable than an octet. 

The octet rule is not sophisticated enough to be correct every time. For example, some molecules that exist in nature have an odd number of valence electrons and thus will not have octets on all their constituent atoms. Some elements tend to form compounds in nature in which they have more (sulfur) or less (boron) than 8 valence electrons. 

There are a few molecules in which an atom will have less than eight valence electrons. The most common examples of these contain H, Be, B, and Al. For example, boron trifluoride, BF3, has a central boron atom surrounded by three fluorine atoms. After filling the octets around the fluorine atoms, there are two possible solutions. One is to leave boron with only six valence electrons, while the second is to draw resonance structures for the molecule. 

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 only nonmetallic element in group 13 (see Topic B2). boron has a strong tendency to covalent bonding. Its uniquely complex structural chemistry arises from the (2s) (2p) configuration, which gives it one less valence electron than the number of orbitals in the valence shell. Simple compounds such as BC13 have an incomplete octet and are strong Lewis acids (see Topics CT and C9), but boron... 

---