Nonbonding electron pairs lone-pair

While the present discussions focus on orbital hybridization relative to bonds between atoms, it is important to recognize that nonbonding electron pairs (lone pairs) also participate in orbital hybridization. Thus, as illustrated in Figure 5.4 and relating to. sp3-hybridized centers, for the purposes of determining orbital hybridization, lone pairs can be treated as bonds between a central atom and nothing. 

This means that the bonding and nonbonding electron pairs (lone pairs) around a given atom are positioned as far apart as possible. 

The same is true for the nitrogen atom in ammonia, which has three covalent N-H bonds and two nonbonding electrons (a lone pair). Atomic nitrogen has five valence electrons, and the ammonia nitrogen also has five—one in each of three shared N-H bonds plus two in the lone pair. Thus, the nitrogen atom in ammonia has no formal charge. 

Fig. 6.7 The tetrachloroiodatc(lll) ion. (a) Octahedral arrangement of bonding and nonbonding electrons with lone pairs cis to each other, (b) Octahedral arrangement of bonding and nonbonding electrons with lone pairs trans to each other, (c) Experimentally determined structure.

When you draw a Lewis structure, make sure that hydrogen atoms are surrounded by no more than two electrons and that C, O, N, and halogen (F, Cl, Br, I) atoms are surrounded by no more than eight electrons—they must obey the octet rule. Valence electrons not used in bonding are called nonbonding electrons or lone-pair electrons. 

Valence electrons that are not used for bonding are called nonbonding electrons, or lone-pair electrons. The nitrogen atom in ammonia, fbr instance, shares six valence electrons in three covalent bonds and has its remaining two valence electrons in a nonbonding lone pair. 

The reacting atoms are represented with numbers of valence electrons equal to their group numbers. In the fluorine molecule, each atom is surrounded by a completed octet. The electron dot picture of the molecule, or Lewis formula, can be simplified by representing the bonding pair of electrons by a line between atoms, and the other pairs as dots surrounding the atoms f F—Ft. The pairs of electrons not shared in the covalent bond are called nonbonded electrons or lone pairs. 

In nonlinear molecules that obey the octet rule, whenever two or more pairs of electrons are localized as a result of their attractive interactions with the nuclei, the a-spin and jff-spin tetrahedra will be brought into coincidence with each other, localizing any nonbonding electrons as lone pairs on the central atom. 

Add multiple bonds to eliminate unpaired electrons. Draw the remaining nonbonding electrons as lone pairs. 

If covalent bonds exist between M atoms, then not all of the e(M) electrons of M can be turned over to X, and the number e(M) in equation (13.1) must be reduced by the number fc(MM) of covalent bonds per M atom. If the M atoms retain nonbonding electrons (lone electron pairs as for Tl+), then e(M) must also be reduced by the number E of these electrons. On the other hand, the X atoms require fewer electrons if they take part in covalent bonds with each other the number e(X) can be increased by the number b(XX) of covalent bonds per X atom ... 

It has been known for some time that the basicities of a heteroatom decrease upon a-silyl substitution [12], For example, alkyl silyl ethers (R3Si-0-R ) are less basic than dialkly ethers. Silylamines are weak bases compared to alkylam-ines. This electron-withdrawing effect of silyl groups has been explained in terms of the interaction between low lying vacant orbitals such as 3d orbitals of silicon or a orbitals with the nonbonding p orbitals (lone pairs) of the heteroatom (Fig. 4). This interaction decreases the HOMO level which in turn lowers the basicity of the heteroatom. Such effect may also cause the increase of the oxidation potentials, but little study has been reported on the electrochemical properties of this type of compounds. 

An electronic transition in which an electron in a nonbonding (e.g., lone pair) orbital (called an n-orbital) is promoted to a w-antibonding orbital. The excited state arising from such a promotion is often referred to as an n-CT state. An electron in an n-orbital typically interacts strongly with a polar solvent this is less hkely to be the case for an electron in a n- orbital. Therefore, the energy difference between n and n orbital electrons will increase when a substance is placed in a more polar solvent this is manifested as a shift to shorter wavelength (often called a blue shift) for light absorption. See Absorption Spectroscopy... 

An electronic transition in which an electron in a nonbonding (e.g., lone-pair) orbital is promoted to an antibonding a orbital. See Absorption Spectroscopy... 

Electrostatic potential maps, shown in Figure 17.7 for pyridine and pyrrole, confirm that the nonbonded electron pair in pyridine is localized on N, whereas the lone pair in pyrrole is part of the delocalized n system. Thus, a fundamental difference exists between the N atoms in pyridine and pyrrole. 

For example, pyridine and pyrrole are both aromatic, but the nonbonded electron pair on the N atom in these compounds is located in different orbitals. Recall from Section 17.8C that the lone pair of electrons in pyridine occupies an sp hybridized orbital, perpendicular to the plane of the molecule, so it is not part of the aromatic system, whereas that of pyrrole resides in a p orbital. 

As a strongly electron deficient species, nitrenes will react with nonbonding electron pairs. Thus, carbethoxynitrene does attack the lone pair of amino groups with the formation of hydrazine derivatives and polyamines. In the reaction of carbethoxynitrene with... 

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