Electron-deficient substances

The One-Electron Bond and the Three-Electron Bond Electron-deficient Substances... 

In a few molecules and crystals it is convenient to describe the interactions between the atoms in terms of the one-electron bond and the three-electron bond. Each of these bbnds is about half as strong as a shared-electron-pair bond each might be described as a half-bond.1 There are also many other molecules and crystals with structures that may be described as involving fractional bonds that result, from the resonance of bonds between two or more positions. Moat of these molecules and crystals have a smaller number of valence electrons than of stable bond orbitals. Substances of this type are called electron-deficient substances. The principal types of electron-deficient substances are discussed in the following sections (and in the next chapter, on metals). 

Electron-deficient substances are substances in which the atoms have more stable orbitals than electrons in the valence shell.66 An example is boron. The boron atom has four orbitals in its valence shell, and three valence electrons. 

A characteristic feature of the structuie of most electron-deficient substances is that the atoms have ligancy that is not only greater than the number of valence electrons but is even greater than the number of stable orbitals.66 Thus most of the boron atoms in the tetragonal form of crystalline boron have ligancy 6. Also, lithium and beryllium, with four stable orbitals and only one and two valence electrons, respectively, have structures in which the atoms have ligancy 8 or 12. All metals can be considered to be electron-deficient substances (Chap. 11). 

A simple theoretical treatment that can be applied to electron-deficient substances in general can be given for the diborane molecule.81 Let us consider the various valence-bond structures that can be written for the molecule8 (with its known configuration) with use of the six... 

Carbonium Ions as Reaction Intermediates.—The properties of electron-deficient substances may be expected to be of great importance in the theory of chemical reactions. For example, a positively charged (and hence electron-deficient) carbon atom in a complex carbonium ion would be expected to cause adjacent atoms to increase their ligancies, as by the formation of a three-membered ring and by the use of bridging hydrogen atoms. The analysis of the mechanisms of chemical reactions may in the course of time permit much more precise principles to be formulated than are now at hand. 

The discussion of electron-deficient substances is continued in the following chapter. 

The principal innovations that have been made in the discussion of the theory of the chemical bond in this edition are the wide application of the electroneutrality principle and the use of an empirical equation (Sec. 7-10) for the evaluation of the bond numbers of fractional bonds from the observed bond lengths. A new theory of the structure of electron-deficient substances, the resonating-valence-bond theory, is described and used in the discussion of the boranes, ferrocene, and other substances. A detailed discussion of the valence-bond theory of the electronic structure of metals and intermetallic compounds is also presented. 

Chemical bonds are formed by electrons, and formation or breakage of bonds requires the migration of electrons. In broad terms, reactive chemical groups function either as electrophiles or as nucleophiles. Electrophiles are electron-deficient substances that react with electron-rich substances nucleophiles are electron-rich substances that react with electron-deficient substances. The task of a catalyst often is to make a potentially reactive group more reactive by increasing its electrophilic or nucleophilic character. In many cases the simplest way to do this is to add or remove a proton. 

In the case of ester hydrolysis the O of the water is an electron-rich substance and the carbonyl carbon of the ester is an electron-deficient substance. Ester hydrolysis occurs in three stages (1) the initial stage in which electrons flow from the water molecule to the ester (2) the intermediate stage in which the ester carbonyl forms a tetrahedral complex involving the hydroxyl group originating from the water molecule that releases a proton and (3) the final stage in which carboxylic acid and alcohol are formed. 

L. Pauling and B. Kamb, The discussion of tetragonal boron by the resonating-valence-bond theory of electron-deficient substances. Z. Kristall. 112, 472-478 (1959). 

That resonance stabilization is just as essential as electron deficiency is shown by the anhydrous nature of both the neutral species and the cation of 4-nitroisoquinoline (40) and3-nitro-l,5-naphthyridine (41), for which hydration can cause no extra stabilization by resonance. These two substances are to be contrasted with equally electron-deficient substances which form strongly resonant hydrates, viz. quinazoline and 1,3,5-triazanaphthalene, respectively. No hydration could be detected in phthalazine or 8-methyl-l,6,7-triazanaph-thalene (a 5-azaphthalazine). ... 

The Lewis categorization of acids and bases uses an even more general picture. In this view an acid is a structure which has an affinity for electron pairs contributed by bases, defined as substances with unshared pairs of electrons. Examples of acids include not only the proton, which can react with the unshared pairs of such bases as H2O, OH , and C2H3O2, but also transition-metal ions, which can react with ligands (bases) to form complexes, and electron-deficient substances such as BF3, which can react with a base like NH3 to form the compound... 

A characteristic feature of the structure of most electron-deficient substances is that the atoms have ligancy that is not only greater than the number of valence electrons but is even greater than the number of... 

Every base has an unshared pair of electrons to share with a proton. The proton is an electron-deficient particle during the process of bonding to the base. However, a proton is not the only species to form bonds with bases other electron-deficient substances do likewise, as exemplified by the reaction of boron trifluoride, BF3, with NHs ... 

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