Vacant valence orbitals

Look closely at the acid-base reaction in Figure 2.5, and note how it is shown. Dimethyl ether, the Lewis base, donates an electron pair to a vacant valence orbital of the boron atom in BF3, a Lewis acid. The direction of electron-pair flow from the base to acid is shown using curved arrows, just as the direction of electron flow in going from one resonance structure to another was shown using curved arrows in Section 2.5. A cuived arrow always means that a pair of electrons moves from the atom at the tail of the arrow to the atom at the head of the arrow. We ll use this curved-arrow notation throughout the remainder of this text to indicate electron flow during reactions. 

Here is a situation we haven t met before. After using the two available partially filled orbitals to form covalent bonds with hydrogen atoms, there remains a vacant valence orbital. In the electron dot formulation (36) we see that the carbon atom finds itself near only six electrons in CH2. The valence orbitals will accommodate eight electrons. Because one valence or-... 

We cannot help noticing, however, that there remains a completely vacant valence orbital. For example, for BH3 the orbital representation would be as shown in (44). 

Therefore we should expect in the gaseous state to find molecules such as BeH2 and BeF2. These molecules have been detected. On the other hand, beryllium has the trouble boron has, only in a double dose. It has two vacant valence orbitals. As a result, BeH2 and BeF2 molecules, as such, are obtained only at extremely high temperatures (say, above 1000°K). At lower temperatures these vacant valence orbitals cause a condensation to a solid in which these orbitals can participate in bonding. We shall discuss these solids in the next chapter. 

There is little new to be said about the bonding capacity of a lithium atom. With just one valence electron, it should form gaseous molecules LiH and LiF. Because of the vacant valence orbitals, these substances will be expected only at extremely high temperatures. These expectations are in accord with the facts, as shown in Table 16-1, which summarizes the formulas and the melting and boiling points of the stable fluorides of the second-row elements. In each case, the formula given in the table is the actual molecular formula of the species found in the gas phase. 

In summary we can say that the metallic bond is a sort of nondirectional covalent bond. It occurs when atoms have few valence electrons compared with vacant valence orbitals and when these valence electrons are not held strongly. 

Melting point. Except for zinc at the end of the row, the melting points are quite high. This is appropriate, since these elements have a large number of valence electrons and also a large number of vacant valence orbitals. Toward the end of the row, in zinc, the 3d orbitals become filled and the melting point drops. 

Ammonia is a prime example of a Lewis base. In addition to its three N—H bonds, this molecule has a lone pair of electrons on its nitrogen atom, as Figure 21-1 shows. Although all of the valence orbitals of the nitrogen atom in NH3 are occupied, the nonbonding pair can form a fourth covalent bond with a bonding partner that has a vacant valence orbital available. 

Ammonia, which has a pair of nonbonding valence electrons, is a typical Lewis base. Trimethylboron, which has a vacant valence orbital, represents one type of Lewis acid. 

Removing electrons from a metal atom always generates vacant valence orbitals. As described in Chapter 20, many transition metal cations form complexes with ligands in aqueous solution, hi these complexes, the ligands act as Lewis bases, donating pairs of electrons to form metal-ligand bonds. The metal cation accepts these electrons, so it acts as a Lewis acid. Metal cations from the p block also act as Lewis acids. For example, Pb ((2 g) forms a Lewis acid-base adduct with four CN anions, each of which donates a pair of electrons Pb ((2 ( ) + 4 CN ((2 q) -> [Pb (CN)4] (a g)... 

Owing to the inherent reactivity of species containing one or more formally vacant valence orbitals, few such transition-metal species are known as isolable compounds. 

Table 4.38. Non-radical reaction types for interactions of transition metals (M) with H2, showing principal donor-acceptor combinations in each case (the symbol denotes a vacant valence orbital [formal hypovalency] on...

The leading NBO Lewis structure of the less strongly bound Au(HCCH)+ complex does indeed correspond to separated Au+ HCCH reactants. Figure 4.89 illustrates the principal NBO donor-acceptor interactions for the Au(HC=CH)+ complex, which are seen to be rather similar to those for the long-range Ti(H2C=CH2) complex (Fig. 4.72). Thus, for a transition metal with only one vacant valence orbital, acetylene and ethylene 7tCc bonds function rather similarly as two-electron donors, and the p2, two-electron complex description is apt. 

In general, both vacant valence orbitals of the zinc centre are nsed and consequently the bonding sitnation of zinc becomes close to sp -hybridization reflected by a tetrahedral coordination geometry of the zinc centre. 

One final point about covalent bonds involves the origin of the bonding electron pair. Although most covalent bonds form when two atoms each contribute one electron, bonds can also form when one atom donates both electrons (a lone pair) to another atom that has a vacant valence orbital. The ammonium ion (NH4+), for example, forms when the two lone-pair electrons from the nitrogen atom of ammonia, NH3, bond to H +. Such bonds are called coordinate covalent bonds. 

Since all proton acceptors have an unshared pair of electrons, and since all electron-pair donors can accept a proton, the Lewis and the Bronsted-Lowry definitions of a base are simply different ways of looking at the same property. All Lewis bases are Bronsted-Lowry bases, and all Bronsted-Lowry bases are Lewis bases. The Lewis definition of an acid, however, is considerably more general than the Bronsted-Lowry definition. Lewis acids include not only H+ but also other cations and neutral molecules having vacant valence orbitals that can accept a share in a pair of electrons donated by a Lewis base. 

Bis-cyclopentadienyltitanium(ll) species are unstable 14-electron d species, with one lone-electron pair and two vacant valence orbitals. The reactivity of Cp2Ti has been compared to carbenes. The interactions between the occupied and unoccupied orbitals can explain why Cp2Ti reacts with unsaturated compounds to give metallacycles (see Metallacycle). 

Bonds involving hydrogen may be fully covalent, as in H2, partiaUy covalent and partially ionic, as in H O, or nearly completely ionic, as in HCl. In the more ionic bonds, the electrons are distributed unevenly, skewed away from hydrogen toward its partner atom. This partial removal of electrons from the hydrogen atom results in partially vacant valence orbitals of hydrogen. The partial vacancy can be... 

Lewis acids are either deficient in electrons (cations) or the central atom has a vacant valence orbital. 

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