Ammonia nonbonded orbital

The model can be extended to predict the direction of adsorbate-induced shifts in cluster IP upon the chemisorption of other reactant molecules as well. To illustrate this, consider the chemisorption of ammonia. In this case the net charge donation is from a filled nonbonding orbital of NH3 to the metal cluster (metal-acceptor interaction), resulting in associative chemisorption of NH3 onto the cluster. In contrast to the situation for H2, adsorption of NH3 can be viewed as a reductive addition process with respect to the metal cluster, thus resulting in an increase in the Fermi level and a decrease in IP. This prediction is in excellent agreement with recent data for NH3 chemisorbed on iron clusters, which indicate that the IPs of the fully ammoniated (saturated) clusters are as much as 2 eV lower than those of the corresponding naked clusters. 

Riled n orbital of ethylene Empty 2/) orbital on BH3 Riled nonbonding orbital of methyl anion Empty o orbital of H—Cl Riled nonbonding orbital of ammonia, 5NH3 Empty o orbital of H—Cl Riled nonbonding orbital of hydroxide Empty o orbital of an O—H bond of H3O ... 

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. 

A Lewis base must have valence electrons available for bond formation. Any molecule whose Lewis stmcture shows nonbonding electrons can act as a Lewis base. Ammonia, phosphorus trichloride, and dimethyl ether, each of which contains lone pairs, are Lewis bases. Anions can also act as Lewis bases. In the first example of adduct formation above, the fluoride ion, with eight valence electrons in its 2 s and 2 p orbitals, acts as a Lewis base. 

This theory explains why BF3 reacts instantaneously with NH3. The nonbonding electrons on the nitrogen in ammonia are donated into an empty orbital on the boron to form a new covalent bond, as shown in the figure below. 

Comparison of orbital structures of the methyl anion and ammonia. Both the methyl anion and ammonia have an sp3 hybridized central atom, with a nonbonding pair of electrons occupying one of the tetrahedral positions. 

These four neutral molecules, ammonia, water, trimethylphosphine, and dimcthylsulfide, all have lone pairs of electrons in sp3 orbitals and in each case this is the donor or nucleophilic orbital. The group VI atoms (O and S) have two lone pairs of equal energy. These are all nonbonding electrons and therefore higher in energy than any of the bonding electrons. 

E2.32 The molecular orbital energy diagram for ammonia is shown in Figure 2.30. The interpretation given in the text was that the 2a) molecular orbital is almost nonbonding, so the electron configuration Iai le 2ai results in only three bonds ((2 + 4)/2 = 3). Since there are three N-H bonds, the average N-H bond order is I (3/3 = 1). 

The spectroscopic and chemical properties of l,4-diazabicyclo[2.2.2]octane (DAB-CO) are consistent with a strong interaction of the in-phase combination of the nonbonded electron pairs of the nitrogen atoms with the symmetric combination of the C—C a bonds, one consequence of which is that the in-phase combination is the HOMO. The two lowest IPs are 7.6 and 9.7 eV [89]. Compare these to the IPs of trimethyl amine (8.44 eV) and ammonia (10.5 eV) [90]. The relative importance of intramolecular orbital interactions through space and through bonds has been reviewed by Hoffmann [6]. 

Whereas the Lewis acid-base theory does not contradict Bronsted theory, as "bases" in Bronsted theory must have a pair of nonbonding electrons in order to accept a proton, it expands the family of compounds that can be called "acids" any compound that has one or more empty valence-shell orbital and provides an explanation for the instantaneous reaction of boron triflouride (BF3) with ammonia (NH3). The nonbonding electrons on the nitrogen in NH3 are donated into an empty orbital on the boron to form a new covalent bond, as shown in Eq. 13. 

The valence bond description of methane, ammonia, and water predicts tetrahedral geometry. In methane, where the carbon valence is four, all the hybrid orbitals are involved in bonds to hydrogen. In ammonia and water, respectively, one and two nonbonding (unshared) pairs of electrons occupy the remaining orbitals. While methane... 

The ammonia molecule has a lone pair of nonbonding electrons, and the BF3 has a vacant orbital—there being only six electrons in the vacant shell of the boron atom. An additive compound, or adduct, is therefore formed ... 

In the reaction of ammonia with borane, not only do the molecules have to collide with enough energy to react, but they must also collide with the orbitals aligned correctly for them to interact. As you saw in Chapter 3, the lone pair of the nitrogen atom resides in a filled, nonbonding sp orbital. This orbital has to overlap with the empty p orbital on B to form a bond. So, a collision like this... 

---