Ammonia group orbitals

Figure 6.1 The mixing of the 2s and 2pz atomic orbitals of the nitrogen atom in the pyramidal ammonia molecule, and the relationship of the hybrid orbitals with the a, group orbital of the three hydrogen atoms...

Consider now the construction of the A i symmetry group orbital of the hydrogen s atomic orbitals in ammonia as an example of the application of the projection operator. (The various kinds of orbitals will be discussed in detail in Chapter 6.) The projection operator for the A irreducible representation in the C3v point group is... 

Table 6-4. The C3v Character Table and the Reducible Representation of the Hydrogen Group Orbitals of Ammonia...

The HOMO of NH3 is slightly bonding, because it contains an electron pair in an orbital resulting from interaction of the 2p orbital of nitrogen with the H orbitals of the hydrogens (from the zero-node group orbital). This is the lone pair of the electron-dot and VSEPR models. It is also the pair donated by ammonia when it functions as a Lewis base (discussed in Chapter 6). 

Figure 6-23. Generation of the A, symmetry orbital of the 3H group orbitals of ammonia.

Figure 6-24. Projection of the two E symmetry group orbitals of the three H ]s orbitals in ammonia.

It is common to refer to ammonia as a cr-only ligand despite the le orbitals (Figure 5.30) that could be used as the basis for a set of tt bonding group orbitals. These e orbitals are assumed to play only a negligible role in the bonding in [M(NH3)5] complexes. 

Ammonia is a colourless gas at room temperature and atmospheric pressure with a characteristic pungent smell. It is easily liquefied either by cooling (b.p. 240 K) or under a pressure of 8-9 atmospheres at ordinary temperature. Some of its physical and many of its chemical properties are best understood in terms of its structure. Like the other group head elements, nitrogen has no d orbitals available for bond formation and it is limited to a maximum of four single bonds. Ammonia has a basic tetrahedral arrangement with a lone pair occupying one position ... 

It is assumed that the reader has previously learned, in undergraduate inorganie or physieal ehemistry elasses, how symmetry arises in moleeular shapes and struetures and what symmetry elements are (e.g., planes, axes of rotation, eenters of inversion, ete.). For the reader who feels, after reading this appendix, that additional baekground is needed, the texts by Cotton and EWK, as well as most physieal ehemistry texts ean be eonsulted. We review and teaeh here only that material that is of direet applieation to symmetry analysis of moleeular orbitals and vibrations and rotations of moleeules. We use a speeifie example, the ammonia moleeule, to introduee and illustrate the important aspeets of point group symmetry. 

Metals and alloys, the principal industrial metalhc catalysts, are found in periodic group TII, which are transition elements with almost-completed 3d, 4d, and 5d electronic orbits. According to theory, electrons from adsorbed molecules can fill the vacancies in the incomplete shells and thus make a chemical bond. What happens subsequently depends on the operating conditions. Platinum, palladium, and nickel form both hydrides and oxides they are effective in hydrogenation (vegetable oils) and oxidation (ammonia or sulfur dioxide). Alloys do not always have catalytic properties intermediate between those of the component metals, since the surface condition may be different from the bulk and catalysis is a function of the surface condition. Addition of some rhenium to Pt/AlgO permits the use of lower temperatures and slows the deactivation rate. The mechanism of catalysis by alloys is still controversial in many instances. 

The polyhedral boranes and carboranes discussed above may be regarded as boron clusters in which the single external orbital of each vertex atom helps to bind an external hydrogen or other monovalent atom or group. Post-transition main group elements are known to form clusters without external ligands bound to the vertex atoms. Such species are called bare metal clusters for convenience. Anionic bare metal clusters were first observed by Zintl and co-workers in the 1930s [2-5], The first evidence for anionic clusters of post-transition metals such as tin, lead, antimony, and bismuth was obtained by potentiometric titrations with alkali metals in liquid ammonia. Consequently, such anionic post-transition metal clusters are often called Zintl phases. 

The ammonia molecule is a trigonal pyramid, belonging to the C3v point group. The 2s and 2p orbitals of the nitrogen atom and the Is orbital group combinations of the three hydrogen atoms transform, with respect to the C3v point group, as indicated in Table 6.1. 

To illustrate this explicitly, let us consider the 2px and 2pv orbitals of the nitrogen atom in ammonia, which belongs to the group C3l>. These orbitals are represented or described by the following eigenfunctions ... 

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