Ammonia orbital hybridization

The valence-bond concept of orbital hybridization described in the previous four sections is not limited to carbon compounds. Covalent bonds formed by-other elements can also be described using hybrid orbitals. Look, for instance, at the nitrogen atom in methylamine, CH3NH2, an organic derivative of ammonia (NH3) and the substance responsible for the odor of rotting fish. 

In the case of ammonia, the five valence electrons surrounding nitrogen— two electrons occupying the outermost 2s orbital and three electrons in three 2p orbitals— hybridize to form four sp orbitals that, if equal, would separate themselves in three dimensional space by pointing at the comers of a tetrahedron. One of these orbitals, however, contains the lone electron pair, and the three remaining sp orbitals make additional space available by pointing at the comers of a pyramid with a triangular base. 

The valence bond model and the accompanying idea of orbital hybridization have applications beyond those of bonding in hydrocarbons. A very simple extension to inorganic chemistry describes the bonding to the nitrogen of ammonia and the oxygen of water. 

Describe the bonding in ammonia assuming sp hybridization of nitrogen In what kind of orbital is the unshared pair What orbital overlaps are involved in the N—H bonds , ... 

Let s look at an example. In ammonia (NH3), the nitrogen atom is sp hybridized, so all four orbitals arrange in a tetrahedral structure, just as we would expect. But only three of the orbitals in this arrangement are responsible for bonds. So, if we look just at the atoms that are connected, we do not see a tetrahedron. Rather, we see a trigonal pyramidal arrangement ... 

The simplest type of Lewis acid-base reaction is the combination of a Lewis acid and a Lewis base to form a compound called an adduct. The reaction of ammonia and trimethyl boron is an example. A new bond forms between boron and nitrogen, with both electrons supplied by the lone pair of ammonia (see Figure 21-21. Forming an adduct with ammonia allows boron to use all of its valence orbitals to form covalent bonds. As this occurs, the geometry about the boron atom changes from trigonal planar to tetrahedral, and the hybrid description of the boron valence orbitals changes from s p lo s p ... 

If nitrogen uses only its p orbitals in bond formation, the angle between N-H bonds would be 90°. However, compounds prefer formations in which electrons are as far apart as possible. For ammonia this is made possible by forming a tetrahedral structure in which the angle between the bonds (M-H) is 107°. This is only possible by undergoing sp3 hybridization. 

The simplest compounds to consider here are ammonia and water. It is apparent from the above electronic configurations that nitrogen will be able to bond to three hydrogen atoms, whereas oxygen can only bond to two. Both compounds share part of the tetrahedral shape we saw with 5/ -hybridized carbon. Those orbitals not involved in bonding already have their full complement of electrons, and these occupy the remaining part of the tetrahedral array (Figure 2.21). These electrons are not inert, but play a major role in chemical reactions we refer to them as lone pair electrons. 

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...

The chemical bonding to the surface is achieved via orbitals of ax symmetry. The adsorbate-substrate hybrid levels exhibiting mainly metal character are represented by the a, states. It has been shown that backdonation into the previously unoccupied ammonia 4at orbital, and a simultaneous 3a, donation into the substrate, plays an important role in the surface chemical bond [112]. 

Pyridines form stable salts with strong acids. Yellow ionic picrates were used for characterization in the past. Pyridine itself is often used to neutralize acid formed in a reaction and as a basic solvent. The basicity of pyridine (as measured by the dissociation constant of its conjugate acid, pKa 5.2) is less than that of aliphatic amines (cf. NH3, pA"a 9.5 NMe3, pKd 9.8). This reduced basicity is probably due to the changed hybridization of the nitrogen atom in ammonia the lone electron pair is in an sp3-orbital, but in pyridine it is in an s/r-orbital. The higher the s character of an orbital, the more it is concentrated near the nucleus, and the less available for bond formation. Nitriles, where the lone electron pair is in an. vp-orbital, are of even lower basicity. 

The final molecule of this series is methane, the tetrahedral structure of which follows if a fourth unit positive charge is removed from the nucleus in the ammonia lone-pair direction. There are now four equivalent bonding orbitals, which may be represented approximately as linear combinations of carbon s-p hybrid and hydrogen Is functions. The transformation from molecular orbitals into equivalent orbitals or vice versa is exactly the same as for the neon atom. 

Some molecules, such as ammonia, have a lone pair of electrons on the central atom. This electron pair occupies an orbital confined to the central atom. According to the VSEPR model, the four electron pairs in NH3 take up a tetrahedral electron arrangement, so we describe the nitrogen atom in terms of four sp3 hybrid orbitals. Because nitrogen has five valence electrons, one of these hybrid orbitals is already doubly occupied (45). The ls-electrons of the three hydrogen atoms pair with the... 

The ammonia molecule owes its capacity as a ligand to the lone-pair orbital on the N atom. In the language of VB theory (which is of limited value in d block and f block chemistry), the bond simply involves the o overlap of the lone-pair orbital with an empty hybrid orbital of the central atom. In MO language, a complex ion M(NH3)"+ has m filled a bonding MOs. There is no reason to suspect that any other orbitals on the NH3 molecule are involved in its bonding to a central atom in a complex. 

The unshared electron pair in ammonia ( NH3) occupies an. (//-hybridized orbital of nitrogen. Each N—H bond corresponds to overlap of a half-filled sp3 hybrid orbital of nitrogen and a l.v orbital of hydrogen. 

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