Ammonia, hybrid orbitals

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. 

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]. 

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. 

Abstraction of an acetylenic proton gives a carbanion that has the lone pair of electrons in the sp hybrid orbital. Electrons in this orbital are close to the nucleus, and there is less charge separation than in carbanions with the lone pair in sp2 or sp3 hybrid orbitals. Ammonia and alcohols are included for comparison note that acetylene can be deprotonated by the amide ( NH2) ion, but not by an alkoxide ion ( OR). 

In the ammonia molecule, for example, the nitrogen atom normally has three unpaired p electrons, but by mixing the 2s and 3p orbitals, we can create four sp3-hybrid orbitals just as in carbon. Three of these can form shared-electron bonds with... 

Both the nitrogen atom in ammonia and the oxygen atom in water form sp3 hybrid orbitals. 

The concept ofhybridisiation described in the prexious four j tions is not r 8triclcarbon compounds. Covalent bunds formed by other elements in the periodic table aliio can be described uains hybrid orbitals. Loot at the nitrogen atom in ammonia, NHg, for example. A nitre en atom haa five Quler hell electrons nnd therefore forms three covalent bonds to complete its valence electron octet. 

The hybrid orbitals used by the metal ion depend on the number and arrangement of the ligands. For example, accommodating the lone pair from each ammonia molecule in the octahedral Co(NH5)63+ ion requires a set of six empty hybrid atomic orbitals with an octahedral arrangement. Recall that an octahedral set of orbitals is formed by the hybridization of two d, one s, and three p orbitals to give six dfsp orbitals (see Fig. 20.19). 

In the nitrogen atom, the same hybridisation of the atomic orbitals occurs and so four sp hybrid orbitals are formed. These are distributed around the central atom in the same tetrahedral manner as in carbon. Thus, suggest the shape of the ammonia molecule. 

The name sp indicates that we are talking about a transformation of one s-orbital and three p-orbitals. The new four hybrid orbitals are then used to host the four electron groups surrounding the nitrogen atom in the ammonia molecule which is sketched in Figure 2- 16b. 

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... 

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