Ammonia orbital structure

The hybridization and bond angles of a simple carbanion also resemble those of an amine. The carbon atom is sp3 hybridized and tetrahedral. One of the tetrahedral positions is occupied by an unshared lone pair of electrons. Figure 4-16 compares the orbital structures and geometry of ammonia and the methyl anion. 

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. 

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 ... 

The tetramethylammonium salt [Me4N][NSO] is obtained by cation exchange between M[NSO] (M = Rb, Cs) and tetramethylammonium chloride in liquid ammonia. An X-ray structural determination reveals approximately equal bond lengths of 1.43 and 1.44 A for the S-N and S-O bonds, respectively, and a bond angle characteristic bands in the IR spectrum at ca. 1270-1280, 985-1000 and 505-530 cm , corresponding to o(S-N), o(S-O) and (5(NSO), respectively. Ab initio molecular orbital calculations, including a correlation energy correction, indicate that the [NSO] anion is more stable than the isomer [SNO] by at least 9.1 kcal mol . ... 

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 ... 

A complex ion is one that contains more than one ion. Because of its effect on mobility, complexation, the process by which complex ions form in solution, is very important for heavy metals and may be significant for organic wastes. Heavy metals are particularly prone to complexation because their atomic structure (specifically the presence of unfilled d-orbitals) favors the formation of strong bonds with polar molecules, such as water and ammonia (NH3), and anions, such as chloride (CO and cyanide (CN ). Depending on the chemistry of an injected waste and existing conditions, complexation can increase or decrease the waste s mobility. 

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. 

These orbital pictures tend to get a little confusing, in that we really need to put in the elemental symbol to distinguish it from carbon, and we usually wish to show the lone pair electrons. We accordingly use a compromise representation that employs the cleaner line drawings for part of the structure and shows the all-important orbital with its lone pair of electrons. These are duly shown for ammonia and water. 

It is assumed that the reader has previously learned, in undergraduate inorganic or physical chemistry classes, how symmetry arises in molecular shapes and structures and what symmetry elements are (e.g., planes, axes of rotation, centers of inversion, etc.). For the reader who feels, after reading this appendix, that additional background is needed, the texts by Cotton and EWK, as well as most physical chemistry texts can be consulted. We review and teach here only that material that is of direct application to symmetry analysis of molecular orbitals and vibrations and rotations of molecules. We use a specific example, the ammonia molecule, to introduce and illustrate the important aspects of point group symmetry. 

Such a bond, in which the donor molecule (or anion) provides both bonding electrons and the acceptor cation provides the empty orbital, is called a coordinate or dative bond. The resulting aggregation is called a complex. Actually, any molecule with an empty orbital in its valence shell, such as the gas boron trifluoride, can in principle act as an electron pair acceptor, and indeed BF3 reacts with ammonia (which has a lone pair, NH3) to form a complex H3N ->BF3. Our concern here, however, is with metal cations, and these usually form complexes with from 2 to 12 donor molecules at once, depending on the sizes and electronic structures of the cation and donor molecules. The bound donor molecules are called ligands (from the Latin ligare, to bind), and the acceptor and donor species may be regarded as Lewis acids and Lewis bases, respectively. 

The distinction between atomic orbitals and basis functions in molecular calculations has been emphasized several times now. An illustrative example of why the two should not necessarily be thought of as equivalent is offered by ammonia, NH3. The inversion barrier for interconversion between equivalent pyramidal minima in ammonia has been measured to be 5.8 kcal mol However, a HF calculation with the equivalent of an infinite, atom-centered basis set of s and p functions predicts the planar geometry of ammonia to be a minimum-energy structure ... 

There are some molecules in which the fourth orbital of boron is used for formation of another bond and is not available to permit double-bond character to be assumed by the B—X bonds. In these molecides the bond lengths should approximate the values calculated for B—X single bonds (Table 9-1 column 2). An example is ammonia-boron trifluoride, with the structure... 

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. 

The monomer species, M, has been described by Kraus (31) as an ion pair. Although he did not elaborate on its possible structure, one may assume that he pictured this species as two ammoniated ions held together by electrostatic forces. Douthit and Dye (12) pointed out that such a picture is consistent with the absorption spectra of sodium-ammonia solutions. Becker, Lindquist, and Alder (2) proposed an expanded metal model in which an electron was assumed to circulate in an expanded orbital on the protons of the coordinated ammonia molecules of an M + ion. The latter model is difficult to reconcile with optical, volumetric, and NMR data (16). 

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