Ammonia molecular shapes

Figure 9-16 shows the molecular shapes of methane, ammonia, and water, all of which have hydrogen ligands bonded to an inner atom. These molecules have different numbers of ligands, but they all have the same steric number. 

Table 4.2 summarizes the molecular shapes that commonly occur. The VSEPR notation used for these shapes adopts the letter A to represent the central atom, the letter X to represent a bonding pair, and the letter E to represent a lone pair of electrons. For example, the VSEPR notation for NH3 is AX3E. This indicates that ammonia has three BPs around its central atom, and one LP. 

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. 

This general scheme of events, although valid for all materials, does not predict the same effects under the same conditions for all species. It depends quite critically on the complexity of the molecule. Large biomolecules or polymers have no gas-phase existence and even in solution, or the liquid state, may have a well-defined molecular shape, while small molecules like ammonia settle into a classical structure only at very low temperatures. 

Compare the boiling points of ammonia, NH3, phosphorus trihydride, PH3, and arsenic trihydride, AsH3. Use the periodic table and the concept of molecular shape and polarity. 

Ammonia (NH3) and water (H2O) both have atoms surrounded by four groups, some of which are lone pairs. In NH3, the three H atoms and one lone pair around N point to the corners of a tetrahedron. The H-N-H bond angle of 107° is close to the theoretical tetrahedral bond angle of 109.5°. This molecular shape is referred to as a trigonal pyramid, because one of the groups around the N is a nonbonded electron pair, not another atom. 

Molecular shapes and 1H chemical shift patterns for chloro-ammine cobalt(III) compounds. Different environments for ammonia molecules in some complexes are defined by eq and ax subscripts, with the two types opposite different types of ligand leading to different magnetic environments and chemical shifts (8). 

Picturing molecular shapes is a great way to visualize what happens during a reaction. For instance, when ammonia accepts the proton from an acid, the lone pair on the N atom of trigonal pyramidal NH3 forms a covalent bond to the and yields the ammonium ion (NH4 ), one of many tetrahedral polyatomic ions. Note how the H—N—H bond angle expands from 107.3° in NH3 to 109.5° in NH4, as the lone pair becomes another bonding pair ... 

Problem For each compound, use the molecular shape and EN values and trends (Figure 9.18) to predict the direction of bond and molecular polarity, if present (a) ammonia, NH3 (b) boron trifluoride, BF3 (c) carbonyl sulfide, COS (atom sequence SCO). 

Figure 4.47 Lewis structures and molecular shapes of the ammonia and water molecules... 

The bonding electrons in ammonia are displaced toward the more electronegative nitrogen atom. The bonds do not cancel in the asymmetrical pyramidal shape, so the molecule is polar. The three-dimensional representation, which attempts to show the molecular shape, better suggests the charge displacement toward the nitrogen atom. 

Molecular Shape The locations of bonds and unshared electrons are shown for molecules of (a) ammonia and (b) water. Although unshared electrons occupy space around the central atoms, the shapes of the molecules depend only on the position of the molecules atoms, as clearly shown by the ball-and-stick models. 

Figure 4.17 Shapes of molecules. These space-filling models show the molecular shapes of a methane, CH, b ammonia, NHj, andc water, H O.

In ammonia and the nitrogen trihalides, there is a non-bonding pair of electrons, leading to a pyramidal molecular shape. 

Electron density surface for ammonia depicts overall molecular size and shape. 

Numerous d cobalt(III) complexes are known and have been studied extensively. Most of these complexes are octahedral in shape. Tetrahedral, planar and square antiprismatic complexes of cobalt(lII) are also known, but there are very few. The most common ligands are ammonia, ethylenediamine and water. Halide ions, nitro (NO2) groups, hydroxide (OH ), cyanide (CN ), and isothiocyanate (NCS ) ions also form Co(lII) complexes readily. Numerous complexes have been synthesized with several other ions and neutral molecular hgands, including carbonate, oxalate, trifluoroacetate and neutral ligands, such as pyridine, acetylacetone, ethylenediaminetetraacetic acid (EDTA), dimethylformamide, tetrahydrofuran, and trialkyl or arylphosphines. Also, several polynuclear bridging complexes of amido (NHO, imido (NH ), hydroxo (OH ), and peroxo (02 ) functional groups are known. Some typical Co(lll) complexes are tabulated below ... 

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