Tetrahedral five-atom molecules

TETRAHEDRAL AND SQUARE-PLANAR FIVE-ATOM MOLECULES 2.6.1. Tetrahedral XY4 Molecules (Tj,)... 

TETRAHEDRAL AND SQUARE-PLANAR FIVE-ATOM MOLECULES... 

We have so far considered valence shells containing four pairs of electrons, but we can extend the same arguments to other numbers of valence shell electron pairs. The most probable arrangements of pairs of opposite spin electrons in the valence shell of an atom in a molecule are two pairs, collinear three pairs, equilateral triangular four pairs, tetrahedral five pairs, trigonal bipyramidal six pairs, octahedral. This is because, as we will now see, these are the arrangements that keep the electron pairs as far apart as possible. We discuss valence shells with more than six electron pairs in Chapter 9. 

We discuss molecules with a valence shell containing five electron pair domains in Section 4.6. The preferred arrangements of five valence shell domains, the trigonal bipyramid and the square pyramid, are not regular polyhedra and therefore exhibit special features not found in tetrahedral and octahedral molecules. Molecules with seven and more electron pair domains in the valence shell of a central atom are not common, although they are of considerable interest. They are restricted to the elements of period 4 and higher periods, with very small ligands such as fluorine, and are discussed in Chapter 9. 

The anion is a regular tetrahedron (isoelectronic with CH4) and might alternatively (and perhaps preferably ) be described as B (H)4 since B and H are of about the same electronegativity, the negative charge should be about equally shared over all five atoms. Many molecules and ions containing tetrahedrally-coordinated boron can be regarded as Lewis acid-Lewis base adducts X3B <— L. 

Once the Lewis structure has been determined, it is possible to know the shape of the molecule or ion. The most important piece of information needed to determine the shape is the total number of groups around the central atom, where a group could be another atom or a lone pair. A central atom connected to one or two other atoms is linear. When the central atom is connected to three atoms, the shape is trigonal planar. When the central atom is connected to four atoms, the shape is tetrahedral. When the central atom is connected to five atoms the shape is trigonal bipyramidal (two triangular-based pyramids joined at the base). When the central atom is connected to six atoms, the shape is octahedral. Other shapes are possible when atoms are replaced with lone pairs. 

There are many ways that five atoms can combine to form a molecule, and thus several different shapes are possible. When four atoms of one type surround one atom of another type, as in the example of CH4, a tetrahedral molecule is formed. To understand the reason for this, you must think in three dimensions again. Each of the four shared pairs of electrons (covalent bonds) repel each other. In order to maximize the distance between electrons, the molecule forms a tetrahedral shape. Once again, the Lewis dot diagram fails to indicate this, but it is clear in a three-dimensional model for the molecule, as Figure 4-5f illustrates. 

After sixty-five years of successful use of the theory of the tetrahedral carbon atom, chemists were astonished in 1939 by the report that two American chemists, R. E. Rundle and J. H. Sturdivant, had discovered carbon atoms with ligancy six in a molecule. 

In addition to tetrahedral, another common shape for AB4 molecules is square planar. All five atoms lie in the same plane, with the B atoms at the corners of a square and the A atom at the center of the square. Which shape in Figure 9.3 could lead to a square-planar shape upon removal of one or more atoms ... 

Practically all known molecules consisting of five atoms have tetrahedral structure. There will be nine normal vibrations, but for reasons of symmetry some of these may have identical frequencies. If the symmetry is that of a regular tetrahedron. 

Lewis structures can be used to explain and predict the shapes of molecules. The basic assumption is that, if the core of an atom is effectively spherical (as for most atoms it is), groups of electrons in the Lewis shell (single bond pairs, double-bond quartets, fractional bond pairs, etc.) get as far apart as possible. Thus, two groups take up a linear arrangement, three a trigonal-planar one, four tetrahedral, five trigonal-bip5Tamidal, six octahedral, and so on. 

The five-atom XY4 molecules and ligands commonly adopt tetrahedral and square-planar shapes. The normal modes of tetrahedral and square-planar XY4 are shown in Figure 5.5. Tetrahedral XY4 molecules show two normal modes that are infrared-active , while the square-planar XY4 molecules show three... 

Strychnine, the most celebrated member of the Strychnos alkaloids, possesses a complex polycyclic structure which is assembled from only twenty-four skeletal atoms. In addition to its obvious architectural complexity, strychnine s structure contains a contiguous array of six unsymmetrically substituted tetrahedral (asymmetric) carbon atoms of which five are included within one saturated six-membered ring. The intimidating structure of the strychnine molecule elicited the following remark by Sir Robert Robinson in 1952 For its molecular size it is the most complex substance known. 5... 

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