Mixed bonding tetrahedron

Bougioukou DJ, Smonou I (2002) Mixed Peroxides from the Chloroperoxidase-Catalyzed Oxidation of Conjugated Dienoic Esters with a Trisubstituted Terminal Double Bond. Tetrahedron Lett 43 4511... 

Sometimes it is illustrative to represent the four bonding types—ionic, covalent, metallic, and van der Waals—on what is called a bonding tetrahedron—a three-dimensional tetrahedron with one of these extreme types located at each vertex, as shown in Figure 2.25a. Furthermore, we should point out that for many real materials, the atomic bonds are mixtures of two or more of these extremes (i.e., mixed bonds). Three mixed-bond types—covalent-ionic, covalent-metallic, and metallic-ionic—are also included on edges of this tetrahedron we now discuss each of them. 

Prior to any work on heteroatom clusters the notion was expressed (20) that heteroatom placement within the polyatomic clusters would lead to a decrease In delocalization and bonding and thence stability. Although this may lessen stability the substitution clearly does not preclude It. Furthermore, many of the likely polyhedra already have Inequlvalent atom positions, the 5, 7, 9 and 10 atom examples already considered here for example, and mixed species especially with elements from different groups may be quite stable within the discrimination provided by Inequlvalent positions. Even the nominally equivalent atom positions In a tetrahedron can obviously accommodate substantial differences. Additional examples of mixed element polycations are certainly to be expected. An Inadequate foresight was revealed In a review of polycations (20) written for a 1974 award symposium, about one year before the crypt discoveries, by the expectation that polycations should be more stable than polyanions for the metallic elements. In hindsight, metallic behavior Is a property of the dense solid state and has little to do with the stability of small clusters where electronic and geometric factors are far more important. 

In sihcon and oxygen compounds, Si has coordination IV or VI. The SiO/ tetrahedron is energetically quite stable, with mixed ionic-covalent bonds. 

In the ground state of the free carbon atom, there are two unpaired electrons in separate 2p orbitals. In order to form four bonds (tetravalence), need four unpaired electrons in four separate but equivalent orbitals. We assume that the single 2s, and the three 2p orbitals of carbon mix into four sp3 hybrid orbitals which are chemically and geometrically identical the latter condition implies that the four hybrid orbitals extend toward the corners of a tetrahedron centered on the carbon atom. 

All X3PO molecules have a pyramidal PX3 group, with the oxygen atom occupying the fourth position to complete a distorted tetrahedron. The P—O bond lengths are about 1.55 A, consistent with the existence of double bonds. Several mixed phosphoryl halides, phosphoryl pseudohalides, as well as X3PS and X3PSe compounds are also known. All of these compounds are prone to ready hydrolysis. 

As we discussed in the previous chapter (Sec. 1.11), each of the four hydrogen atoms is bonded to the carbon atom by a covalent bond, that is, by the sharing of a pair of electrons. When carbon is bonded to four other atoms, its bonding orbitals (sp orbitals, formed by the mixing of one s and three/ orbitals) are directed to the comers of a tetrahedron (Fig. 2.1a). This tetrahedral arrangement is the one that permits the orbitals to be as far apart as possible. For each of these orbitals to... 

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