Tetravalent Atoms

Chiral carbon atoms are common, but they are not the only possible centers of chirality. Other possible chiral tetravalent atoms are Si, Ge, Sn, N, S, and P, while potential trivalent chiral atoms, in which non-bonding electrons occupy the position of the fourth ligand, are N, P, As, Sb, S, Se, and Te. Furthermore, a center of chirality does not even have to be an atom, as shown in the structure represented in Figure 2-70b, where the center of chirality is at the center of the achiral skeleton of adamantane. 

In most common chiral molecules, chirality arises from chiral tetravalent atoms. A conformation-independent chirality code (CICC) was developed that encodes the molecular chirality originating from a chiral tetravalent atom [42], For more generality, a conformation-dependent chirality code (CDCC) is used [43]. CDCC ti cats a molecule as a rigid set of points (atoms) linked by bonds, and it accounts for chirality generated by chirality centers, chirality axes, or chirality planes. 

The numerals I-IV designate the numbers of the mono-, di-, tri-, and tetravalent atoms, respectively. 

The Fermi level is a theoretical energy of electrons in a semiconductor, such that the probability of occupation of the VB and CB is 50%. In an intrinsic semiconductor this Fermi level is about half-way between the VB and the CB, but it can be displaced substantially in doped semiconductors. An intrinsic semiconductor would be for example a crystal of pure Si or Ge, all tetravalent atoms being linked together in a three-dimensional array. In a doped semiconductor of n -type some of the Si atoms are replaced by pentavalent atoms such as As, and these will release electrons into the CB. A p -type semiconductor, however, contains some trivalent atoms like A1 which are electron deficient. The Fermi level moves closer to the CB in the n-doped semiconductor, while it comes closer to the VB in the p-type semiconductor (Figure 3.46). 

Figure 3.45 In a doped semiconductor some tetravalent atoms are replaced by trivalent atoms (p-type) or pentavalent atoms (n-type)...

In stereoisomerism, three dimensions must be considered. In stereoisomerism (also termed optical isomerism), there is no plane of symmetry in the molecule, so that the two forms are mirror-images, and thus cannot be turned into a position of coincidence. Thus, compounds containing a carbon atom (or other tetravalent atom) to which four different atoms or radicals are bonded are optical isomers. They receive this name from the fact that one isomer rotates the plane of polarized light to the right (d extra form) the other rotates it to the left (leva form). Lactic acid is an example. See also Lactic Acid, and formulas below ... 

Some interesting effects associated to the presence of well-defined structural units appear on a broad class of binary alloys formed by mixing an alkali metal (Li, Na, K, Rb, Cs) with a tetravalent metal like Sn or Pb. Due to the large difference in electronegativities it is normally assumed that one electron is transferred from the alkali to the tetravalent atom. As the Sn- or Pb-anions are isoelectronic with the P and As atoms, which in the gas phase form tetrahedral molecules P4 and AS4, in the same way the anions group in the crystal compounds forming (Sn4)4- and (Pb4)4- tetrahedra, separated by the alkali cations. This building principle was developed by Zintl in the early thirties [1], and the presence of such tetrahedra has been detected in the equiatomic solid compounds of Pb and Sn with Na, K, Rb and Cs, but not with Li [2, 3, 4]. In this paper we focus on alkali-lead alloys. 

If H = number of univalent atoms (H, halogen), N = number of trivalent atoms (N, P), and C = number of tetravalent atoms, then... 

Two calix[4]arenes may be connected also by a spiro-linker derived from pentaerythritol (D with X = — (CH2)20(CH2)20CH2—) ° or by two tetravalent atoms (E, X = Si, Ti). In the latter case, centro-symmetric molecules with two open cavities pointing in opposite directions (koilands) are obtained (SiCLt, NaH, THF, 52%) and may form one-dimensional networks in the crystalline state (koilates) when a suitable connector (e.g. hexadiyne) is included in their cavities. ... 

In the phthalocyanine field, the octupolar route provides additional degrees of freedom to help in the design of efficient nonlinear molecules. One of the possible methodologies to reach Pc-based octupolar architectures is the arrangement of the Pc cores into D or structures by means of attaching the macrocycles to benzene [58] or to a tetravalent atom such as phosphorus. Thus, for example, aryl trisphthalocyanine phosphonium salt (Figure 5) has been prepared and the second-order NLO response at the molecular level has been measured by HRS [59]. The jShrs values at X = 1.06/a,m(189 x 10 esu) is superior to those available for other related unsymmetrically substituted phthalocyanines with dipolar characteristics. 

Figure 1. Topological transition from a diamond network (left) to polyhedral homoatomic clusters (right). Some bonds between the tetravalent atoms break by adding electrons (middle). The formation of progressively more and more lone pairs eventually results in discrete cluster anions (right).

In a given chemical transformation, reactive atoms may undergo changes in hybridization and valency. For example, a tetravalent sp atom in a reactant molecule may change to a (a) dsp pentavalent, (b) sp trivalent or (c) different sp-" tetravalent atom in the conjunctive state or product. The first of these is a junctive process (Figure 16.1, l->2). The second one is a disjunctive process (1 3). In the third case, the process is substitutive (1 4, as in an 5 2 transformation) -it is simultaneously "lytic" and "genic". 

The A1 centers can be replaced by trivalent atoms such as B, Fe, Cr, Sb, As, and Ga, and the Si centers by tetravalent atoms such as Ge, Ti, Zr and Hf. Silicon enrichment up to a pure Si02 pentasil zeolite (sUicaUte) is also possible [4],... 

Ge is directly below Si in the periodic table, and both SiOi and GeOz may crystallise as quartz, cristobalite and rutile phases. This tetravalent atom can adopt coordination numbers of 4-6 in contrast to Si that is normally tetrahedral in its oxides, therefore, it is not surprising that Ge substitution into several zeolite frameworks including THO, FAU, LTA and PHI topologies was successfully attempted five decades ago. Until the late 1990s, however, no attempts to synthesise new microporous germa-nate frameworks in which all Ge atoms are tetrahedral had been successful.f " " ... 

In the following we shall confine our discussion to bond distances between Lewis-valent atoms, i.e. trivalent atoms in Group 13, tetravalent atoms in Group 14, trivalent in Group... 

Since none of the lanthanides forms a tetravalent metal, it is somewhat more difficult to obtain accurate values for A , iv However, from fig. 1 it is quite clear that cerium as a tetravalent metal should have a cohesive energy close to 145 kcal/ mol relative to its tetravalent atomic state d s. For the tetravalent element after lutetium in the Periodic Table, i.e. hafnium, the experimental cohesive energy is 148.4 kcal/mol (Brewer 1975). Therefore all the lanthanides should in their hypothetical tetravalent metallic state have a cohesive energy of about 145-148 kcal/mol relative to the proper tetravalent atomic state. This means that if the atomic excitations f" ds d s were known for the lanthanides we could easily... 

---