Oxidation tetrahedral arrangement

The concept of oxidation states is best applied only to germanium, tin and lead, for the chemistry of carbon and silicon is almost wholly defined in terms of covalency with the carbon and silicon atoms sharing all their four outer quantum level electrons. These are often tetrahedrally arranged around the central atom. There are compounds of carbon in which the valency appears to be less than... 

In P", (a), the oxide packing arrangement comprises close packed layers in a cubic close packed,. .. ABC. .., stacking sequence, which continues throughout the structure. The spinel blocks are four oxide layers thick and contain AP" ions (and stabilising Li, Mg ions) distributed over tetrahedral and octahedral sites, as in spinel, MgAl204. These sites... 

Figure 3.33a shows the diamond structure. Each of the carbon atoms in the giant structure is covalently bonded to four others. They form a tetrahedral arrangement similar to that found in silicon(iv) oxide (p. 50). This bonding scheme gives rise to a very... 

Paramagnetic compounds can also be difficult NMR subjects because the unpaired electron(s) couple(s) with the nuclear spin to provide rapid relaxation. Relaxation times can, however, yield useful information about a system certain parameters which contribute to the magnitude of 7) are directly related to the oxidation number and spin state of the metal ion, the nature of the metal ion and its coordination geometry. For example, high-spin Com in octahedral complexes has a relaxation time more than an order of magnitude faster than the same ion in a tetrahedral arrangement of ligands (Banci et at., 1992). 

The spinel structure is based on a face-centered cubic arrangement of O2- ions that results in cation sites that can be described as octahedral and tetrahedral holes. In the structure of spinel, the aluminum ions are surrounded by six oxide ions in an octahedral arrangement and magnesium ions are surrounded by four oxide ions in a tetrahedral arrangement. It should be noted that the +3 ions reside in octahedral holes, whereas the +2 metal ions reside in tetrahedral holes (see Chapter 3). In a variation of this structure, compounds of this type form an arrangement in which half of the M 3+ ions reside in tetrahedral holes and the M2+ and half of the M 3+ ions reside in octahedral holes. Such a structure is known as... 

Several copper-containing NiRs have been identified, but the most extensive structural and mechanistic studies have focused on the enzyme from Achromobacter cycloclastes (17-25). A 2.3-A resolution X-ray crystal structure for this NiR in its oxidized form at pH 5.2 has been reported (17), and a representation of the active site is shown in Figure 1. Each monomer in the trimeric protein contains two copper ions, one of which (Cu-1) is ligated to a cysteine, a methionine, and two histidine residues in a geometry similar to that of type 1 copper centers in proteins such as plastocyanin (26). The second type 2 copper ion in NiR (Cu-2) is only 12.5-A distant from the first and is bound to three histidine imidazoles (two from one monomer, the third from an associated subunit) and a fourth small ligand in an unusual tetrahedral arrangement. The... 

The oxophilic character of lanthanide is evident from the characterization of the complex (MeCp)3Yb4(/i-Cl)(,(/i3-Cl)(/i4-0)(THF)3 by the reaction of YbCb and YbOCl with MeCp in THF. The crystal structure of the complex shows an oxygen-centred tetrahedral arrangement of Yb atoms with p.-C bridging each edge and p -C 1 over the triangular face formed by the (MeCp)Yb units [137]. Yb-0(/x4-oxide) bond distance range is 2.13(1)— 2.29 (1) A. 

All the metals or their oxides react with carbon at high temperatures to give carbides MQ.15 These are ionic acetylides whose properties are discussed in Chapter 7. The carbide Mg2C316 has been obtained relatively pure by reacting Mg dust with pentane at ca. 680°C. This has Mg atoms coordinated by four linear C3 groups in a tetrahedral arrangement, three via terminal C atoms and one by a... 

This complex catalyzes the reaction through the Q cycle (Section 18.3.4). In the first half of the Q cycle, plastoquinol is oxidized to plastoquinone, one electron at a time. The electrons from plastoquinol flow through the Fe-S protein to convert oxidized plastocyanin into its reduced form. Plastocyanin is a small, soluble protein with a single copper ion bound by a cysteine residue, two histidine residues, and a methionine residue in a distorted tetrahedral arrangement (Figure 19.17). This geometry facilitates the interconversion between the Cu2+ and the Cu+ states and sets the reduction potential at an appropriate value relative to that of plastoquinol. Plastocyanin is intensely blue in color in its oxidized form, marking it as a member of the "blue copper protein," or type I copper protein family. 

In its simple molecules and ions Si does not exhibit a covalency of less than four except (possibly) in the silyl ion (see later). Unlike carbon it does not form a small number of multiple bonds, as does carbon in CO, COj, -CN, etc. Only one oxide of Si is stable at ordinary temperatures, and this exists in a number of crystalline forms, in all of which-with the exception of stishovite (p. 804)-there is a tetrahedral arrangement of four bonds from each Si atom, and in every case the Si and 0 atoms form an infinite 3D network. The lower oxide SiO has nothing in common with the gaseous CO. It is produced by heating SiOj with Si at... 

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