Oxides and sulfides

CLi4 has a tetrahedral ground state. In contrast, SiLi4, GeLi4 and SnLLq have a C2v ground state structure (7)164. The energy differences (at RHF, in parentheses at 

Miriam Kami, Yitzhak Apeloig, Jtirgen Kapp and Paul von R. Schleyer 

FIGURE 8. Optimized geometry of C2V MLi4 (M = Si, Ge, Sn, given in this order from top to bottom), calculated at MP2/6-31G (M = Si, Ge) and RHF (M = Sn)164 bond lengths in pm, bond angles in deg 

The binaiy hydrides (p. 64), borides (p. 145), carbides (p. 299) and nitrides (p. 417) are hard, refractory, nonstoichiometric materials with metallic conductivities. They have already been discussed in relation to comparable compounds of other metals in earlier chapters. 

The main oxides are the dioxides. In fact, Ti02 is by far the most important compound formed by the elements of this group, its importance arising predominantly from its use as a white pigment (see Panel, p. 959). It exists at room temperature in three forms — rutile, anatase and brookite, each of which occurs naturally. Each contains 6-coordinate titanium but rutile is the most common form, both in nature and as produced commercially, and the others transform into it on heating. The rutile 

Finally, oxygen is soluble in metallic titanium up to a composition of TiOo.s with the oxygen atoms occupying octahedral sites in the hep metal lattice distinct phases that have been crystallographically characterized are TieO, TisO and Ti20. It seems likely that in all these reduced oxide phases there is extensive metal-metal bonding. 

Mohr and H. MiJller-Buschbaum, Z. anorg. allg. Chem. 620, 1175-8 (1994). 

The sulfides have been less thoroughly examined than the oxides but it is clear that a number of stable phases can be produced and nonstoichiometry is again prevalent (p. 679). The most important are the disulfides, which are semiconductors with metallic lustre. TiS2 and ZrS2 have the Cdl2 structure (p. 1211) in which the cations occupy the octahedral sites between alternate layers of hep anions. 

Space will not be devoted here to emphasis of the vast technological importance of iron and the steels nor to the discussion of ferrous metallurgy. However, typical processes for obtaining cobalt and nickel from natural sources are outlined in Table 24-1. The process for cobalt is somewhat oversimplified, for cobalt ores often contain, in addition to iron and arsenic, nickel, silver, or copper, which must also be removed. Note that nickel is conveniently purified by conversion to its volatile carbonyl, Ni(CO)4, unstable at high temperatures Mond process). 

Co CoAsS 4- FeS (cobaltite) A1 a, Os ). IC03O4 — Co metal NaNO. feS r + SioT Fe2° + Sl°s + NasCO. iejOj + Na3As04 1 Na3As04 (slag, pour off) 

In addition to its importance in alloys (for example, alnico, vicalloy, and stellite), cobalt is of use as a catalyst in the Fisher-Tropsch process in which carbon monoxide is hydrogenated to a mixture of hydrocarbons. It appears likely here that one or more carbonyl derivatives of cobalt act as intermediates. Nickel is of importance in a number of alloys Monel metal, alnico, stainless steel, etc.). In a very finely divided state Raney nickel), it is of use to the organic chemist in hydrogenation reactions, for it will absorb large quantities of hydrogen gas with probable breakage of the molecules to atoms (p. 27). 

The monoxides of iron, of cobalt, and (above 200° C) of nickel have the sodium chloride structure. However, it should be noted that FeO 

The familiar monosulfides, FeS, CoS, and NiS, may, like the monoxides, show considerable deviation from stoichiometry without exhibiting heterogeneity. These three sulfides have the same structure, different from that common to the monoxides, but similar to the structure of a group of compounds which may be considered intermediate in character between alloys and salts (for example, NiAs, CoTe, CrSb). Indeed, the pronounced metallic lustre and appreciable conductivity of large crystals of FeS raa r be regarded as distinctly alloylike.  

Bulk structures of oxides are best described by assuming that they are made up of positive metal ions (cations) and negative O ions (anions). Locally the major structural feature is that cations are surrounded by O ions and oxygen by cations, leading to a bulk structure that is largely determined by the stoichiometry. The ions are, in almost all oxides, larger than the metal cation. It does not exist in isolated form but is stabilized by the surrounding positive metal ions. 

2 Structure of Metals, Oxides and Sulfides and their Su faces 

The cations in transition metal oxides often occur in more than one oxidation state. Molybdenum oxide is a good example, as the Mo cation may be in the 6-r, 5-r, and 4+ oxidation states. Oxide surfaces with the cation in the lower oxidation state are usually more reactive than those in the highest oxidation state. Such ions can engage in reactions that involve changes in valence state. 

Cations at the surface possess Lewis acidity, i.e. they behave as electron acceptors. The oxygen ions behave as proton acceptors and are thus Bronsted bases. This has consequences for adsorption, as we will see. According to Bronsted s concept of basicity, species capable of accepting a proton are called a base, while a Bronsted acid is a proton donor. In Lewis concept, every species that can accept an electron is an acid, while electron donors, such as molecules possessing electron lone pairs, are bases. Hence a Lewis base is in practice equivalent to a Bronsted base. However, the concepts of acidity are markedly different. 

Sulfides play an important role in hydrotreating catalysis. Whereas oxides are ionic structures, in which cations and anions preferably surround each other to minimize the repulsion between ions of the same charge, sulfides have largely covalent bonds as a consequence there is no repulsion which prevents sulfur atoms forming mutual bonds and hence the crystal structures of sulfides differ, in general, greatly from those of oxides. 

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