Carbon interstitial carbides

The interstitial carbides These are formed by the transition metals (e.g. titanium, iron) and have the general formula M, C. They are often non-stoichiometric—the carbon atoms can occupy some or all of the small spaces between the larger metal atoms, the arrangement of which remains essentially the same as in the pure metal (cf. the interstitial hydrides). 

Carbon is the only Group 14/IV element that forms both monatomic and polyatomic anions. There are three classes of carbides saline carbides (saltlike carbides), covalent carbides, and interstitial carbides. The heavier elements in Group 14/IV form polyatomic anions, such as Si44 and Sn52, in which the atoms form a tetrahedron and trigonal bipyramid, respectively. 

The interstitial carbides are compounds formed by the direct reaction of a d-block metal and carbon at temperatures above 2000°C. In these compounds, the C atoms occupy the gaps between the metal atoms, as do the H atoms in metallic hydrides (see Fig. 14.9). Here, however, the C atoms pin the metal atoms together into a rigid structure, resulting in very hard substances with melting points often well above 3000°C. Tungsten carbide, WC, is used for the cutting surfaces of drills, and iron carbide, FesC, is an important component of steel. 

Carbon forms ionic carbides with the metals of Groups 1 and 2, covalent carbides with nonmetals, and interstitial carbides with d-block metals. Silicon compounds are more reactive than carbon compounds. They can act as Lewis acids. 

These carbides, also known as interstitial carbides, are crystalline compounds of a host metal and carbon. The host-metal atoms are generally arranged in a close-packed structure and the carbon occupies specific interstitial sites in that structure. Such a structure sets size restrictions on the two elements in order for the carbon atom to fit into the available sites and the population of these sites (if all are occupied) determines the stoichiometry of the carbide. 

Carbides. As might he expecled from its position in the periodic table, carbon forms binary compounds with the metals in which it exhibits a negative valence, and binary compounds with the non-metals in which it exhibits a positive valence. A convenient classification of the binary compounds of carbon is into ionic or salt-like carbides, intermediate carbides, interstitial carbides, and cuvalent binary carbon compounds. 

FIGURE 14.48 The structure of an interstitial carbide, in which the carbon atoms (represented by the black spheres) lie between metal atoms (the gray spheres), thereby producing a rigid structure. 

In virtually all its stable compounds carbon forms four bonds and has coordination numbers of 2 (=C— or =C=), 3 (=CQ, or 4, with linear, triangular (planar), and tetrahedral geometries, respectively CO has coordination number 1. In interstitial carbides (Section 7-3), certain metal cluster compounds1 (Section 7-9), and very stable trigonal bipyramidal and octahedral penta- and hexa(aurio)methanium cations of the type (LAu)5C+ and (LAu)6C2+, where L is a phosphine,2 carbon atoms are found with coordination numbers of 4, 5, or 6. Coordination number 5 is also found in compounds with bridging alkyls such as Al2Me6, in some carboranes (Section 5-12), and in reactive carbocations.3... 

The main structural features of the transition-metal interstitial carbides are as follows. The maximum carbon content depends on the c.p. layer sequence. The two octahedral interstices on either side of an h layer are located directly above one another, and only one of these is ever occupied. This restriction gives the following limiting formulae ... 

Class 4 The carbides of this class do not possess the extreme properties of the interstitial carbides. For example, whereas titanium carbide is not attacked by water or HQ even at 600°C, these carbides are decomposed by dilute acids (Fe3C and NisC) or even water (Mn3C). Although the carbon is present as discrete C atoms the products include, in addition to hydrogen, complex mixtures of hydrocarbons. 

The carbon atoms of most binary metal carbides have hypercoordinated environments like those shown in Figure 1.5. In particular, octahedral carbon coordination is common in the interstitial carbides formed by many transition metals, materials of variable composition in which carbon atoms... 

Interstitial carbides, which are interstitial compounds of carbon with transition metals. Titanium carbide (TiC) is an example. These compounds are all hard high-melting solids, with metallic properties. Some carbides (e.g. nickel carbide... 

Interstitial carbides are formed by many transition metals. The carbon atoms occupy open spaces (interstices) between the metal atoms in a manner analogous to the interstitial hydrides (Section 22.2). This process generally makes the metal harder. Tungsten carbide, for example, is very hard and very heat-resistant and, thus, used to make cutting tools. 

A very successful method for synthesizing interstitial carbide dusters is based on the metal assisted disproportionation of carbon monoxide into a carbide atom and carbon dioxide, which is related to the Bouduard reaction. Thus, most of the Re, Fe, Ru, and Os carbide dusters have been obtained from the thermal decomposition of a carbonyl precursor and the evolution of CO2 has been confirmed in one case. [95]... 

Whereas certain bonding features are considered most unusual in a molecule, similar features are quite normal in solid state compounds, lb give an example, the unusual coordination of the carbon atom in the discrete molecule [ligQ is suggestive of hypervalency, [1, 2] however the occurrence of octahedrally coordinated carbon atoms is often observed in transition metal carbides having the rocksalt type structure, thus a quite normal situation. From a simple ionic representation as [(Ii )5C (e )2], the only surprise is the surplus of electrons. Yet, even this feature is familiar with such interstitial carbides as V2C in which, the valence electrons of V (li) which are not used for heteronuclear V-C (li-Q bonds are used to form homonuclear V-V (li-Ii) bonds, as suggested by the (8-N) rule. [3]... 

The difference in electronegativity between carbon and the other element forming a carbide is an important factor in determining the nature of the compound. As shown in Table 2.1, that difference in the interstitial carbides is large (Box A) while it is much less pronounced in the covalent carbides (Box B). 

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