Compounds interstitial carbides

Metallic Carbides. This class of compounds comprises the interstitial carbides of the transition metals of Groups 4—6 (see Industrial,... 

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. 

Hafnium carbide (HfC) is an interstitial carbide which, with tantalum carbide, is the most refractory compound known. Its characteristics and properties are summarized in Table 9.4. 

Boron and carbon form one compound, boron carbide [12069-32-8], B4C, although excess boron may dissolve in boron carbide, and a small amount of boron may dissolve in graphite (5). Usually excess carbon appears as graphite, except for the special case of boron diffused into diamonds at high pressures and temperatures, eg, 5 GPa (50 kbar) and 1500°C, where boron may occupy both interstitial and substitutional positions in the diamond lattice, a property utilized in synthetic diamonds (see Carbon, diamond, synthetic). 

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. 

Periodic Table to justify the leaving of such slighted elements for a more advanced course taken by students with special interests in inorganic chemistry. Similarly, I have passed over many interesting but somewhat exotic compounds of the more usual elements (for example, the nitrides of sulfur, metallic nitrosyl compounds, heteropoly acids, and interstitial carbides and nitrides). 

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... 

Compounds with the sodium chloride structure range from the essentially ionic halides and hydrides of the alkali metals and the monoxides and monosulphides of Mg and the alkaline-earths, through ionic-covalent compounds such as transition-metal monoxides to the semi-metallic compounds of B subgroup metals such as PbTe, InSb, and SnAs, and the interstitial carbides and nitrides (Table 6.1). Unique and different distorted forms of the structure are adopted by the Group IIIB... 

Wo might here deal briefly with the structure of the well-known interstitial compound iron carbide, or cementite. The largest interatomic distance in iron is 2-52A, which figure may be taken for the atomic diameter of iron, while that of carbon is 1-54A, so that the diameter ratio is 1 54/2 52 — (Mil i.e. the diameter ratio is greater than 0 59. The relatively small size of the iron atom therefore induces the formation of an interstitial structure which is more complex than the simple cubic or hexagonal types described above. ... 

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 carbide compounds of transition metals (such as WC and TiC) do not readily hydrolyze. However, these compounds when dissolved in H2O2 solutions give oxalato complexes of W(VI), etc. This implies formation of a... 

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). 

This switch in the metal structure occurring when an interstitial carbide is formed has been used to assert that these compounds are not truly interstitial structures since the basic metal fiamework is modified.This interpretation may be semantically correct but the use of the term interstitial is widespread and, if not entirely accurate, provides a visual and easy-to-grasp representation of the structure. 

As seen above, the atomic structure of interstitial carbides is a mixture of ionic, covalent, and metallic bonding. As a result, the properties of these compounds reflect this structural mix and combine metallic and ceramic characteristics as summarized in Table 3.11. 

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