Carbides, interstitial ionic

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. 

Other Inorganic Carbon Compounds Carbon combines with metals to form carbides. In many cases, the carbon atoms occupy the holes or voids, also called interstitial sites, in metal structures, forming interstitial carbides. With active metals, the carbides are ionic. Calcium carbide forms in the high-temperature reaction of lime and coke ... 

Attempts to classify carbides according to structure or bond type meet the same difficulties as were encountered with hydrides (p. 64) and borides (p. 145) and for the same reasons. The general trends in properties of the three groups of compounds are, however, broadly similar, being most polar (ionic) for the electropositive metals, most covalent (molecular) for the electronegative non-metals and somewhat complex (interstitial) for the elements in the centre of the d block. There are also several elements with poorly characterized, unstable, or non-existent carbides, namely the later transition elements (Groups 11 and 12), the platinum metals, and the post transition-metal elements in Group 13. 

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. 

Compounds containing carbon in a negative oxidation state are properly called carbides, and many such compounds are known. In a manner analogous to the behavior of hydrogen and boron, carbon forms three types of binary compounds, which are usually called ionic, covalent, and interstitial... 

When discussing metal alloys (Section 4.3), we saw that atoms of non-metallic elements such as H, B, C, and N can be inserted into the interstices (tetrahedral and octahedral holes) of a lattice of metal atoms to form metal-like compounds that are usually nonstoichiometric and have considerable technological importance. These interstitial compounds are commonly referred to as metal hydrides, borides, carbides, or nitrides, but the implication that they contain the anions H, B3, C4, or N3- is misleading. To clarify this point, we consider first the properties of truly ionic hydrides, carbides, and nitrides. 

Why do carbides and nitrides exhibit the properties that make them so useful in industrial applications It is well accepted that these properties are related to the strength of interatomic bonding.2 In transition metal carbides and nitrides, bonding is believed to have both covalent and ionic contributions.3 The carbon or nitrogen atoms occupy interstitial sites in the metal lattice and are believed to promote strong metal-to-nonmetal and metal-to-metal bonds.1 More detailed bonding explanations require... 

As for hydrides, borides, and carbides, different types of nitrides are possible depending on the type of metallic element. The classifications of nitrides are similarly referred to as ionic (salt-like), covalent, and interstitial. However, it should be noted that there is a transition of bond types. Within the covalent classification, nitrides are known that have a diamond or graphite structure. Principally, these are the boron nitrides that were discussed in Chapter 8. 

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... 

Can be ionic saline), molecular, or interstitial -> Saline carbides are most commonly formed from Group 1 and 2 metals, Al, and a few other metals. 5-Block metals form saline carbides when their oxides are heated with C. 

A curious compromise is reached in many ionic crystals. The crystal NaCl,f or example, is based on two interpenetrating close-packed (fee) lattices. The positions of one lattice are occupied by positive ions, while those of the other are occupied by negative ions. Consider the unit cube of the fee structure in Fig. 27.7(a). There is a void, or hole, outlined by the octahedron, at the center of the cube. An identical octahedral hole is centered on each edge of the unit cube (Fig. 27.7b). Each hole is at the center of an octahedron, which has atoms at each of the six apices. The centers of the octahedral holes occupy the positions of an fee lattice, which interpenetrates the lattice on which the atoms are located. Small foreign atoms, such as H, B, C, N, can occupy these holes. Many carbides, hydrides, borides, and nitrides of the metals are interstitial compounds formed in this way. 

Ni3C) have properties intermediate be-tw een those of interstitial and ionic carbides. 

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]... 

This overall trend of TMNs in structure and physical properties is also consistent with the observation that these interstitial compounds exhibit surface reactivity and catalytic properties resembling those of noble metals, although the difference between carbides and nitrides is not always clear. The bonding in TMNs can be described as a mixture of metallic, covalent, and ionic components like TMCs. 

Interstitial carbides, which are INTERSTITIAL COMPOUNDS of carbon with transition metals. Titanium carbide (TiC) is an example. These compounds are all hard solids with high melting points and metallic properties. Some carbides (e.g. nickel carbide M3C) have properties intermediate between those of interstitial and ionic carbides. 

Covalence. Carbon bonding is covalent, that is, the atoms share a pair of electrons. Such covalent bonds are strong since the carbon atom is small and four of its six electrons (the four sp valence electrons) form bonds. This is the case for the two covalent carbides, silicon carbide and boron carbide (see Ch. 7). The bonding in interstitial carbides is not as straightforward and is a combination of covalent, metallic, and ionic bonding as reviewed in Sec. 6.0. 

As opposed to ionic bonding which involves electron transfer, covalent bonding means the sharing of electrons. Interstitial carbides have some degree of covalent bonding (M-C and M-M) resulting mostly from interaction between the Ip state of the carbon (see Ch. 2, Sec. 3.1) and the d state of the metal (the unfilled d orbitals mentioned in Sec. 2.2), and also from interaction between metal atoms. 

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. 

The third factor governing the structure of nitrides is the nature of the bond between the nitrogen atom and the other element forming the compound. As mentioned in Ch. 2, the bond is the force of attraction that holds together the atoms of a molecule.l The bonds in refractory carbides can be ionic (saltlike nitrides), covalent (covalent nitrides), or a combination of metallic, covalent, and ionic (interstitial nitrides) (for a discussion of electronic bonding, see Ch. 2, Sec. 5.0). 

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