Carbides

Classifying carbides is not simple, but some useful categories are:

• saline (salt-like) carbides which produce mainly CH₄ when hydrolysed;
• those containing the [C≡C]^2- ion;
• those containing the [C=C=C]^4- ion;
• interstitial carbides;
• solid state carbides with other lattice structures;
• fulleride salts
• endohedral metallofullerenes

     Examples of saline carbides are Be₂C and Al₄C₃, both made by heating the  constituent elements at high temperatures. Although their solid state structures contain isolated C centres which are converted to CH₄ on reaction with H₂O, it is unlikely that the ‘C^4-’ ion is present since the interelectronic repulsionenergy would be enormous. Carbides containing the [C≡C]^2- (acetylide) ion include Na₂C₂, K₂C₂, MC₂ (M=Mg, Ca, Sr, Ba), Ag₂C₂ and Cu₂C₂; they evolve C₂H₂ when treated with water. Calcium carbide is manufactured as a grey solid by heating CaO with coke at ≈ 2300 K, and when pure, it is colourless. It adopts a distorted NaCl lattice, the axis along which the [C≡C]^2- are aligned being lengthened; the C^_C bond distance is 119 pm, compared with 120pm in C₂H₂. The reaction between CaC₂ and N₂ is used commercially for the production of calcium cyanamide, a nitrogenous fertilizer. The cyanamide ion is iso electronic with CO₂.

   Equations 13.30 and 13.31 show syntheses of Na₂C₂, Ag₂C₂ and Cu₂C₂; the group 11 carbides are heat- and shock-sensitive, and explosive when dry.

    Carbides of formula MC₂ do not necessarily contain the acetylide ion. The room temperature form of ThC₂ (Th is an actinoid metal, see Chapter 24) adopts an NaCl lattice but is not isostructural with CaC₂. In ThC₂ , the C₂ –units (d_CC = 133 pm) in alternating layers lie in different orientations.The solid state structure of LaC₂ contains C₂-units with d_CC = 129 pm. Unlike CaC₂ which is an insulator, ThC₂ and LaC₂ have metallic appearances and are electrical conductors. The C^_C bond lengths can be rationalized in terms of structures approximating to Th₄‏[C₂]^4-  and La₃‏[C₂]^3-; compared with [C₂]^2-, the extra electrons in [C₂]^4- and [C₂]^3- reside in antibonding MOs, thus weakening the C^_C interaction. However, the conducting properties and diamagnetism of ThC₂ and LaC₂ show that this is an oversimplified description since electron delocalization into a conduction band (see Section 5.8) must occur. Hydrolysis of these carbides is also atypical of a [C₂]^2--containing species, e.g. the reaction of ThC₂ and H₂O yields mainly C₂H₂, C₂H₆ and H₂. Carbides containing [C=C=C]^4- are rare; they include Mg₂C₃ which liberates propyne upon hydrolysis. The structures of the so-called interstitial carbides (formed by heating C with d-block metals having rmetal > 130 pm, e.g. Ti, Zr, V, Mo, W) may be described in terms of a closepacked metal lattice with C atoms occupying octahedral holes.

  In carbides of type M₂C (e.g. V₂C, Nb₂C) the metal atoms are in an hcp lattice and half of the octahedral sites are occupied; in the MC type (e.g. TiC and WC), the metal atoms adopt a ccp structure and all the octahedral holes are occupied. These interstitial carbides are important refractory materials; characteristically they are very hard and infusible, have melting points >2800K and, in contrast to the acetylide derivatives, do not react with water. Tungsten carbide, WC, is one of the hardest substances known and is widely used in cutting tools and dies. Although TiC, WC, V₂C, Nb₂C and related compounds are commonly described as interstitial compounds, this does not imply weak bonding. To convert solid carbon into isolated carbon atoms is a very endothermic process and this must be compensated by the formation of strong W^_C bonds. Similar considerations apply to interstitial nitrides . Transition metals with rmetal < 130 pm (e.g. Cr, Fe, Co, Ni) form carbides with a range of stoichiometries (e.g. Cr₃C₂, Fe₃C) which possess complicated structures involving C^_C bonding. In Cr₃C₂ (formed by reaction 13.32), the Cr atoms form a lattice of edge-sharing trigonal prisms each occupied by a C atomsuch that carbon chains run through the structure with C^_C distances comparable to single bonds.

Carbides of this type are hydrolysed by water or dilute acid to give mixtures of hydrocarbons and H₂.