Carbide

[1] The carbides of the group 4, 5 and 6 transition metals (with the exception of chromium) are often described as interstitial compounds.

Some exhibit a range of stoichiometries, being a non-stoichiometric mixture of various carbides arising due to crystal defects.

the body centered cubic structure adopted by vanadium, niobium, tantalum, chromium, molybdenum and tungsten is not a close-packed lattice.)

For a long time the non-stoichiometric phases were believed to be disordered with a random filling of the interstices, however short and longer range ordering has been detected.

These compounds share features with both the inert interstitials and the more reactive salt-like carbides.

[8] Methanides are a subset of carbides distinguished by their tendency to decompose in water producing methane.

Transition metal carbides are not saline: their reaction with water is very slow and is usually neglected.

For example, depending on surface porosity, 5–30 atomic layers of titanium carbide are hydrolyzed, forming methane within 5 minutes at ambient conditions, following by saturation of the reaction.

Several carbides are assumed to be salts of the acetylide anion C2−2 (also called percarbide, by analogy with peroxide), which has a triple bond between the two carbon atoms.

The bonding in LaC2 has been described in terms of LaIII with the extra electron delocalised into the antibonding orbital on C2−2, explaining the metallic conduction.

Metallocarbohedrynes (or "met-cars") are stable clusters with the general formula M8C12 where M is a transition metal (Ti, Zr, V, etc.).