Answer to Question #74554 in Inorganic Chemistry for srikant behera

Question

Why iron, cobalt and nickel do not form typical interstitial carbides? Write the type of carbides they form.

Answer 1

Interstitial carbides have metallic properties and are refractory. The long-held view is that the carbon atoms fit into octahedral interstices in a close-packed metal lattice when the metal atom radius is greater than approximately 135 pm.

Iron, cobalt and nickel do not form typical interstitial carbides, because they have atomic radius smaller than 135 pm. Iron, cobalt and nickel form intermediate transition metal carbides.

In intermediate transition metal carbides the transition metal ion is smaller than the critical 135 pm, and the structures are not interstitial but are more complex. Multiple stoichiometries are common; for example, iron forms a number of carbides, Fe3C, Fe7C3 and Fe2C. The best known is cementite, Fe3C, which is present in steels. These carbides are more reactive than the interstitial carbides; for example, the carbides of Fe, Co and Ni are all hydrolysed by dilute acids and sometimes by water, to give a mixture of hydrogen and hydrocarbons. These compounds share features with both the inert interstitials and the more reactive salt-like carbides.