section_path: Chemistry and compounds/Nitrides, azides, and nitrido complexes
section_name: Nitrides, azides, and nitrido complexes
paragraph_text: Nitrides, azides, and nitrido complexes
  
  Nitrogen bonds to almost all the elements in the periodic table except the first two noble gases, helium and neon, and some of the very short-lived elements after bismuth, creating an immense variety of binary compounds with varying properties and applications. Many binary compounds are known: with the exception of the nitrogen hydrides, oxides, and fluorides, these are typically called nitrides. Many stoichiometric phases are usually present for most elements (e.g. MnN, Mn N, Mn N, Mn N, Mn N, and Mn N for 9.2 < x < 25.3). They may be classified as "salt-like" (mostly ionic), covalent, "diamond-like", and metallic (or interstitial), although this classification has limitations generally stemming from the continuity of bonding types instead of the discrete and separate types that it implies. They are normally prepared by directly reacting a metal with nitrogen or ammonia (sometimes after heating), or by thermal decomposition of metal amides:
  
  Many variants on these processes are possible. The most ionic of these nitrides are those of the alkali metals and alkaline earth metals, Li N (Na, K, Rb, and Cs do not form stable nitrides for steric reasons) and M N (M = Be, Mg, Ca, Sr, Ba). These can formally be thought of as salts of the N anion, although charge separation is not actually complete even for these highly electropositive elements. However, the alkali metal azides NaN and KN, featuring the linear N anion, are well-known, as are Sr(N) and Ba(N). Azides of the B-subgroup metals (those in groups 11 through 16) are much less ionic, have more complicated structures, and detonate readily when shocked.
  
  Many covalent binary nitrides are known. Examples include cyanogen ((CN)), triphosphorus pentanitride (P N), disulfur dinitride (S N), and tetrasulfur tetranitride (S N). The essentially covalent silicon nitride (Si N) and germanium nitride (Ge N) are also known: silicon nitride, in particular, would make a promising ceramic if not for the difficulty of working with and sintering it. In particular, the group 13 nitrides, most of which are promising semiconductors, are isoelectronic with graphite, diamond, and silicon carbide and have similar structures: their bonding changes from covalent to partially ionic to metallic as the group is descended. In particular, since the B–N unit is isoelectronic to C–C, and carbon is essentially intermediate in size between boron and nitrogen, much of organic chemistry finds an echo in boron–nitrogen chemistry, such as in borazine ("inorganic benzene "). Nevertheless, the analogy is not exact due to the ease of nucleophilic attack at boron due to its deficiency in electrons, which is not possible in a wholly carbon-containing ring.
  
  The largest category of nitrides are the interstitial nitrides of formulae MN, M N, and M N (although variable composition is perfectly possible), where the small nitrogen atoms are positioned in the gaps in a metallic cubic or hexagonal close-packed lattice. They are opaque, very hard, and chemically inert, melting only at very high temperatures (generally over 2500 °C). They have a metallic lustre and conduct electricity as do metals. They hydrolyse only very slowly to give ammonia or nitrogen.
  
  The nitride anion (N) is the strongest π donor known among ligands (the second-strongest is O). Nitrido complexes are generally made by the thermal decomposition of azides or by deprotonating ammonia, and they usually involve a terminal {≡N} group. The linear azide anion (N), being isoelectronic with nitrous oxide, carbon dioxide, and cyanate, forms many coordination complexes. Further catenation is rare, although N (isoelectronic with carbonate and nitrate) is known.