Chemistry of group 4 transition metal amides

In this article are summarized general properties, synthesis, reactivity, and catalysis of Group 4 transition metal amide complexes. Thermally robust but moisture sensitive compounds of the type M (NR₂)₄ and MY_n(NR₂)_4-n [M=Ti, Zr, Hf; R=H, alkyl, aryl; Y=X(halogens), OR; n=1-3) are generally synthesized from MX₄ and amines or alkali metal amides, by disproportionation reactions of M(NR₂)₄ with MY₄, or by ligand exchange reactions of M(NR₂)₄ and MY_n(NR₂)_4-n. Since nitrogen atoms in these compounds form both σ- and π-donor bonds to the metal center, the amide complexes typically have planar MNR₂ units with shorter M-N lengths. The amide complexes are highly reactive towards carbonyl compounds or Brønsted acids such as cyclopentadiene. These reactions are useful for chemical transformation of carbonyl compounds and synthesis of metallocene derivatives; some of the latter are excellent catalysts for olefin polymerization. Besides metallocene derivatives, Group 4 transition metal amide complexes themselves have recently received much attention as catalysts for ethylene and α-olefins and for hydroamination of acetylenes.