Ligands multiple bond linkages

The General Paucity of Rare Earth-Ligand Multiple Bond Linkages in an Organometalhc Context... 

Titanium amido and imido complexes are of increasing interest as reagents in organic transformations and catalysis.1 1 For ease of representation, the terminal titanium-imido or titanium-oxo linkages are generally drawn as Ti=X (X = NR, O). Nevertheless, the formal metal-ligand multiple bonds in these complexes are better described as a2-tt4 triple bonds. 

Crystal structure analysis of [WCl(diphos)2(N2H2)]BPh4 368 clearly shows that the N2H2 ligand is indeed coordinated as a hydrazido(2-) group. The W—N—N linkage is linear with W—N and N—N distances of 1.73 and 1.37 A respectively. The short W—N distance clearly suggests a considerable multiple bond character to the W—N bond. 

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