Tantalum complexes oxygen ligands

Tantalum complexes are excellent models in understanding how effective well-defined W and Mo catalysts might be designed that mimic metathesis catalysts based on W and Mo [6]. The first example of a stable M=CHR complex is the Ta(CH-t-Bu)(CH2-t-Bu)3 complex. In this complex, four bulk covalently bound ligands stabilize even an electronically unsaturated specie toward bimolecular decomposition. This compound is sensitive to oxygen, water, and a variety of functionalities, among them are ketones and aldehydes, with which it reacts to yield polymeric (t-BuCH2)3Ta=0 and the expected olefin [26]. It was established that d alkylidene species were responsible for olefin metathesis and drew attention to... 

Even fewer complexes with nitrogen donor ligands have been reported and all are methyl cyanide adducts (Tables X and XI). Protactinium pentabromide forms a soluble 1 3 complex in contrast to the 1 1 complexes formed by niobium and tantalum pentahalides (46). Other actinide pentahalide-methyl cyanide complexes are still unknown. Protactinium tetrachloride, tetrabromide, and tetraiodide react with anhydrous, oxygen-free methyl cyanide to form slightly soluble 1 4 complexes (44, 48) which are isostructural with their actinide tetrahalide analogs. 

Basset and his group have observed that propane and propene metathesis give similar Cn+i/Cn+2 ratios of cross-metathesis products on silica-supported tantalum-neopentylidene catalyst at 150°C. The olefin-metathesis activity of these Schrock-type supported complexes results from the presence of the silyloxy ligand (vide infra) - Organometallic complexes are bound to silica or alumina by reaction of soluble complexes and involve die formation of one or several bonds between the central metal and the oxygen atom of the oxide support. 

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