Tantalum complexes with mono

Imidazolium ligands, in Rh complexes, 7, 126 Imidazolium salts iridium binding, 7, 349 in silver(I) carbene synthesis, 2, 206 Imidazol-2-ylidene carbenes, with tungsten carbonyls, 5, 678 (Imidazol-2-ylidene)gold(I) complexes, preparation, 2, 289 Imidazopyridine, in trinuclear Ru and Os clusters, 6, 727 Imidazo[l,2-a]-pyridines, iodo-substituted, in Grignard reagent preparation, 9, 37—38 Imido alkyl complexes, with tantalum, 5, 118—120 Imido-amido half-sandwich compounds, with tantalum, 5,183 /13-Imido clusters, with trinuclear Ru clusters, 6, 733 Imido complexes with bis-Gp Ti, 4, 579 with monoalkyl Ti(IV), 4, 336 with mono-Gp Ti(IV), 4, 419 with Ru half-sandwiches, 6, 519—520 with tantalum, 5, 110 with titanium(IV) dialkyls, 4, 352 with titanocenes, 4, 566 with tungsten... 

Tantalum enolate chemistry shows the dichotomy for the carbonylation reaction " of Cp Ta(CH2R)Cl3 with CO which results in the mono-THF adduct of rj -acyl complex Cp Ta(0=CCH2R)Cl3(THF) for R = t-Bu (the acyl group is anionic) but the isomeric enolate Cp Ta((Z)-7j -OCH=CHR)Cl3 for R = p-Tol. This invites the question of the relative thermodynamic stabilities of metal complexes of RCH2CO and RCHCHO and additionally the question of Z vs. E enolate stabilities. Only for organometalhc compounds (X = [M]) do we find examples where RCH2COX is less stable than RCH=CHOX. 

Scheme 19 intramolecular hydroaminoalkylation of unactivated olefins with secondary amines catalyzed by mono- and bis(amidate) tantalum ami do complexes... 

Although group 5 organometallic systems have been found to be of relevance in transition-metal catalyzed hydroboration reactions, structurally authenticated group 5 boryl complexes remain relatively few in number. Smith and co-workers, for example, have probed the mechanisms for the formation of niobium and tantalum mono- and bis(boryls) from propylene complex precursors, with concomitant formation of propyl boronate esters [31,32]. Of particular interest from a structural viewpoint are the relative merits of alternative bonding descriptions for metal(V) boryl bis(hydrides) as borohydride complexes or as mono(hydride) a-borane systems [31-34]. 

The polymerization is effected by a complex catalyst system. One such system has three main components. The first of these is typically a halide or acetyl-acetonate of tungsten, molybdenum, tantalum or rhenium the second a trialkyl aluminium or dialkyl aluminium chloride and the third component an activator such as epichlorhydrin or 2-chloroethanol. The polymerization may be carried out in bulk in the temperature range -50 to 0°C. The molecular weight may be regulated by the addition of small amounts of butene. The final polymer molecules are not necessarily macrocyclic in nature due to bond scission. This explanation has been shown to be consistent with the existence of vinyl end-groups even where no chain terminating non-cyclic mono-olefins have been specificaUy added. 

Vanadium, Niobium and Tantalum An entry into Cp 2Ta chemistry has been developed with the reaction of Cp TaCl3PMe3 with LiCp. The complex Cp 2Nb(BH4) provides an appropriate entry into Cp 2NbH3 chemistry,A study of the electrochemistry of Cp2NbCl2 has led to the identification of the one-electron reduction product (Cp2NbCl3NbCp2l and (Cp2NbCl]2 on the way to niobocene. Reactions of mono and dihydrides of Cp2TaH(C0)... 

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