Cyclopentadienyls tantalum

The reaction between pentahalides or aUcoxides and carboxylic acids gives oxochloro- or oxoaUcoxocarboxylates. Chiral tantalum cyclopentadienyl carboxylates mediate acylation reactions of amides, giving dipeptides in good yield with high stereochemical purity. ... 

Schrock-type carbenes are nucleophilic alkylidene complexes formed by coordination of strong donor ligands such as alkyl or cyclopentadienyl with no 7T-acceptor ligand to metals in high oxidation states. The nucleophilic carbene complexes show Wittig s ylide-type reactivity and it has been discussed whether the structures may be considered as ylides. A tantalum Schrock-type carbene complex was synthesized by deprotonation of a metal alkyl group [38] (Scheme 7). 

Terminal to interior double olefins catalyst cyclopentadienyl-tantalum. Isomerisation catalyst based on Al-silicate 55... 

Triazacyclohexane also gives rise to very active catalysts with the use of chromium [13] as do ligands of the type RS(CH2)2NH(CH2)2SR [14], The latter coordinate in a meridional fashion, while the former can only coordinate in a facial fashion. Recently examples using cyclopentadienyl titanium complexes [15] and tantalum have been reported [16], The diversity of the chromium systems and the new metal systems show that very likely more catalysts will be discovered that are useful for this reaction, including 1-octene producing catalysts (1-octene is in high demand as a comonomer for ethene polymerisation for certain grades of polyethylene). 

Reaction with cyclopentadienyl sodium yields bis(cyclopentadienyl) tantalum... 

Table 13 Niobium(V) and Tantalum(V) Nitrene Derivatives (without Cyclopentadienyl Ligands)...

Bis(cyclooctene)—iridium(I) complexes, preparation, 7, 316 Bis(cyclopentadienyl) alkenes, with tantalum, 5, 157 Bis(cyclopentadienyl) alkyne niobium complexes, characteristics, 5, 81... 

Mono(cyclopentadienyl) complexes alkenes and tantalum, 5, 157 alkyne and niobium, 5, 80 calcium, strontium, barium, 2, 133... 

Note added in proof Very recent work by Schrock has extended this tantalum ylide chemistry very considerably and some cyclopentadienyl tantalum methylenes and benzylidenes could be isolated and fully characterized (82b). Among the new compounds the complex (C5H4CH3)2Ta(CH3)(CH2) is particularly noteworthy. Its crystal structure has been determined and the temperature dependence of its nmr spectra has been carefully investigated (17b). The ylidic carbon is in a trigonal planar configuration ... 

R. R. Schrock, and L. J. Guggenberger, Structure of Bis(cyclopentadienyl)methyl-methylenetantalum and the Estimated Barrier to Rotation about the Tantalum-Methylene Bond, J. Am. Chem. Soc. 97, 6578-6579 (1975). 

Monomeric, paramagnetic chlorohydrides snch as MH2CI2 (PR3)4 derive from facile oxidative addition of H2 to MCl2(PR3)4 complexes. Of these 17e dodecahedral complexes, (32) is the most interesting (eqnation 17). Its thermolysis in solution offers a qnadruply bridged hydride (33) and it can also be rednced to a monomeric Ta 16e species (34). Ta hydrides display a hydridic character (see Hydride Complexes of the Transition Metals), as shown by the reduction of CO by cyclopentadienyl derivatives. (see Niobium Tantalum Organometallic Chemistry)... 

The number of niobium and tantalum coordination compounds in oxidation states lower than II is limited, and they often are organometallic derivatives involving carbonyl, cyclopentadienyl, or alkenic ligands. 

Bis(cyclopentadienyl)cobalt(II), CoCp2, or hexacarbonyhnetalate anions of niobium and tantalum, [M(CO)e] , have been used as reducing agents for the preparation of titanium(ni) , and niobium(I) and tantalum(I) derivatives . ... 

Tetrachloro(cyclopentadienyl) complexes of niobium and tantalum, and their ring-substituted derivatives can be prepared by two general, methods. One method uses tin reagents of cyclopentadienes and the other uses trimethylsilyl reagents, which are reacted with niobium or tantulum pentachloride. Here the former method (Method 1) is described for the synthesis of Ta(ii -C5Me5)Cl4, and the latter (Method 2) for the synthesis of Nb(T -C5Me5)Cl4. 

Bis(7r-cyclopentadienyl)rhenium hydride was the first hydride to be recognized for 20 years after the isolation of the carbonyl hydrides of iron and cobalt 22). Within the last few years the related hydrides of molybdenum, tungsten, and tantalum have been isolated 80, 99, 106) (Table VIII). 

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