Tantalum complexes, reaction with

The reactivity of a remarkable electronically unsaturated tantalum methyli-dene complex, [p-MeCgH4C(NSiMe3)2]2Ta( = CH2)CH3, has been investigated. Electrophilic addition and olefination reactions of the Ta = CH2 functionality were reported. The alkylidene complex participates in group-transfer reactions not observed in sterically similar but electronically saturated analogs. Reactions with substrates containing unsaturated C-X (X = C, N, O) bonds yield [Ta] = X compounds and vinylated organic products. Scheme 117 shows the reaction with pyridine N-oxide, which leads to formation of a tantalum 0x0 complex. ... 

The chemistry of tantalum is different. The tantalum pentaalkyl complex does not exists, because it transforms easily into a carbene complex by a-elimination. This complex reacts also with silica, leading to a supported tantalum complex.262,263 Their reaction proceeds first by the addition of the silanol OH group across the tantalum-carbon double bond followed by elimination of an alkane (Scheme 7.19). 

Although monodentate isocyanide-tantalum complexes are available by alternative routes,204 the majority of studies are concerned with the resultant 2-imines. Conversely, similar reactions with nitriles give monodentate imine moieties, which can undergo reactions while remaining attached to the metal.195,203-209... 

In 1982, Cotton et al prepared the first phosphinocarbene tantalum complex 99 from the reaction of Ta2Cl6(SMe2)3 with bis(diphenylphosphi-no)acetylene. The most remarkable feature of this reaction is the formal dimerization of the alkyne moiety.91... 

TaCl593 reacted with metallic sodium in neat trimethylphosphine to give the phosphinocarbene tantalum complexes 100 and 101, respectively. These reactions are the first examples of double activation of coordinated trimethylphosphine via oxidative cleavage of a substituent methyl C-H bond. A similar process was also observed in the reduction of tantalum pentabrom-ide with magnesium turnings in the presence of dimethylphenylphosphine.94... 

The tantalum(V) calix[4]arene complex 44 provides an interesting scaffold for the construction of an j -PhC2C=CPh ligand upon reaction with an excess of phenylethynyllithium (Scheme 10). The coupling reaction is presumed to proceed via bis-alkynyl 45, which is subsequently attacked at the a-carbon of one of the acetylide ligands to give anion 46, isolated as its lithium salt. The addition of a further equivalent of LiC CPh is probably prohibited by orbital constraints. ... 

In a similar manner, the reaction of 6.28 with CuCl in CH2CI2-CH3CN produces the spherical complex [ Cp Ta(CO)2(t " -P4) 6 CuCl g] (6.31). The tantalum complex 6.31 is comprised of 32 core atoms forming four-membered P4 rings and six-membered P4CU2 rings in an extended cubic arrangement with... 

Tantalum carbene complexes such as 5 and Ta(=CHCMe3)(S—C6H2-i-Pr3-2,4,6)3 (py) are effective, provided the conditions are such as to allow the coordinated base (THF or py) to give way to monomer (M). In the first example the initially formed tantalacyclobutane complex has been isolated and shown to have a trigonal-bipyramidal structure, and to polymerize NBE at a rate that is independent of [M], In this case the rearrangement of the intermediate tantalacyclobutane complex, to form the tantalum carbene complex, controls the rate of polymerization. In contrast, in the second example the rate is first-order in monomer here the reaction of the tantalum carbene complex with the monomer is the slower step. In both cases the polymer, after termination by reaction with benzaldehyde, is nearly monodisperse93,94. 

Although there are few five-membered metallacycles reported for group 5, there are some examples of metallacyclopentane compounds for niobium and tantalum. The reaction of the labile vanadium naphthalene complex [CpV(Ci0H8)] with ethene provided the unusual binuclear complex [Cp2V2(p-C4H8)2] (34 [Eq. (13)], Fig. 9) in which the two vanadium centres are bridged by two butanediyl groups.38... 

Di(carbene)gold(I) salts, oxidation, 2, 293—294 Dicarbido clusters, with decarutheniums, 6, 1036 Dicarbollide amides, with tantalum, 5, 184 Dicarbollide thorium complexes, synthesis and characterization, 4, 224—225 Dicarbollyl ligands, in nickel complexes, 8, 185 Dicarbonyl complexes arylation with lead triacetates diastereoselectivity, 9, 389 enantioselectivity, 9, 391 mechanisms, 9, 387 reaction examples, 9, 382 indium-mediated allylation, 9, 675 with iridium, 7, 287 reductive cyclization, 10, 529 in Ru and Os half-sandwiches, 6, 508 with Zr—Hf(II), 4, 700... 

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