Transition metal complexes tantalum

In contrast, Schrock carbenes are electron deficient [10 to 16 valence electrons (VE)] early transition metal complexes with the metal atom in a high oxidation state and carbene substituents that are limited to alkyl groups and hydrogen [131]. Their bonding situation can be described in terms of the interaction of a triplet carbene with a triplet metal fragment resnlting in a covalent double bond [132], Tantalum complexes like [(np)3Ta=CHBu ] and [Cp2(Me)Ta=CH2] are representative of Schrock carbenes. 

Acetonitrile is a convenient solvent in which to study, by pulse radiolysis, the one-electron reduction of transition metal complexes that are not stable in water or hy-droxylic media. For example, the tantalum compound [Ta2Cl6(4-methylpyridine)4] has been shown to be reduced by CH3CN with A = 1.2 x 10 dm mol s [24], The absorption spectrum of the product shows the characteristic features of d-d transitions and it was suggested that the added electron is delocalized over the double-bonded Ta=Ta moiety. Another example is the radiolytic reduction of the vanadium(III) complex [VCl3(y-pic)3], where y-pic is 4-methylpyridine, to the derivative via the V complex [25]. It was shown by pulse radiolysis that the electron adduct of the complex decayed in a first-order process with k= 1.3 x 10 s which is thought to involve loss of Cl". The intermediate V complex had a... 

It has been shown that solutions of transition metal complexes of metals, such as chromium and iron, in organolithium or Grignard reagents will also react with molecular nitrogen [125, 126]. The only other non-bio-logical systems which are known to react with nitrogen under mild conditions are metals in the pure state such as lithium and many of the transition metals, e.g. tantalum. 

The inteimolecular hydroamination of allenes is readily catalyzed by early transition metal complexes to yield imines. An addition of aromatic and ahphatic amines to aUene requires high reaction temperatures (90-135°C) and long reaction times (1-6 days) when mediated by zirconocene- [41] and tantalum-imido [178] catalysts. The more efficient titanium half-sandwich imido-amide complex 42 operates under significantly milder reaction conditions (27) [179], Because the metal-imido species are prone to dimerization, sterically more hindered aliphatic and aromatic amines are more reactive. Simple, sterically unencumbered aliphatic amines add to aUenes in the presence of the bis(amidate) titanium complex 43 (28), although higher reaction temperatures are required [180]. 

One of the most important parameters that defines the structure and stability of inorganic crystals is their stoichiometry - the quantitative relationship between the anions and the cations [134]. Oxygen and fluorine ions, O2 and F, have very similar ionic radii of 1.36 and 1.33 A, respectively. The steric similarity enables isomorphic substitution of oxygen and fluorine ions in the anionic sub-lattice as well as the combination of complex fluoride, oxyfluoride and some oxide compounds in the same system. On the other hand, tantalum or niobium, which are the central atoms in the fluoride and oxyfluoride complexes, have identical ionic radii equal to 0.66 A. Several other cations of transition metals are also sterically similar or even identical to tantalum and niobium, which allows for certain isomorphic substitutions in the cation sublattice. 

Even the addition of 4 equiv. of dimethylzinc to a bis(dicarbollide)tantalum dichloride, Scheme 23, produced only the monomethyl complex 26 in an isolated yield of 75%.63 In these reactions, the oxidation state of the transition metal and the steric bulk of its ligands obviously play a role in the degree of alkylation. 

The cationic tantalum dihydride Cp2(CO)Ta(H)2]+ reacts at room temperature with acetone to generate the alcohol complex [Cp2(C0)Ta(H01Pr)]+, which was isolated and characterized [45]. The mechanism appears to involve protonation of the ketone by the dihydride, followed by hydride transfer from the neutral hydride. The OH of the coordinated alcohol in the cationic tantalum alcohol complex can be deprotonated to produce the tantalum alkoxide complex [Cp2(C0)Ta(01Pr)]. Attempts to make the reaction catalytic by carrying out the reaction under H2 at 60 °C were unsuccessful. The strong bond between oxygen and an early transition metal such as Ta appears to preclude catalytic reactivity in this example. 

The chemistry of alkylidene and alkylidyne complexes of early transition metals was developed by Schrock and co-workers and these complexes turned out to be of crucial importance to alkene and alkyne metathesis. Initially their research focused on tantalum complexes of the type CpTaCEIE, which after a-elimination (Figure 16.6) led to alkylidene complexes Cp(R)Cl2Ta=CHR [11]. 

COMPLEXES OF THE TRANSITION METALS 13.6.3.1 Titanium, Zirconium, Vanadium, Niobium, Tantalum... 

C-M bond addition, for C-C bond formation, 10, 403-491 iridium additions, 10, 456 nickel additions, 10, 463 niobium additions, 10, 427 osmium additions, 10, 445 palladium additions, 10, 468 rhodium additions, 10, 455 ruthenium additions, 10, 444 Sc and Y additions, 10, 405 tantalum additions, 10, 429 titanium additions, 10, 421 vanadium additions, 10, 426 zirconium additions, 10, 424 Carbon-oxygen bond formation via alkyne hydration, 10, 678 for aryl and alkenyl ethers, 10, 650 via cobalt-mediated propargylic etherification, 10, 665 Cu-mediated, with borons, 9, 219 cycloetherification, 10, 673 etherification, 10, 669, 10, 685 via hydro- and alkylative alkoxylation, 10, 683 via inter- andd intramolecular hydroalkoxylation, 10, 672 via metal vinylidenes, 10, 676 via SnI and S Z processes, 10, 684 via transition metal rc-arene complexes, 10, 685 via transition metal-mediated etherification, overview,... 

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