Tantalum complexes bonding

Closely related mixed amido/imido/guanidinato tantalum complexes of the type Ta(NR R )[(R R2N)C(NR )2]( = NR ) (R R = Me, Et R = Cy, Pr R = Pr", BuO were synthesized by the insertion of carbodiimides into to tantalum-amide bonds in imidotantalum triamide precursors, and the effects of ligand substitution on thermal properties were studied by TGA/DTA measurements. In addition, selected compounds were pyrolyzed at 600 °C and the decomposition products were studied by GC-MS and NMR spectroscopy. ... 

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). 

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... 

Alkilidine tantalum complexes have also been utilized for the preparation of ni-trido Tav. Reaction of ammonia with 31 or 32 afforded nitride 33 in moderate yields [14]. The structure of 33 has been determined by single-crystal X-ray crystallography and shown to consist of a pentameric complex, as depicted in Eq. (9). All of the Ta-N bond lengths are equivalent (2.00 A), suggesting a structure with extensive delocalization of the multiple bonds. 

Alkylidene complexes are of two types. The ones in which the metal is in a low oxidation state, like the chromium complex shown in Fig. 2.4, are often referred to as Fischer carbenes. The other type of alkylidene complexes has the metal ion in a high oxidation state. The tantalum complex is one such example. For both the types of alkylidene complexes direct experimental evidence of the presence of double bonds between the metal and the carbon atom comes from X-ray measurements. Alkylidene complexes are also formed by a-hydride elimination. An interaction between the metal and the a-hydrogen atom of the alkyl group that only weakens the C-H bond but does not break it completely is called an agostic interaction (see Fig. 2.5). An important reaction of alkylidene complexes with alkenes is the formation of a metallocycle. 

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. 

AUcyl ligands in niobium and tantalum complexes are susceptible to attack by electrophiles (see Electrophilic Reaction). Hydrogenation (see Hydrogenation) of niobium or tantalum M R bonds to provide the metal hydrides is an important reaction of synthetic utility. Insertion reactions of unsaturated reagents into Nb- or Ta bonds are common. The unsaturated reagents include aUcenes, aUcynes, CO, NO, RN=C=NR, CNR, and others. 

As regards the SiC/Ta interface, the peak shifts (Si, C) reflect the changes in chemical environment of these elements and the existence of complex bonds in the Ta-Si-C-0 and Ta-C systems. Note that Auger measurements performed on the denuded parts of the substrate obtained during scratch tests, and on these same parts obtained during bending tests, give evidence of a failure surface made of tantalum. 

Tantalum complexes 38 were prepared through the reaction in Equation (22). The authors postulate coordination of one isocyanide to the boron with concomitant loss of methane, azaallyl formation, and finally coordination of the second isocyanide to boron followed by its insertion into the B-C bond <2002OM2422>. 

Carbon monoxide insertion into the tantalum-methyl bonds of a tetramethyl derivative of tantalum(V), Taf j -CsMesjMe, reportedly" gives the following >j -acetone complex ... 

Treatment of tantalum hydride complexes with hexamethylsilacyclopropane at 65 °C in the presence of a Lewis base gave the corresponding silyl-tantalum compounds in excellent yield. It appears that this reaction takes place via direct insertion of dimethylsilylene into the tantalum-hydride bond, which would make it the first example of silylene trapping by a transition-metal substrate (Equation (15)) <87JA6210>. 

Heterometallacyclopropenes are rare but tantalum complex (15) is an example of a metallaphos-phoniumcyclopropene. The heterocycle is best described as having a Ta—C double bond but the C—P bond (172 pm) also appears to possess some double-bond character. Metallathiacyclopropenes are normally described as metal-coordinated thioketones and their structures will not be covered in this chapter. The few examples of thioniummetallacyclopropenes are, however, described as heterocycles. A diffraction study of cationic thioniumtungstacyclopropene (36) reveals a short W—C... 

Reduction of Ta(silox)3Cl2 with Na/Hg leads to a three-coordinate alkoxide complex Ta(silox)3. The coordinatively unsaturated tantalum complex is capable of cleaving H2 and O2 bonds resulting in the hydride and 0x0 complexes as illustrated in Scheme 7.14. Carbon monoxide is also split upon carbonylation of Ta(silox)3 generating the 0x0 and p-dicarbide complexes. This reaction models the C—O bond cleavage and C—C bond formation believed to occur in the Fischer-Tropsch reaction, and the ketenylidene complex Ta(silox)3(=C=C=0) was postulated as the key intermediate. On the other hand, when Ta(silox)3 was treated with pyridine and benzene, remarkable T -coordinated complexes were formed. 

Another type of carbene complex is characterized by a high oxidation state of the central metal and an a-carbon atom that does not usually bear a hetero atom. It is called an alkylidene complex or Schrock-type complex, since R. R. Schrock first synthesized a tantalum complex of this type [14]. Formation of the tantalum carbon double bond is based on the a-elimination reaction of a neopentyl ligand as shown in eq.(2). 

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