Tantalum complexes adducts

Interestingly, co-ordination of isocyanide ligand to the endo-hydride olefin tantalum complexes gives adducts containing organic groups [equation (6)]. ... 

Tantalum pentachloride forms adducts with donor solvent molecules. Also, it forms several complexes and organometaUic derivatives. 

Reactions of Tantalum(V) and Niobium(V) Iodides with Pyridine. Both niobium(V) and tantalum(V) iodide were reduced in pyridine. In each case the di-adduct of pyridine with the metal tetraiodide was produced, along with elemental iodine as its pyridine complex. The two reduction products were identified by analysis after washing with chloroform to remove the iodine liberated in the reaction. Further identification of the tantalum product was provided by x-ray diffraction data, which compared favorably to those obtained for samples of the tetraiododi(pyridine)niobium (IV) as shown in Table V. 

The latter deep-purple THF adducts of niobium and tantalum have been prepared independently by the photolysis of the corresponding [M(CO)6] " in tetrahydrofuran at — 70°C (54). They are even more reactive than the corresponding ammine complexes (vide infra) with respect to ligand substitution reactions. 

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. 

Even fewer complexes with nitrogen donor ligands have been reported and all are methyl cyanide adducts (Tables X and XI). Protactinium pentabromide forms a soluble 1 3 complex in contrast to the 1 1 complexes formed by niobium and tantalum pentahalides (46). Other actinide pentahalide-methyl cyanide complexes are still unknown. Protactinium tetrachloride, tetrabromide, and tetraiodide react with anhydrous, oxygen-free methyl cyanide to form slightly soluble 1 4 complexes (44, 48) which are isostructural with their actinide tetrahalide analogs. 

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... 

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