Tantalum clusters

Reactions between tantalum cluster ions Espenson and McCarley have estimated the rate coefficient for the reaction Ta6Clt +Ta6Cl 2 = 2 TagCl ... 

Nepijko SA, Klimenkov M, Kuhlenbeck H, Zemlyanov D, Herein D, Schldgl R, Ereund H-J. 1998. TEM study of tantalum clusters on Al2O3/NiAl(110). Surf Sci 412/413 192-201. 

Ni and Co or of oxophilic metals, for example. Re, is still poorly studied the surface-mediated synthesis of bimetallic carbonyl clusters is limited to a few examples the surface-mediated synthesis of metal compounds without carbonyl ligands has just begun with the silica-mediated synthesis of [RhH2(PMe3)4] by treatment of bis (allyl) rhodium with PMe3 followed by H2 [121] the silica-mediated synthesis of tantalum clusters has been investigated recently but the products were not extracted from the surface-for example, treatment of silica physisorbed Ta(CH2Ph)5 in H2 at 523 K for 20 h led to tri-tantalum clusters, as shown by EXAFS spectroscopy [122]. 

Table 43 Various Niobium and Tantalum Clusters with Non-integral Oxidation States...

Table 45 Niobium and Tantalum Cluster Adducts Containing the [M6X12]2+ Core...

The fourth step is the preparation of isomorphous crystals of heavy metal-containing derivatives. The heavy metal may be allowed to react with the protein before crystallization or may be diffused into preformed crystals. A variety of both cationic and anionic metal complexes, even large Ta6Br122+ tantalum clusters, have been used 405 Two or more different heavy metal derivatives are often required for calculation of the phases. The heavy metal atoms must be present at only a very small number of locations in the unit cell. 

This mechanism can be found in the electrochemistry of compounds with two or more oxidation states. Oxidation of the tantalum cluster compound Ta6Br122+ is a typical case [5] ... 

Inner-Transition Metal to Transition and Inner-Transition Metal Bond 65 9.2.8. Formation of Heavy Transition Metal Group V Metal-Metal Bonds 9.2.8.1. Synthesis of Hexanuclear Niobium and Tantalum Clusters... 

Although the behavior of niobium and tantalum clusters in ethanolic solutions is expected to be similar to that in methanol, experiments indicate significantly faster reaction with methanol. 

Another approach using supported, early transition metals involves the use of tantalum clusters on oxide surfaces. Gates et al. reported the formation of trinuclear tantalum by exposing partially dehydroxylated silica treated with... 

Ta(CH2Ph)5 to ethane or hydrogen [48]. EXAFS spectroscopy confirmed the existence of a Ta-Ta species, demonstrating the formation of tantalum clusters on silica. This supported that Ta sample catalyzed the metathesis of ethane at high temperature (250 "C) [49]. The initial conversion of ethane metathesis was 24% the molar ratio of methane to propane exceeded 1, and small quantities of butanes were also observed. This catalytic system was also found to convert propane into ethane and butanes [48, 50, 51]. The authors found that the observed catalytic activity was dependent on average cluster size. Increasing the average cluster diameter by 0.2-0.3 nm led to a decrease in activity [51]. 

The alkali alkoxide method has been extended to the preparation of alkoxides of the hexanuclear niobium and tantalum cluster units, e.g., [M6Xi2](OMe)2.4MeOH (where M = Nb or Ta and X = Cl or Br), and M2[Ta2Cli2](OMe)6.6MeOH. 

Mild oxidant agents often permit oxidations with retention of the cluster structures. One example of this kind of simple chemical electron-transfer reactions are the oxidations of niobium and tantalum clusters [M6Xi2] to the species with charges 3+ and 4+. The reaction with iron(III) chloride and cerium (IV) salts leads directly to the cations [M6Xi2] . ... 

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