Pentahalides metal

However, most complexes of Nb and Ta are derived from the pentahalides. NbFs and TaFs dissolve in aqueous solutions of HF to give [MOFs] " and, if the concentration of HF is increased, [MFg]. This is normally the highest coordination number attained in solution though some [NbFy] - may form, and [TaFv] " definitely does form, in very high concentrations of HF. However, by suitably regulating the concentration of metal, fluoride ion and HF, octahedral... 

M(SCH2CH2S)3] with stereochemistry midway between octahedral and trigonal-prismatic, are known for both Nb and Ta. The pentahalides of these two metals act as Lewis acids and form complexes of the type MX5L with O, S, N, P. and As donor ligands. 

Protactinium metal was first prepared in 1934 by thermal decomposition of a pentahalide on a hot filament 50). It has since been prepared from PaF4 by metallothermic reduction (Section II,A) with barium 26, 27, 34,102), lithium 40), and calcium 73, 74). However, the highest purity metal is achieved using the iodide transport (van Arkel-De Boer) process (Section II,D). 

Caution. During the heatup period the reaction tube may explode in the furnace if the temperature is raised too fast, if the tube has not been filled under strictly anhydrous conditions, or if the metal pentahalide is partially hydrolyzed. 

The chemistry of niobium and tantalum ranges from oxidation state +V to —III, but with no species of oxidation state —II presently known (Table 1). The largest number of molecular compounds, by far, is found for oxidation state V. The very reactive pentahalides provide the most convenient entry to the molecular chemistry of these metals. 

They are usually prepared by reacting the ligand with the tetrahalide or by reduction of the pentahalides, either electrolytically or with metals like Na, Zn, Al, Mg, and Nb. Substitution of weakly coordinating THF in NbCLi(THF)2 is also a convenient route to NbCb adducts.42 Typical homoleptic complexes are the octahedral [MX ]2 ions (X = Cl, Br, and SCN), pentagonal bipyramidal [NbF7]3 , and dodecahedral [Nb(CN)8]4-. 

They are obtained by reduction of the pentahalides by the metal alone, or in the presence of sodium chloride. Their key structural unit, an [M6X12]"+ group, is shown in Fig. 18-B-ll. A typical reaction is... 

The compounds known are summarized in Table 10.1. The only compound of an early actinide in the -1-2 state is Thl2, a metallic conductor which is probably Th + (e )2 (D)2-Certain heavier actinides form MX2 (Am, Cf, Es), which usually have the structure of the corresponding EuX2 and are thus genuine M + compounds. All four trihalides exist for all the actinides as far as Es, except for thorium and protactinium. Tetrafluorides exist for Th-Cm and the other tetrahalides as far as NpX4 (and in the gas phase in the case of PuCE). Pentahalides are only known for Pa, U, and Np whilst there are a few MFe (M = U-Pu), uranium is the only actinide to form a hexachloride. The known actinide halides are generally stable compounds most are soluble in (and hydrolysed by) water. 

Pentahalides form adducts with nearly all types of neutral or anionic donors. " The high Lewis acidity of MF5 is illustrated by facile fluoride transfer and formation of anionic adducts. Species such as MF ", n = 1-4) and M2Fn have been detected in HF solutions of the metals using a selective fluoride ion electrode or by F NMR. These complexes... 

Halodialkylamido derivatives are accessible by several methods, especially reactions between pentahalides and amines or trimethylsilyl reactants. They are obtained as dimers or as monomeric adducts in the presence of an excess of amines. A common feature of these derivatives is the hexacoordina-tion of the metal. The MNC2 units are planar with short M-N distances ( 1.86A), and thus suggest strong r-bonding by interaction of the nitrogen p orbitals and empty A orbitals of the metal. 

Niobium and tantalum belong to the metals that are able to bind dinitrogen. The development of this area was first due to Schrock and coworkers, who in 1982 could obtain stable tantalum /X-N2 compounds in high yields by reducing Alkylidene complexes under dinitrogen at normal pressure.This route can be circumvented in a single pot synthesis by reacting the pentahalides with sUylated hydrazines (equation 12). 

Vanadium Pentafluoride, VFs. Vanadium pentafluoride is the only pentahalide so far reported. It is best prepared by direct reaction between metallic vanadium and fluorine in a flow system at 300 °C under moisture free conditions. It is an extremely reactive white solid at room temperature and an extremely powerfiil oxidizing and fluorinating agent. 

M(CH2SiMe3)2(/i-CSiMe3)]2 Simple MPh, derivatives have not been isolated but several anions [MPh ]"" (n = l-4) are the reported products of reacting metal pentahalides with LiPh... 

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