Niobium iodide

Cesium niobium iodide complexes, 46 34 Cesium pseudohalide complexes, 46 42 Cesium salt (CS2MCy, lattice energy, 22 22 Cesium tetraiodide, structure of, 3 152 Cesium trifluoroacetates, 17 8, 31, 32 CF, 33 110... 

Redox Chemistry of the Group 5 Clusters Ligand Substitution Electronic and Molecular Structure Niobium Iodide Clusters NbJJ"-Materials Chemistry Derived from Soluble Metal Halide Clusters A. Higher Nuclearity Clusters Supported Cluster Materials Charge-Transfer Salt Complexes Extended Solids Chemically Modified Surfaces... 

For Nb or Ta clusters MgLig units are formed when the ligands are Cl or Br for niobium, and Cl, Br or I for the tantalum. In contrast, the niobium iodides have the MeLi4 unit, owing to the important steric effect of the iodine, as discussed later. Such MeLis units can also be prepared from valence-electron-poor transition metals such as zirconium, but in this case an interstitial element located at the center of the octahedral cluster is necessary to stabilize the cluster. ... 

MgLi4 units also appear in very specific examples of niobium iodides, instead of MeLig which is usual for this transition element. In these Nbg iodides, for example Nbfilii or Nb6l9S, the units are not, however, discrete and they will not be discussed in detail. 

All elements that form isochains can also form heterochains with other elements. In addition, simple heterochains can sometimes be formed by the next higher row of the periods III, IVB, and VB, namely aluminum, iron, and bismuth. Compounds with multicenter bonds are also formed by beryllium and a few elements from higher rows, e.g., niobium and vanadium. In these compounds, as well as the OH and H groups, the elements of period VIIB can also occur as the central atoms in multicenter bridge bonding, that is, in borohydrides, niobium iodide, etc. These elements, however, do not represent true chain atoms. 

Niobium Halides and Oxyhalides. AH possible haUdes of pentavalent niobium are known and preparations of lower valent haUdes generally start with the pentahaUde. Ease of reduction decreases from iodide to fluoride. 

The known halides of vanadium, niobium and tantalum, are listed in Table 22.6. These are illustrative of the trends within this group which have already been alluded to. Vanadium(V) is only represented at present by the fluoride, and even vanadium(IV) does not form the iodide, though all the halides of vanadium(III) and vanadium(II) are known. Niobium and tantalum, on the other hand, form all the halides in the high oxidation state, and are in fact unique (apart only from protactinium) in forming pentaiodides. However in the -t-4 state, tantalum fails to form a fluoride and neither metal produces a trifluoride. In still lower oxidation states, niobium and tantalum give a number of (frequently nonstoichiometric) cluster compounds which can be considered to involve fragments of the metal lattice. 

The niobium(V) and tantalum (V) iodides were prepared by the method reported by Corbett and Seabaugh for niobium(V) iodide (3). 

Excess pyridine was distilled from the flask, and the resulting product was dried under high vacuum at room temperature to a constant weight. The amount of pyridine utilized in the reaction could then be determined by weight difference before and after reaction. Complete analytical results for the crude products obtained with the chlorides and oromides are given in Table II. Because the tantalum(V) and niobium(V) iodides both yielded free iodine in the reaction with pyridine, their products were washed with chloroform, as described later, before analysis. 

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. 

This mode of reaction for the iodides is consistent with the lower stability of the pentaiodides. For example, it has been shown that niobium (IV) iodide can be prepared by dissociation at temperatures as low as 270° (Equation 8). 

Apparently the dissociation is enhanced in pyridine, owing to formation of the stable complexes of both niobium (IV) iodide and elemental iodine. Although some reduction of tantalum (V) iodide also occurred in pyridine, the analytical data suggest that this reaction was not complete, and that some unreduced tantalum (V) iodide remained in the washed product. By comparison with niobium (V) iodide, the smaller extent of reduction of tantalum (V) iodide may be accounted for in terms of the greater stability of the latter toward the dissociation shown in Equation 8. 

Pentafluorocthyl iodide is of practical interest, particularly as a precursor of higher perfluoroal-kyl iodides. There are several patents for the preparation of the key compound from tetra-fluoroethene, iodine pentafluoride and iodine at 75-80 C in the presence of catalysts anti-mony(III) fluoride, titanium(lV) chloride, boron trifluoride, vanadium(V) fluoride, niobium(V) fluoride, and molybdenum(Vl) fluoride.11-13 The agents iodine monofluoride" and bromine monofluoride" can add to branched pcrfluoroalkcnes, e.g. perfluoro-2-methylbut-2-ene gives perfluoro-2-iodo-2-methylbutane.1415... 

The pentahalides of niobium and tantalum are predominantly covalent. Their volatility decreases from the fluorides to the iodides. Nb and Ta belong to the few elements which form stable pentaiodides. Selected physical and thermodynamic properties for the halides are listed in Table 2. All are sensitive to moist air, water or hydroxylic solvents. 

Definite compounds of niobium and iodine are unknown, although tantalum pentiodide and vanadium tri-iodide have been prepared. 

Definite compounds of niobium and iodine are unknown. The preparation of an iodide from the pentabromide has been reported2 but no details are supplied. A pyridine addition compound of the pentiodide, NbIB.(C5H5N.HI)6, has been described,2 but its existence lacks confirmation.4... 

D.3 (a) potassium phosphate (b) iron(II) iodide, ferrous iodide (c) niobium(V) oxide (d) copper(II) sulfate, cupric sulfate... 

As in the case of Na3[V(CO)5], liquid ammonia solutions of the trisodium salts of [Nb(CO)5]3 and [Ta(CO)5]3 were also quite air and moisture sensitive but showed no tendency to decompose violently at or below - 33°C. A summary of our results on the syntheses of the tricesium salts are shown in Eq. (19). Unlike Cs3[V(CO)5], which is nearly insoluble in liquid ammonia, the niobium and tantalum analogs are appreciably soluble in this medium even at — 78°C. Because the solid tricesium salts must be washed with liquid ammonia to remove all but traces of sodium iodide, the isolated pure... 

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