Niobium tetravalent

No available data was found on the precipitation from fluoride solutions of niobium and tantalum fluoride compounds containing tri- and tetravalent metals. 

Tetravalent niobium is believed to occur in die form of NbOCl " ions in a solution obtained, with color change, by reduction of HC1 solution of NbCls, and by inference in similarly reduced solutions of the other pentahalides, Tetravalent niobium also is found in the dioxide (see above) and the carbide. NbC. 

The salts of tetravalent titanium, zirconium, and thorium and of pentavalent niobium and tantalum are diamagnetic, as would be expected, and have colors ranging from white to orange, depending upon the halogen. 

The two most important sources of uranium are the minerals carnotite, where uranium occurs in the hexavalent oxide or hydrated oxide, and pitchblende, where uranium occurs mostly in the tetravalent state as a compound salt with other metals. It also occurs as a mixed oxide with titanium, thorium, and niobium in the tetravalent form. The tetravalent uranium minerals appear to have been geologically formed in the presence of reducing agents such as hydrocarbon minerals, graphite, native metals, and sulfide minerals, while such association is rarely observed with the hexavalent uranium minerals. 

Properties Dense, silvery solid. D 19.0, mp 1132C, bp3818C, heat of fusion 4.7 kcal/mole, heat capacity 6.6 cal/mole/C. Strongly electropositive, ductile and malleable, poor conductor of electricity. Forms solid solutions (for nuclear reactors) with molybdenum, niobium, titanium, and zirconium. The metal reacts with nearly all nonmetals. It is attacked by water, acids, and peroxides, but is inert toward alkalies. Green tetravalent uranium and yellow uranyl ion (U()2") are the only species that are stable in solution. 

Minerals of the third group of Table 5.15 contain relatively small proportions of tetravalent uranium combined with a refractory oxide of titanium, niobium, or tantalum. To free the uranium from these minerals, they must be leached with hot, concentrated sulfuric acid. Davidite is one of the principal ores at Radium Hill in South Australia. Brannerite is found in the Blind River district of Ontario. Pyrochlore occurs in the Lake Nipissing district of Ontario and in Nigeria. 

From the above discussion it follows that tetravalent and hexavalent thorium, uranium, and plutonium can be separated from the trivalent rare-earth fission products by taking advantage of differences in complexing properties. More highly charged cation fission products, such as tetravalent cerium and the fifth-period transition elements zirconium, niobium, molybdenum, technetium, and ruthenium, complex more easily than the trivalent rare-earths and are more difficult to separate from uranium and plutonium by processes involving complex formation. 

The synthesis of M(OEt)4 shows that, contrary to previous observations, tetravalent alkoxides are stable in the presence of alcohol, at least for a short time. The alcoholysis of [Nb(NR2)4] usually led to [Nb(OR )s], but oxidation state +IV was retained with bulky alcohols, e.g. for tetra(l-adamanto)niobium and tetra(l adamantylmethoxo)niobium, illustrating the close dependence of the metal s oxidation state on steric requirements. [Nb(l-ado)4] was reported to be monomeric, but the absence of ESR signals down to — 150°C, as well as its reluctance to form complexes (MczNH, PMcs, THT) is puzzling. Dinuclear and tetranuclear diamagnetic niobiiun chlorocatecholates have also been obtained. The Ta alkoxides are limited to some diamagnetic chloro alkoxide adducts. 

Replacement of Si with other tetravalent species (Ti in particular) has been attempted, which does not bring about Bronsted acidity per se, but promotes catalytic behavior [ 11 ]. The same holds for pentavalent atoms hke vanadium or niobium [11,12]. 

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