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Some properties of vanadium and niobium pentafluorides Cavell, Ronald George

Abstract

The reactions of various amines with vanadium and niobium pentafluoride have been studied. Pyridine and ammonia reacted with vanadium pentafluoride reducing vanadium from the pentavalent to the tetravalent state and forming monosolvated complexes of vanadium tetrafluoride. The products isolated were pyridinetetrafluorovanadium IV, (Py) VF₄, and amminotetrafluorovanadium IV, (NH₃) VF₄. Ethylenediamine also reduced vanadium pentafluoride to the tetravalent state but formed a more highly solvated denvative of vanadium tetrafluoride. The product was identified as tris (ethylenediamine) tetrafluorovanadium IV and has been formulated as the octahedral complex [V(en)₃] F₄. Niobium pentafluoride reacted with ammonia to form a 2:1 adduct of niobium pentafluoride, diammonopentafluoroniobium V, (NH₃) ₂NbF₅. The product formed on reaction of ethylenediamine with niobium pentafluoride had the composition NbF₅ (en)₁.₆. It has been considered as a highly solvated 1:1 adduct ethylenediaminepentafluoroniobium V. No reduction of niobium has been observed in amine reactions. The infrared spectra and magnetic suceptibilities of the complexes have been measured and the dissociation pressure of diamminopentafluoroniobium V has been determined. The solubilities and melting points of the complexes were also investigated. The pentafluorides of vanadium and niobium, in contrast to the highest chlorides of the same metals, are not solvolysed by the amines studied here. This common behaviour of fluorides, coupled with an apparent maximum coordination number of six for vanadium has been used to explain the reduction of vanadium pentafluoride by the amines. The reduction of vanadium occurs because the association of vanadium pentafluoride in the liquid state has saturated the coordination number of vanadium, so that any solvated product formed by the amines has an unstable coordination number which must be lowered. The fact that solvolysis, which provides a method of reducing the coordination of the central atom without reducing its valence state, cannot occur with fluorides, requires that the reduction in coordination number of vanadium must proceed through an alternate mechanism with concomitant reduction of the valence of vanadium. This interpretation is consistent with the behaviour of niobium pentafluoride which forms solvated derivatives in which niobium has increased its coordination number to seven or eight, the probable maximum coordination number of niobium. Structures have been suggested for the products of the amine reactions. The properties of the pyridine and ammonia derivatives of vanadium pentafluoride, (Py)VF₄ and (NH₃) VF₄, plus the improbable coordination of five for vanadium in the monomer suggests that the compounds are polymers. The most probable structure is a long chain of VF₄. Base units linked by fluorine bridge bonds such as have been proposed to explain association of vanadium pentafluoride. The ethylenediamine derivative of vanadium pentafluoride, [V(en)₃] F₄, is probably a monomeric compound, for there is no need to postulate association to increase the coordination number of vanadium to six. The many complex ions formed by ethylenediamine suggest that it is reasonable to postulate the formation of the [V(en)₃]⁺⁴ ion. This is the first hexacoordinate cationic complex of tetravalent vanadium, with the exception of vanadyl complexes, to be reported. The niobium pentafluoride complexes (NH₃) ₂NbF₅, (en)NbF₅ may be septacoordinate monomers, isostructural with iodine heptafluoride, or octacoordinate fluorine bridged polymers. The proposed structures are based only on the few qualitative properties that are available for the compounds. Complete structure determinations are required for verification of the structures proposed. A nuclear magnetic resonance study of vanadium pentafluoride has yielded only the F¹⁹ chemical shift of vanadium pentafluoride of -335 ppm relative to trifluoroacetic acid. The broad nuclear resonance absorption obtained has been explained in terms of rapid fluorine exchange occuring in conjunction with quadrupole broadening due to the electric quadrupole moment of vanadium.

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