Editing Group 5 element (section)

==== Halides and their derivatives ====

Twelve binary [[halides]], compounds with the formula VX<sub>n</sub> (n=2...5), are known. VI<sub>4</sub>, VCl<sub>5</sub>, VBr<sub>5</sub>, and VI<sub>5</sub> do not exist or are extremely unstable; the only known pure V{{Sup|5+}} halide compound is {{Chem2|VF5|link=vanadium pentafluoride}}.<ref>{{Citation |title=Vanadium series products and functional materials |date=2021 |work=Vanadium |pages=395–413 |url=https://linkinghub.elsevier.com/retrieve/pii/B9780128188989000140 |access-date=2024-11-11 |publisher=Elsevier |language=en |doi=10.1016/b978-0-12-818898-9.00014-0 |isbn=978-0-12-818898-9|url-access=subscription }}</ref> In combination with other reagents, [[vanadium(IV) chloride|VCl<sub>4</sub>]] is used as a catalyst for polymerization of [[diene]]s. Like all binary halides, those of vanadium are [[Lewis acid]]ic, especially those of V(IV) and V(V). Many of the halides form octahedral complexes with the formula VX<sub>''n''</sub>L<sub>6−''n''</sub> (X= halide; L= other ligand).<ref>{{Cite journal |last=VonDreele |first=Robert B. |last2=Fay |first2=Robert C. |date=November 1972 |title=Octahedral vanadium(IV) complexes. Synthesis and stereochemistry of vanadium(IV) .beta.-diketonates |url=https://pubs.acs.org/doi/abs/10.1021/ja00777a052 |journal=Journal of the American Chemical Society |language=en |volume=94 |issue=22 |pages=7935–7936 |doi=10.1021/ja00777a052 |issn=0002-7863|url-access=subscription }}</ref><ref>{{Cite journal |last=Halepoto |first=Dost M |last2=Larkworthy |first2=Leslie F |last3=Povey |first3=David C |last4=Smith |first4=Gallienus W |last5=Ramdas |first5=Vijayalaksmi |date=June 1995 |title=Some complex halides of vanadium(II) and vanadium(III). The crystal and molecular structure of tetrakis(methylammonium) hexachlorovanadate(III) chloride |url=https://linkinghub.elsevier.com/retrieve/pii/0277538794004139 |journal=Polyhedron |language=en |volume=14 |issue=11 |pages=1453–1460 |doi=10.1016/0277-5387(94)00413-9|url-access=subscription }}</ref>

Many vanadium [[oxyhalide]]s (formula VO<sub>m</sub>X<sub>n</sub>) are known.<ref>{{Greenwood&Earnshaw|page=993}}</ref> The oxytrichloride and oxytrifluoride ([[vanadium oxytrichloride|VOCl<sub>3</sub>]] and [[Vanadium(V) oxytrifluoride|VOF<sub>3</sub>]]) are the most widely studied. Akin to POCl<sub>3</sub>, they are volatile, adopt tetrahedral structures in the gas phase, and are Lewis acidic.<ref name=":0">{{Cite book |url=https://www.degruyter.com/document/doi/10.1515/9783110495904/html |title=Nebengruppenelemente, Lanthanoide, Actinoide, Transactinoide |date=2016-12-19 |publisher=De Gruyter |isbn=978-3-11-049590-4 |editor-last=Holleman |editor-first=Arnold F. |pages=1819-1825 |language=de |chapter=Kapitel XXVI. Die Vanadiumgruppe |doi=10.1515/9783110495904}}</ref>

[[File:Niobium pentachloride solid.jpg|thumb|right|upright=0.8|A very pure sample of niobium pentachloride|alt=Watch glass on a black surface with a small portion of yellow crystals]]
[[File:Niobium-pentachloride-from-xtal-3D-balls.png|thumb|right|upright=0.8|Ball-and-stick model of [[niobium pentachloride]], which exists as a [[Dimer (chemistry)|dimer]]]]

Niobium forms halides in the oxidation states of +5 and +4 as well as diverse [[nonstoichiometric compound|substoichiometric compounds]].<ref name="HollemanAF" /><ref name="Aguly">{{cite book|first = Anatoly|last = Agulyansky|title = The Chemistry of Tantalum and Niobium Fluoride Compounds|pages = 1–11|publisher = Elsevier|date=2004| isbn = 978-0-444-51604-6}}</ref> The pentahalides ({{chem|NbX|5}}) feature octahedral Nb centres. Niobium pentafluoride ({{chem2|NbF5}}) is a white solid with a melting point of 79.0&nbsp;°C and [[niobium pentachloride]] ({{chem2|NbCl5}}) is yellow (see image at left) with a melting point of 203.4&nbsp;°C. Both are [[hydrolyzed]] to give oxides and oxyhalides, such as {{chem2|NbOCl3}}. The pentachloride is a versatile reagent used to generate the [[organometallic]] compounds, such as [[niobocene dichloride]] ({{chem|(C|5|H|5|)|2|NbCl|2}}).<ref>{{cite book|author = Lucas, C. R. |author2 = Labinger, J. A. |author3 = Schwartz, J. |title = Inorganic Syntheses |chapter = Dichlorobis(η <sup>5</sup> -Cyclopentadienyl) Niobium(IV) |editor1-link=Robert Angelici|editor-first = Robert J.|editor-last = Angelici|date = 1990|volume = 28|pages = 267–270|isbn = 978-0-471-52619-3|doi = 10.1002/9780470132593.ch68|location = New York}}</ref> The tetrahalides ({{chem|NbX|4}}) are dark-coloured polymers with Nb-Nb bonds; for example, the black [[hygroscopic]] niobium tetrafluoride ({{chem2|NbF4}})<ref>{{Cite journal |last=Gortsema |first=F. P. |last2=Didchenko |first2=R. |date=February 1965 |title=The Preparation and Properties of Niobium Tetrafluoride and Oxyfluorides |url=https://pubs.acs.org/doi/abs/10.1021/ic50024a012 |journal=Inorganic Chemistry |language=en |volume=4 |issue=2 |pages=182–186 |doi=10.1021/ic50024a012 |issn=0020-1669|url-access=subscription }}</ref> and dark violet niobium tetrachloride ({{chem2|NbCl4}}).<ref name="Macintyre">Macintyre, J.E.; Daniel, F.M.; Chapman and Hall; Stirling, V.M.  Dictionary of Inorganic Compounds.  1992, Cleveland, OH: CRC Press, p. 2957</ref>

Anionic halide compounds of niobium are well known, owing in part to the [[Lewis acid]]ity of the pentahalides. The most important is [NbF<sub>7</sub>]<sup>2−</sup>, an intermediate in the separation of Nb and Ta from the ores.<ref name="ICE">{{cite journal|title = Staff-Industry Collaborative Report: Tantalum and Niobium|author=Soisson, Donald J.|author2=McLafferty, J. J.|author3=Pierret, James A.| journal = Industrial and Engineering Chemistry|date = 1961|volume = 53|issue = 11|pages = 861–868|doi = 10.1021/ie50623a016}}</ref> This heptafluoride tends to form the oxopentafluoride more readily than does the tantalum compound. Other halide complexes include octahedral [{{chem2|NbCl6}}]{{sup|−}}:
:{{chem2|Nb2Cl10}} + 2 Cl{{sup|−}} → 2 [{{chem2|NbCl6}}]{{sup|−}}

As with other metals with low atomic numbers, a variety of reduced halide cluster ions is known, the prime example being [{{chem2|Nb6Cl18}}]{{sup|4−}}.<ref name="Greenwood&Earnshaw2nd"/>

Tantalum halides span the oxidation states of +5, +4, and +3. [[Tantalum pentafluoride]] (TaF<sub>5</sub>) is a white solid with a melting point of 97.0&nbsp;°C. The anion [TaF<sub>7</sub>]<sup>2-</sup> is used for its separation from niobium.<ref name="ICE" /> The chloride [[tantalum(V) chloride|{{chem|TaCl|5}}]], which exists as a dimer, is the main reagent in synthesis of new Ta compounds. It hydrolyzes readily to an [[oxychloride]]. The lower halides {{chem|TaX|4}} and {{chem|TaX|3}}, feature Ta-Ta bonds.<ref name="HollemanAF" /><ref name="Aguly" />