Editing Group 5 element (section)

== Chemical properties ==

Like other groups, the members of this family show patterns in its [[electron configuration]], especially the outermost shells. (The expected 4d<sup>3</sup> 5s<sup>2</sup> configuration for niobium is a very low-lying excited state at about 0.14&nbsp;eV.)<ref>[https://physics.nist.gov/PhysRefData/ASD/levels_form.html NIST Atomic Spectra Database]</ref>

{| class="wikitable" style="white-space:nowrap;"
|+
! colspan=4 | [[Electron configuration]]s of group 5 elements
|-
! {{abbr|1=[[Atomic number|''Z'']]|2=Atomic number}} !! [[Chemical element|Element]] !! Electrons per [[Electron shell|shell]] !! [[Electron configuration]]
|-
| style="text-align:right" | 23 || V, vanadium || {{mono|2, 8, 11, 2}}    || {{mono|1=[Ar] &nbsp;&nbsp;<sup>&nbsp;&nbsp;</sup> 3d<sup>3</sup> 4s<sup>2</sup>}}
|-
| style="text-align:right" | 41 || Nb, niobium   || {{mono|2, 8, 18, 12, 1}}|| {{mono|1=[Kr] &nbsp;&nbsp;<sup>&nbsp;&nbsp;</sup> 4d<sup>4</sup> 5s<sup>1</sup>}}
|-
| style="text-align:right" | 73 || Ta, tantalum || {{mono|2, 8, 18, 32, 11, 2}} ||  {{mono|1=[Xe] 4f<sup>14</sup> 5d<sup>3</sup> 6s<sup>2</sup>}}
|-
| style="text-align:right" | 105 || Db, dubnium || {{mono|2, 8, 18, 32, 32, 11, 2}} || {{mono|1=[Rn] 5f<sup>14</sup> 6d<sup>3</sup> 7s<sup>2</sup>}}
|}

Most of the chemistry has been observed only for the first three members of the group (the chemistry of dubnium is not very established, but what is known appears to match expectations for a heavier congener of tantalum). All the elements of the group are reactive metals with a high melting points (1910&nbsp;°C, 2477&nbsp;°C, 3017&nbsp;°C). The reactivity is not always obvious due to the rapid formation of a stable oxide layer, which prevents further reactions, similarly to trends in group 3 or group 4. The metals form different oxides: vanadium forms [[vanadium(II) oxide]], [[vanadium(III) oxide]], [[vanadium(IV) oxide]] and [[vanadium(V) oxide]], niobium forms [[niobium(II) oxide]], [[niobium(IV) oxide]] and [[niobium(V) oxide]], but out of tantalum oxides only [[tantalum(V) oxide]] is characterized. Metal(V) oxides are generally nonreactive and act like acids rather than bases, but the lower oxides are less stable. They, however, have some unusual properties for oxides, such as high electric conductivity.<ref name="HollemanAF" />

All three elements form various [[inorganic chemistry|inorganic compounds]], generally in the oxidation state of +5. Lower oxidation states are also known, but in all elements other than vanadium,<ref name="HollemanAF2">{{cite book |last=Holleman |first=Arnold F. |title=Lehrbuch der Anorganischen Chemie |author2=Wiberg, Egon |author3=Wiberg, Nils |date=1985 |publisher=Walter de Gruyter |isbn=978-3-11-007511-3 |edition=91–100 |pages=1071–1075 |language=de |chapter=Vanadium}}</ref> they are less stable, decreasing in stability with atomic mass increase.<ref name="Greenwood956" />

=== Compounds ===

==== Oxides ====
Vanadium forms oxides in the +2, +3, +4 and +5 [[oxidation state]]s, forming [[vanadium(II) oxide]] (VO), [[vanadium(III) oxide]] (V<sub>2</sub>O<sub>3</sub>), [[vanadium(IV) oxide]] (VO<sub>2</sub>) and [[vanadium(V) oxide]] (V<sub>2</sub>O<sub>5</sub>). Vanadium(V) oxide or vanadium pentoxide is the most common, being precursor to most alloys and compounds of vanadium, and is also a widely used industrial catalyst.<ref name=Ullmann>{{Cite book|doi = 10.1002/14356007.a27_367|chapter = Vanadium and Vanadium Compounds|title = Ullmann's Encyclopedia of Industrial Chemistry|year = 2000|last1 = Bauer|first1 = Günter|last2 = Güther|first2 = Volker|last3 = Hess|first3 = Hans|last4 = Otto|first4 = Andreas|last5 = Roidl|first5 = Oskar|last6 = Roller|first6 = Heinz|last7 = Sattelberger|first7 = Siegfried|isbn = 3527306730}}</ref>

Niobium forms oxides in the oxidation states +5 ([[Niobium pentoxide|{{chem2|Nb2O5}}]]),<ref>{{Cite web|url=https://pubchem.ncbi.nlm.nih.gov/compound/Niobium_oxide#section=Top|title=Niobium oxide {{!}} Nb2O5 – PubChem|last=Pubchem|website=pubchem.ncbi.nlm.nih.gov|access-date=29 June 2016|archive-date=16 August 2016|archive-url=https://web.archive.org/web/20160816070526/https://pubchem.ncbi.nlm.nih.gov/compound/Niobium_oxide#section=Top|url-status=live}}</ref> +4 ([[Niobium dioxide|{{chem2|NbO2}}]]), and the rarer oxidation state, +2 ([[niobium monoxide|NbO]]).<ref name=Greenwood&Earnshaw2nd /> Most common is the pentoxide, also being precursor to almost all niobium compounds and alloys.<ref name="HollemanAF" /><ref name="Cardarelli">{{cite book|first = Francois|last = Cardarelli|date = 2008|title = Materials Handbook |publisher = Springer London|isbn = 978-1-84628-668-1}}</ref>

[[Tantalum pentoxide]] (Ta<sub>2</sub>O<sub>5</sub>) is the most important compound from the perspective of applications. Oxides of tantalum in lower oxidation states are numerous, including many [[defect structure]]s, and are lightly studied or poorly characterized.<ref name="Greenwood&Earnshaw2nd">{{Greenwood&Earnshaw2nd}}</ref>
<!-- speculations about dubnium oxides -->

==== Oxyanions ====

[[File:decavanadate polyhedra.png|thumb|The [[decavanadate]] structure]]
<!-- [[File:Ammonium-metavanadate-chains-3D.png|thumb|upright|Metavanadate chains]] -->In aqueous solution, vanadium(V) forms an extensive family of [[oxyanion]]s as established by [[Vanadium-51 nuclear magnetic resonance|<sup>51</sup>V NMR spectroscopy]].<ref name="Rehder">{{cite book |doi=10.1016/S0066-4103(07)62002-X|title=Vanadium-51 NMR|series=Annual Reports on NMR Spectroscopy|year=2007|last1=Rehder|first1=D.|last2=Polenova|first2=T.|last3=Bühl|first3=M.|volume=62|pages=49–114|isbn=9780123739193}}</ref> The interrelationships in this family are described by the [[predominance diagram]], which shows at least 11 species, depending on pH and concentration.<ref>{{Greenwood&Earnshaw|page=984}}</ref> The tetrahedral orthovanadate ion, {{chem|VO|4|3−}}, is the principal species present at pH 12–14. Similar in size and charge to phosphorus(V), vanadium(V) also parallels its chemistry and crystallography. [[Sodium orthovanadate|Orthovanadate]] V{{chem|O|4|3−}} is used in [[protein crystallography]]<ref>{{cite journal|volume= 577|issue= 3|doi= 10.1016/j.febslet.2004.10.022|pmid= 15556602|date= 2004|title= The power of vanadate in crystallographic investigations of phosphoryl transfer enzymes|first1= Irmgard|last1= Sinning|journal= FEBS Letters|last2= Hol|first2= Wim G. J.|pages= 315–21|s2cid= 8328704|doi-access= free|bibcode= 2004FEBSL.577..315D}}</ref> to study the [[biochemistry]] of phosphate.<ref>{{cite journal|volume= 181|pmc= 1161148|date= 1979|title= Inhibition of human alkaline phosphatases by vanadate|first= Lorne E.|last= Seargeant|author2=Stinson, Robert A. |journal= Biochemical Journal|pmid=486156|issue=1|pages= 247–50|doi= 10.1042/bj1810247}}</ref> Beside that, this anion also has been shown to interact with activity of some specific enzymes.<ref>{{Cite journal |last1=Crans |first1=Debbie C. |last2=Simone |first2=Carmen M. |date=1991-07-09 |title=Nonreductive interaction of vanadate with an enzyme containing a thiol group in the active site: glycerol-3-phosphate dehydrogenase |url=https://pubs.acs.org/doi/abs/10.1021/bi00241a015 |journal=Biochemistry |language=en |volume=30 |issue=27 |pages=6734–6741 |doi=10.1021/bi00241a015 |pmid=2065057 |issn=0006-2960|url-access=subscription }}</ref><ref>{{Cite journal |last1=Karlish |first1=S. J. D. |last2=Beaugé |first2=L. A. |last3=Glynn |first3=I. M. |date=Nov 1979 |title=Vanadate inhibits (Na+ + K+)ATPase by blocking a conformational change of the unphosphorylated form |url=https://www.nature.com/articles/282333a0 |journal=Nature |language=en |volume=282 |issue=5736 |pages=333–335 |doi=10.1038/282333a0 |pmid=228199 |bibcode=1979Natur.282..333K |s2cid=4341480 |issn=1476-4687|url-access=subscription }}</ref> The tetrathiovanadate [VS<sub>4</sub>]<sup>3−</sup> is analogous to the orthovanadate ion.<ref>{{Greenwood&Earnshaw|page=988}}</ref>

At lower pH values, the monomer [HVO<sub>4</sub>]<sup>2−</sup> and dimer [V<sub>2</sub>O<sub>7</sub>]<sup>4−</sup> are formed, with the monomer predominant at vanadium concentration of less than c. 10<sup>−2</sup>M (pV > 2, where pV is equal to the minus value of the logarithm of the total vanadium concentration/M). The formation of the divanadate ion is analogous to the formation of the [[dichromate]] ion. As the pH is reduced, further protonation and condensation to [[vanadate|polyvanadates]] occur: at pH 4–6 [H<sub>2</sub>VO<sub>4</sub>]<sup>−</sup> is predominant at pV greater than ca. 4, while at higher concentrations trimers and tetramers are formed. Between pH 2–4 [[decavanadate]] predominates, though its formation from orthovanadate is optimized at pH 4–7, represented by this reaction:<ref name="one">{{cite book |author1=Johnson, G. |title=Inorganic Syntheses |author2=Murmann, R. K. |date=1979 |isbn=978-0-471-04542-7 |volume=19 |pages=140–145 |chapter=Sodium and Ammonium Decayanadates(V) |doi=10.1002/9780470132500.ch32}}</ref>
:{{Chem2|10 Na3[VO4] + 24 HOAc → Na6[V10O28] + 12 H2O + 24 NaOAc}}

In decavanadate, each V(V) center is surrounded by six oxide [[ligand]]s.<ref name="HollemanAF" /> Vanadic acid, H<sub>3</sub>VO<sub>4</sub> exists only at very low concentrations because protonation of the tetrahedral species [H<sub>2</sub>VO<sub>4</sub>]<sup>−</sup> results in the preferential formation of the octahedral [VO<sub>2</sub>(H<sub>2</sub>O)<sub>4</sub>]<sup>+</sup> species. In strongly acidic solutions, pH&nbsp;<&nbsp;2, [VO<sub>2</sub>(H<sub>2</sub>O)<sub>4</sub>]<sup>+</sup> is the predominant species, while the oxide V<sub>2</sub>O<sub>5</sub> precipitates from solution at high concentrations.<ref>{{Cite journal |last=Sadoc |first=Aymeric |last2=Messaoudi |first2=Sabri |last3=Furet |first3=Eric |last4=Gautier |first4=Régis |last5=Le Fur |first5=Eric |last6=le Pollès |first6=Laurent |last7=Pivan |first7=Jean-Yves |date=2007-06-01 |title=Structure and Stability of VO 2 + in Aqueous Solution: A Car−Parrinello and Static ab Initio Study |url=https://pubs.acs.org/doi/10.1021/ic0614519 |journal=Inorganic Chemistry |language=en |volume=46 |issue=12 |pages=4835–4843 |doi=10.1021/ic0614519 |issn=0020-1669|url-access=subscription }}</ref> The oxide is formally the [[acidic oxide|acid anhydride]] of vanadic acid. The structures of many [[vanadate]] compounds have been determined by X-ray crystallography.<ref>{{Cite journal |last=Davies |first=Douglas R. |last2=Hol |first2=Wim G.J. |date=2004-11-19 |title=The power of vanadate in crystallographic investigations of phosphoryl transfer enzymes |url=https://febs.onlinelibrary.wiley.com/doi/10.1016/j.febslet.2004.10.022 |journal=FEBS Letters |language=en |volume=577 |issue=3 |pages=315–321 |doi=10.1016/j.febslet.2004.10.022 |issn=0014-5793}}</ref>

[[File:VinwaterPourbaixdiagram2.svg|thumb|right|The [[Pourbaix diagram]] for vanadium in water, which shows the [[redox]] potentials between various vanadium species in different oxidation states.<ref>{{cite journal|journal= Electrochimica Acta|volume= 42|date= 1997|pages= 579–586|doi= 10.1016/S0013-4686(96)00202-2|title= Electrochemical behavior of vanadium in aqueous solutions of different pH|first= F. M.|last= Al-Kharafi|author2=Badawy, W. A. |issue= 4}}</ref>]]

Vanadium(V) forms various peroxo complexes, most notably in the active site of the vanadium-containing [[bromoperoxidase]] enzymes. The species VO(O)<sub>2</sub>(H<sub>2</sub>O)<sub>4</sub><sup>+</sup> is stable in acidic solutions. In alkaline solutions, species with 2, 3 and 4 peroxide groups are known; the last forms violet salts with the formula M<sub>3</sub>V(O<sub>2</sub>)<sub>4</sub> nH<sub>2</sub>O (M= Li, Na, etc.), in which the vanadium has an 8-coordinate dodecahedral structure.<ref>{{Greenwood&Earnshaw}}, p994.</ref><ref>{{cite book|date=1992|url=https://books.google.com/books?id=Lmt3x9CyfLgC&pg=PA128|page=128|title=Catalytic oxidations with hydrogen peroxide as oxidant|author=Strukul, Giorgio|publisher=Springer|isbn=978-0-7923-1771-5}}</ref>

Niobates are generated by dissolving the pentoxide in [[Base (chemistry)|basic]] [[hydroxide]] solutions or by melting it in alkali metal oxides. Examples are [[lithium niobate]] ({{chem2|LiNbO3}}) and lanthanum niobate ({{chem2|LaNbO4}}). In the lithium niobate is a trigonally distorted [[Perovskite (structure)|perovskite]]-like structure, whereas the lanthanum niobate contains lone {{chem|NbO|4|3-}} ions.<ref name="HollemanAF" />

Tantalates, compounds containing [TaO<sub>4</sub>]<sup>3−</sup> or [TaO<sub>3</sub>]<sup>−</sup> are numerous. [[Lithium tantalate]] (LiTaO<sub>3</sub>) adopts a perovskite structure. [[Lanthanum]] tantalate (LaTaO<sub>4</sub>) contains isolated {{chem|TaO|4|3−}} tetrahedra.<ref name="HollemanAF" />

==== 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" />