Editing Niobium (section)

==Compounds==
{{see also|Category:Niobium compounds}}
In many ways, niobium is similar to [[tantalum]] and [[zirconium]]. It reacts with most nonmetals at high temperatures; with [[fluorine]] at room temperature; with [[chlorine]] at 150&nbsp;°C and [[hydrogen]] at 200&nbsp;°[[Celsius|C]]; and with [[nitrogen]] at 400&nbsp;°C, with products that are frequently interstitial and nonstoichiometric.<ref name="Nowak" /> The metal begins to [[oxidation|oxidize]] in air at 200&nbsp;°[[Celsius|C]].<ref name="HollemanAF">{{cite book|publisher = Walter de Gruyter|date = 1985|edition = 91–100|pages = 1075–1079|isbn = 978-3-11-007511-3|title = Lehrbuch der Anorganischen Chemie|author=Holleman, Arnold F.|author2=Wiberg, Egon|author3=Wiberg, Nils|chapter = Niob| language = de}}</ref> It resists corrosion by acids, including [[aqua regia]], [[hydrochloric acid|hydrochloric]], [[sulfuric acid|sulfuric]], [[nitric acid|nitric]] and [[phosphoric acid]]s.<ref name="Nowak" /> Niobium is attacked by hot concentrated sulfuric acid, [[hydrofluoric acid]] and hydrofluoric/nitric acid mixtures. It is also attacked by hot, saturated alkali metal hydroxide solutions.

Although niobium exhibits all of the formal oxidation states from +5 to −1, the most common compounds have niobium in the +5 state.<ref name="Nowak" /> Characteristically, compounds in oxidation states less than 5+ display Nb–Nb bonding. In aqueous solutions, niobium only exhibits the +5 oxidation state. It is also readily prone to hydrolysis and is barely soluble in dilute solutions of [[hydrochloric acid|hydrochloric]], [[sulfuric acid|sulfuric]], [[nitric acid|nitric]] and [[phosphoric acid]]s due to the precipitation of hydrous Nb oxide.<ref name="Aguly" /> Nb(V) is also slightly soluble in alkaline media due to the formation of soluble polyoxoniobate species.<ref>{{Cite journal|last1=Deblonde|first1=Gauthier J. -P.|last2=Chagnes|first2=Alexandre|last3=Bélair|first3=Sarah|last4=Cote|first4=Gérard|date=1 July 2015|title=Solubility of niobium(V) and tantalum(V) under mild alkaline conditions|journal=Hydrometallurgy|volume=156|pages=99–106|doi=10.1016/j.hydromet.2015.05.015|bibcode=2015HydMe.156...99D |issn=0304-386X}}</ref><ref>{{Cite journal|last=Nyman|first=May|date=2 August 2011|title=Polyoxoniobate chemistry in the 21st century|journal=Dalton Transactions|language=en|volume=40|issue=32|pages=8049–8058|doi=10.1039/C1DT10435G|pmid=21670824|issn=1477-9234}}</ref>

===Oxides, niobates and sulfides===
Niobium forms [[oxide]]s in the [[oxidation state]]s +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>{{Greenwood&Earnshaw}}</ref> Most common is the pentoxide, 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> 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" /> The layered niobium sulfide ({{chem2|NbS2}}) is also known.<ref name="Nowak" />

Materials can be coated with a thin film of niobium(V) oxide [[chemical vapor deposition]] or [[atomic layer deposition]] processes, produced by the thermal decomposition of [[niobium(V) ethoxide]] above 350&nbsp;°C.<ref>{{cite thesis | title = Atomic Layer Deposition of High Permittivity Oxides: Film Growth and In Situ Studies |  author = Rahtu, Antti | publisher = University of Helsinki | date = 2002 | isbn = 952-10-0646-3| hdl = 10138/21065 }}</ref><ref>{{cite journal | doi = 10.1149/1.2059247 | title = Electrochromic Properties of Niobium Oxide Thin Films Prepared by Chemical Vapor Deposition | date = 1994 | last1 = Maruyama | first1 = Toshiro | journal = Journal of the Electrochemical Society | volume = 141 | issue = 10 | pages = 2868–2871| bibcode = 1994JElS..141.2868M }}</ref>

===Halides===
[[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 right) 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 = Robert J. 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(IV) fluoride|niobium tetrafluoride]] ({{chem2|NbF4}})<ref>{{Cite journal |last1=Gortsema |first1=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}}</ref> and dark violet [[Niobium(IV) chloride|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>{{Greenwood&Earnshaw2nd}}</ref>

===Nitrides and carbides===
Other [[binary compound]]s of niobium include [[niobium nitride]] (NbN), which becomes a [[superconductor]] at low temperatures and is used in detectors for infrared light.<ref><!--highly specialized vanity paper, it appears:-->{{cite journal|doi = 10.1080/09500340410001670866|title = Ultrafast superconducting single-photon detectors for near-infrared-wavelength quantum communications|author=Verevkin, A.|display-authors=4|author2=Pearlman, A.|author3=Slstrokysz, W.|author4=Zhang, J.|author5=Currie, M.|author6=Korneev, A.|author7=Chulkova, G.|author8=Okunev, O.|author9=Kouminov, P.|author10=Smirnov, K.|author11=Voronov, B.|author12=N. Gol'tsman, G.|author13=Sobolewski, Roman|journal = Journal of Modern Optics|volume = 51|issue = 12|date = 2004|pages = 1447–1458}}</ref> The main [[niobium carbide]] is NbC, an extremely [[hardness|hard]], [[refractory]], [[ceramic]] material, commercially used in cutting [[tool bit]]s.