Editing Titanium (section)

== Compounds ==
{{Category see also|Titanium compounds|Titanium minerals}}
{{see also|Titanium alloy}}
[[File:Titanium nitride coating.jpg|thumb|upright=0.25|alt=A steel colored twist drill bit with the spiral groove colored in a golden shade.|TiN-coated [[drill bit]]]]
The +4 [[oxidation state]] dominates titanium chemistry,<ref name="Greenwood1997p958">{{harvnb|Greenwood|Earnshaw|1997|p=958}}</ref> but compounds in the +3 oxidation state are also numerous.<ref name="Greenwood1997p970">{{harvnb|Greenwood|Earnshaw|1997|p=970}}</ref> Commonly, titanium adopts an [[octahedral coordination geometry]] in its complexes,<ref name="Greenwood1997p960">{{harvnb|Greenwood|Earnshaw|1997|p=960}}</ref><ref name="Greenwood1997p967">{{harvnb|Greenwood|Earnshaw|1997|p=967}}</ref> but tetrahedral TiCl<sub>4</sub> is a notable exception. Because of its high oxidation state, titanium(IV) compounds exhibit a high degree of [[covalent bond]]ing.<ref name="Greenwood1997p958" />

=== Oxides, sulfides, and alkoxides ===
The most important oxide is TiO<sub>2</sub>, which exists in three important [[polymorphism (materials science)|polymorphs]]; anatase, brookite, and rutile. All three are white diamagnetic solids, although mineral samples can appear dark (see [[rutile]]). They adopt polymeric structures in which Ti is surrounded by six [[oxide]] ligands that link to other Ti centers.<ref name="Greenwood1997p961">{{harvnb|Greenwood|Earnshaw|1997|p=961}}</ref>

The term ''[[titanate]]s'' usually refers to titanium(IV) compounds, as represented by [[barium titanate]] (BaTiO<sub>3</sub>). With a perovskite structure, this material exhibits [[piezoelectric]] properties and is used as a transducer in the interconversion of [[sound]] and [[electricity]].<ref name="TICE6th" /> Many minerals are titanates, such as ilmenite (FeTiO<sub>3</sub>). [[Star sapphire (jewel)|Star sapphires]] and [[ruby|rubies]] get their [[asterism (gemmology)|asterism]] (star-forming shine) from the presence of titanium dioxide impurities.<ref name="Emsley2001p453" />

A variety of reduced oxides ([[suboxide]]s) of titanium are known, mainly reduced [[stoichiometry|stoichiometries]] of titanium dioxide obtained by [[atmospheric plasma spraying]]. Ti<sub>3</sub>O<sub>5</sub>, described as a Ti(IV)-Ti(III) species, is a purple semiconductor produced by [[reduction (chemistry)|reduction]] of TiO<sub>2</sub> with hydrogen at high temperatures,<ref>{{cite journal |last1=Liu |first1=Gang |last2=Huang |first2=Wan-Xia |last3=Yi |first3=Yong |title=Preparation and Optical Storage Properties of λTi<sub>3</sub>O<sub>5</sub> Powder |journal=Journal of Inorganic Materials |date=26 June 2013 |volume=28 |issue=4 |pages=425–430|doi=10.3724/SP.J.1077.2013.12309|doi-broken-date=1 November 2024 }}</ref> and is used industrially when surfaces need to be vapor-coated with titanium dioxide: it evaporates as pure TiO, whereas TiO<sub>2</sub> evaporates as a mixture of oxides and deposits coatings with variable refractive index.<ref>{{cite journal |last1=Bonardi |first1=Antonio |last2=Pühlhofer |first2=Gerd |last3=Hermanutz |first3=Stephan |last4=Santangelo |first4=Andrea |year=2014 |title=A new solution for mirror coating in {{mvar|γ}}-ray Cherenkov Astronomy |journal=Experimental Astronomy |volume=38 |issue=1–2 |pages=1–9 |doi=10.1007/s10686-014-9398-x |bibcode=2014ExA....38....1B |s2cid=119213226 |arxiv=1406.0622}}</ref> Also known is [[titanium(III) oxide|Ti<sub>2</sub>O<sub>3</sub>]], with the [[corundum]] structure, and [[titanium(II) oxide|TiO]], with the [[rock salt structure]], although often [[nonstoichiometric]].{{sfn|Greenwood|Earnshaw|1997|p=962}}

The [[alkoxide]]s of titanium(IV), prepared by treating TiCl<sub>4</sub> with [[Alcohol (chemistry)|alcohol]]s, are colorless compounds that convert to the dioxide on reaction with water. They are industrially useful for depositing solid TiO<sub>2</sub> via the [[sol-gel process]]. [[Titanium isopropoxide]] is used in the synthesis of chiral organic compounds via the [[Sharpless epoxidation]].<ref>{{cite journal |author1=Ramón, Diego J. |author2=Yus, Miguel |year=2006 |title=In the arena of enantioselective synthesis, titanium complexes wear the laurel wreath |journal=Chem. Rev. |volume=106 |issue=6 |pages=2126–2308 |doi=10.1021/cr040698p |pmid=16771446}}</ref>

Titanium forms a variety of sulfides, but only [[titanium disulfide|TiS<sub>2</sub>]] has attracted significant interest. It adopts a layered structure and was used as a cathode in the development of [[lithium batteries]]. Because Ti(IV) is a [[HSAB theory|"hard cation"]], the sulfides of titanium are unstable and tend to hydrolyze to the oxide with release of [[hydrogen sulfide]].<ref>{{cite book | last1 = McKelvy | first1 = M.J. | last2 = Glaunsinger | first2 = W.S. | year = 1995 | title = Inorganic Syntheses | chapter = Titanium Disulfide | volume = 30 | pages = 28–32 | doi = 10.1002/9780470132616.ch7 | isbn = 978-0-470-13261-6 }}</ref>

=== Nitrides and carbides ===
[[Titanium nitride]] (TiN) is a refractory solid exhibiting extreme hardness, thermal/electrical conductivity, and a high melting point.<ref>{{Cite journal |last=Saha |first=Naresh |year=1992 |title=Titanium nitride oxidation chemistry: An x-ray photoelectron spectroscopy study |journal=Journal of Applied Physics |volume=72 |issue=7 |pages=3072–3079 |doi=10.1063/1.351465 |bibcode=1992JAP....72.3072S}}</ref> TiN has a hardness equivalent to [[sapphire]] and [[carborundum]] (9.0 on the [[Mohs scale]]),<ref>{{cite web |author=Schubert, E.F. |title=The hardness scale introduced by Friederich Mohs |series=Educational resources |publisher=[[Rensselaer Polytechnic Institute]] |place=Troy, NY |url=https://www.ecse.rpi.edu/~schubert/Educational-resources/Materials-Hardness.pdf |url-status=live |archive-url=https://web.archive.org/web/20100603075632/http://www.rpi.edu/~schubert/Educational-resources/Materials-Hardness.pdf |archive-date=3 June 2010}}</ref> and is often used to coat cutting tools, such as [[drill bit]]s.<ref>{{cite magazine |last=Truini |first=Joseph |date=May 1988 |title=Drill bits |magazine=[[Popular Mechanics]] |volume=165 |issue=5 |page=91 |issn=0032-4558 |url=https://books.google.com/books?id=Z-QDAAAAMBAJ}}</ref> It is also used as a gold-colored decorative finish and as a [[Copper interconnects#Barrier metal|barrier layer]] in [[semiconductor fabrication]].<ref>{{cite book|last=Baliga |first=B. Jayant |year=2005 |title=Silicon carbide power devices |publisher=World Scientific |page=91 |isbn=978-981-256-605-8 |url=https://books.google.com/books?id=LNLVwAzhN7EC}}</ref> [[Titanium carbide]] (TiC), which is also very hard, is found in cutting tools and coatings.<ref>{{cite web |title=Titanium carbide product information |publisher=H.C. Starck |url=http://www.hcstarck.com/titanium_carbide_tic  |access-date=16 November 2015 |archive-url=https://web.archive.org/web/20170922194330/https://www.hcstarck.com/titanium_carbide_tic |archive-date=22 September 2017}}</ref>

=== Halides ===
[[File:TiCl3.jpg|thumb|right|upright=0.75|Titanium(III) compounds are characteristically violet, illustrated by this aqueous solution of [[titanium trichloride]].]]
[[Titanium tetrachloride]] (titanium(IV) chloride, TiCl<sub>4</sub><ref>{{cite report |author1=Seong, S. |author2=Younossi, O. |author3=Goldsmith, B.W. |year=2009 |title=Titanium: Industrial base, price trends, and technology initiatives |publisher=Rand Corporation |isbn=978-0-8330-4575-1 |page=10 |url=https://books.google.com/books?id=tIPFfYW304IC&pg=PA10}}</ref>) is a colorless volatile liquid (commercial samples are yellowish) that, in air, hydrolyzes with spectacular emission of white clouds. Via the [[Kroll process]], TiCl<sub>4</sub> is used in the conversion of titanium ores to titanium metal. Titanium tetrachloride is also used to make titanium dioxide, e.g., for use in white paint.<ref>{{cite book |last=Johnson |first=Richard W. |year=1998 |title=The Handbook of Fluid Dynamics |publisher=Springer |pages=38–21 |isbn=978-3-540-64612-9 |url=https://books.google.com/books?id=JBTlucgGdegC}}</ref> It is widely used in [[organic chemistry]] as a [[Lewis acids and bases|Lewis acid]], for example in the [[Mukaiyama aldol condensation]].<ref>{{cite book |last=Coates |first=Robert M. |author2=Paquette, Leo A. |year=2000 |title=Handbook of Reagents for Organic Synthesis |publisher=John Wiley and Sons |page=93|isbn=978-0-470-85625-3|url=https://books.google.com/books?id=xxYjJgupBSMC}}</ref> In the [[van Arkel–de Boer process]], [[titanium tetraiodide]] (TiI<sub>4</sub>) is generated in the production of high purity titanium metal.<ref name="Greenwood1997p965">{{harvnb|Greenwood|Earnshaw|1997|p=965}}</ref>

Titanium(III) and titanium(II) also form stable chlorides. A notable example is [[titanium(III) chloride]] (TiCl<sub>3</sub>), which is used as a [[catalyst]] for production of [[polyolefin]]s (see [[Ziegler–Natta catalyst]]) and a reducing [[reagent|agent]] in organic chemistry.<ref>{{cite encyclopedia |first1=Lise-Lotte |last1=Gundersen |first2=Frode |last2=Rise |first3=Kjell |last3=Undheim |first4=José |last4=Méndez Andino |year=2007 |title=Titanium(III) Chloride |encyclopedia=[[Encyclopedia of Reagents for Organic Synthesis]] |doi=10.1002/047084289X.rt120.pub2 |isbn=978-0-471-93623-7 }}</ref>

=== Organometallic complexes ===
{{Main|Organotitanium chemistry}}
Owing to the important role of titanium compounds as [[polymerization]] catalyst, compounds with Ti-C bonds have been intensively studied. The most common organotitanium complex is [[titanocene dichloride]] ((C<sub>5</sub>H<sub>5</sub>)<sub>2</sub>TiCl<sub>2</sub>). Related compounds include [[Tebbe's reagent]] and [[Petasis reagent]]. Titanium forms [[metal carbonyl|carbonyl complexes]], e.g. [[titanocene dicarbonyl|(C<sub>5</sub>H<sub>5</sub>)<sub>2</sub>Ti(CO)<sub>2</sub>]].<ref>{{cite book |author-link=John F. Hartwig |author=Hartwig, J.F. |year=2010 |title=Organotransition Metal Chemistry, from Bonding to Catalysis |publisher=University Science Books |place=New York, NY |isbn=978-1-891389-53-5}}</ref>

=== Anticancer therapy studies ===
Following the success of [[cisplatin|platinum-based]] chemotherapy, titanium(IV) complexes were among the first non-platinum compounds to be tested for cancer treatment. The advantage of titanium compounds lies in their high efficacy and low toxicity ''[[in vivo]]''.<ref name=Tshuva-Miller/> In biological environments, hydrolysis leads to the safe and inert titanium dioxide. Despite these advantages the first candidate compounds failed clinical trials due to insufficient efficacy to toxicity ratios and formulation complications.<ref name=Tshuva-Miller/> Further development resulted in the creation of potentially effective, selective, and stable titanium-based drugs.<ref name=Tshuva-Miller>{{cite book |last1=Tshuva |first1=Edit Y. |last2=Miller |first2=Maya |editor1-last=Sigel |editor1-first=Astrid |editor2-last=Sigel |editor2-first=Helmut|editor3-last=Freisinger |editor3-first=Eva |editor4-last=Sigel |editor4-first=Roland K.O. |year=2018 |title=Metallo-drugs: Development and action of anticancer agents |series=Metal Ions in Life Sciences |volume=18 |doi=10.1515/9783110470734-014 |pmid=29394027 |publisher=de Gruyter GmbH |location=Berlin, DE |chapter=Chapter&nbsp;8. Coordination complexes of titanium(IV) for anticancer therapy |pages=219–250 |isbn=978-3-11-047073-4 |chapter-url=https://books.google.com/books?id=4nBLDwAAQBAJ}}</ref>