View source for Aluminium(I) compounds

In chemistry, '''aluminium(I)''' refers to [[Monovalent ion|monovalent]] [[aluminium]] (+1 oxidation state) in both [[Ionic bonding|ionic]] and [[Covalent bond|covalent]] bonds. Along with aluminium(II), it is an extremely unstable form of aluminium.

While late [[Group 13 element]]s such as [[thallium]] and [[indium]] prefer the +1 oxidation state, aluminium(I) is rare. Aluminium does not experience the [[inert-pair effect]], a phenomenon where valence s electrons are poorly shielded from nuclear charge due to the presence of filled d and f orbitals.<ref name="Tacke 1989">{{Cite journal|last1=Tacke|first1=Matthias|last2=Schnoeckel|first2=Hansgeorg|date=1989-07-01|title=Metastable aluminum chloride, AlCl, as a solid and in solution|journal=Inorganic Chemistry|language=en|volume=28|issue=14|pages=2895–2896|doi=10.1021/ic00313a039|issn=0020-1669}}</ref> As such, aluminium (III) (<chem>Al^3+</chem>) is the much more common oxidation state for aluminium.

Aluminium(I) compounds are both prone to [[disproportionation]] and difficult to prepare.<ref name="Dohmeier 1996">{{Cite journal|last1=Dohmeier|first1=Carsten|last2=Loos|first2=Dagmar|last3=Schnöckel|first3=Hansgeorg|date=1996-02-02|title=Aluminum(I) and Gallium(I) Compounds: Syntheses, Structures, and Reactions|journal=Angewandte Chemie International Edition in English|language=en|volume=35|issue=2|pages=129–149|doi=10.1002/anie.199601291|issn=0570-0833}}</ref> At standard conditions, they readily oxidize to the aluminium(III) form.

== Characteristics ==
[[File:Homo lumo.png|thumb|HOMO and LUMO Orbitals for AlCl.<ref>{{Cite journal|last1=van der Woerd|first1=Mark J.|last2=Lammertsma|first2=Koop|last3=Duke|first3=Brian J.|last4=Schaefer|first4=Henry F.|date=1991-07-15|title=Simple mixed hydrides of boron, aluminum, and gallium: AlBH6, AlGaH6, and BGaH6|journal=The Journal of Chemical Physics|language=en|volume=95|issue=2|pages=1160–1167|doi=10.1063/1.461146|issn=0021-9606}}</ref>]]
Al(I) appears to be red, as solutions of [[Aluminium monobromide|AlBr]] and [[Aluminium monochloride|AlCl]] in organic solvents are both red.<ref name="Mocker 1994">{{Cite journal|last1=Mocker|first1=Mario|last2=Robl|first2=Christian|last3=Schnöckel|first3=Hansgeorg|date=1994-05-02|title=Donor-Stabilized Aluminum(II) Bromide|journal=Angewandte Chemie International Edition in English|language=en|volume=33|issue=8|pages=862–863|doi=10.1002/anie.199408621|issn=0570-0833}}</ref> The presence of this color implies a relatively small HOMO/LUMO gap that is accessible by green light.<ref>{{Cite book|title=Fundamentals of Atmospheric Radiation: An Introduction with 400 Problems|url=https://archive.org/details/fundamentalsatmo00bohr|url-access=limited|last=Bohren|first=Craig|publisher=Wiley-VCH|year=2006|isbn=978-3-527-40503-9|pages=[https://archive.org/details/fundamentalsatmo00bohr/page/n225 214]}}</ref>

The geometry of compounds can be determined by analysis of the fine structure of the electronic spectra.<ref name="Dohmeier 1996" /> Matrix isolation spectroscopy prevents disproportionation of aluminium monohalides and thus allows for the measuring of transitional vibrations as well as reactivity with molecules such as O<sub>2</sub>.<ref name="Dohmeier 1996" /><ref name="Schnockel 2000">{{Cite journal|last=Schnockel|date=2000-09-01|title=Low valent aluminium and gallium compounds — structural variety and coordination modes to transition metal fragments|journal=Coordination Chemistry Reviews|language=en|volume=206–207|pages=285–319|doi=10.1016/S0010-8545(00)00339-8|issn=0010-8545}}</ref>

Analysis by <sup>27</sup>Al [[Nuclear magnetic resonance spectroscopy|NMR spectroscopy]] of AlCl, AlBr, and [[Aluminium monoiodide|AlI]] in toluene/diethyl ether at room temperature reveal two signals: one very broad signal at δ = 100-130 ppm (regardless of the halogen), and one at higher field strength (AlCl: δ = + 30, AlBr: δ = + 50, AlI: δ = + 80).<ref name="Dohmeier 1996" /> The first signal corresponds to a donor-stabilized four-coordinate aluminium species, while the identity of the latter is inconclusive.<ref name="Dohmeier 1996" />

== Monohalides ==
The aluminium(I) cation reacts with hydrogen halides to form the following aluminium monohalides:<ref name="Tacke 1989" />

* [[aluminium monofluoride]] (AlF)
* [[aluminium monochloride]] (AlCl)
* [[aluminium monobromide]] (AlBr)
* [[aluminium monoiodide]] (AlI)

These compounds are only thermodynamically stable at high temperatures and low pressures in the singlet ground state.<ref>{{Cite journal|last1=Nagendran|first1=Selvarajan|last2=Roesky|first2=Herbert W.|date=2008-02-05|title=The Chemistry of Aluminum(I), Silicon(II), and Germanium(II)|journal=Organometallics|language=en|volume=27|issue=4|pages=457–492|doi=10.1021/om7007869|issn=0276-7333}}</ref> However, decomposition can be prevented by making disproportionation kinetically unfavorable. Under cold temperatures (below 77 [[Kelvin|K]]), disproportionation is slow enough that the AlCl solid can be kept for long periods of time.<ref name="Tacke 1989" />

=== Synthesis ===
AlCl is synthesized by reaction of liquid aluminium with gaseous [[Hydrogen chloride|HCl]] at 1200 K and 0.2 mbar to yield gaseous AlCl and hydrogen gas.<ref name="Tacke 1989" /> At 77 K, AlCl is a dark red solid which turns black upon disproportionation at temperatures higher than 180 K. At temperatures under 77 K and dissolved in a matrix of polar and non-polar solvents, it exists as a metastable solution whose reactivity can be studied. AlBr, a red oil, is prepared similarly from liquid aluminium and gaseous [[Hydrogen bromide|HBr]].<ref name="Mocker 1994" />

:<chem>Al (l) + HCl (g) -> AlCl (g) + 1/2 H2 (g)</chem>

Due to the nature of [[Hydrogen fluoride|HF]], which possesses a bond much stronger than that of its congeners,<ref>{{Cite journal|last=Benson|first=Sidney W.|title=III - Bond energies|journal=Journal of Chemical Education|volume=42|issue=9|pages=502|doi=10.1021/ed042p502|issn=0021-9584|year=1965|bibcode=1965JChEd..42..502B }}</ref> AlF is synthesized instead by the [[comproportionation]] of Al and [[Aluminium fluoride|AlF<sub>3</sub>]] which are pressed and mixed into pellets.<ref name="Dyke 1984">{{Cite journal|date=1984-08-01|title=A study of aluminium monofluoride and aluminium trifluoride by high-temperature photoelectron spectroscopy|journal=Chemical Physics|language=en|volume=88|issue=2|pages=289–298|doi=10.1016/0301-0104(84)85286-6|issn=0301-0104|last1=Dyke|first1=J.M.|last2=Kirby|first2=C.|last3=Morris|first3=A.|last4=Gravenor|first4=B.W.J.|last5=Klein|first5=R.|last6=Rosmus|first6=P.|bibcode=1984CP.....88..289D}}</ref> The pellets are then loaded into a graphite furnace and heated to 1050 K.<ref name="Dyke 1984" />

:<chem>2 Al (s) + AlF3 (s) -> 3 AlF (g)</chem>

Stability increases with mass: while AlCl decomposes at 77 K or above, AlBr remains stable up to 253 K.<ref name="Tacke 1989" /><ref name="Mocker 1994" /> Remarkably, it has been discovered that at any given temperature, the vapor pressure of AlF is orders of magnitude lower than that of other aluminium monohalides.<ref name="Dyke 1984" />

=== Disproportionation ===
At room temperature, AlX compounds tend to disproportionate to Al and AlX<sub>3</sub>. When dark red, solid AlCl is allowed to warm up, it turns black to yield aluminium and the more stable aluminium (III) chloride salt.<ref name="Tacke 1989" />

:<chem>2 AlCl (s)-> Al (s) +AlCl3 (s)</chem>

Dohmier et al. documented that the exception is AlBr. AlBr is stable enough at temperatures under -30 C that it comproportionates to AlBr<sub>2</sub> in the presence of AlBr<sub>3</sub>.<ref name="Dohmeier 1996" />

:<chem>AlBr (s) + AlBr3 (s) -> 2 AlBr2 (s)</chem>

=== Oligomerization ===
In Lewis basic solutions, AlX compounds have a tendency to oligomerize.<ref name="Dohmeier 1996" />
[[File:Albr in solution with correct coefficients.png|thumb|Molecular structure of the adduct of NEt<sub>3</sub> with AlBr, [Al<sub>4</sub>Br<sub>4</sub>(NEt<sub>3</sub>)<sub>4</sub>)], in the solid state.<ref name="Dohmeier 1996" /> |alt=|none|449x449px]]

== Complex chemistry ==
Aluminium is not only the [[Abundance of elements in Earth's crust|most abundant metal in the Earth's crust]], but also an element of low toxicity.  As such, aluminium (I) complexes attract considerable interest. These complexes can be supported by various ligands and used to activate small molecules.

=== β-Diketiminato systems ===
[[File:Chem structure of Hnacnac.png|thumb|β-Diketiminato ligands, an example of a NacNac ligand. It is anionic and bidentate. Shown are tautomers of a substituted HNacNac ligand precursor, and an idealized complex (right) of the conjugate base (M = metal, L = other ligand)]]In 2018, Liu et al. reviewed the chemistry of aluminium (I) with β-diketiminato ligands,<ref name="Liu 2018" /> widely used ligands with immense versatility in electronic and steric properties. These aluminium (I) complexes have immense potential for small molecule activation.<ref name="Liu 2018" />

'''Synthesis'''

β-diketiminato aluminium alkyls and aluminium halides are synthesized by adding a trialkyl aluminium compound to the initial β-diketiminate ligands, adding iodine, and the reducing with potassium.<ref name="Liu 2018">{{Cite journal|date=2018-11-01|title=The chemistry of aluminum(I) with β-diketiminate ligands and pentamethylcyclopentadienyl-substituents: Synthesis, reactivity and applications|journal=Coordination Chemistry Reviews|language=en|volume=374|pages=387–415|doi=10.1016/j.ccr.2018.07.004|issn=0010-8545|last1=Liu|first1=Yashuai|last2=Li|first2=Jia|last3=Ma|first3=Xiaoli|last4=Yang|first4=Zhi|last5=Roesky|first5=Herbert W.|s2cid=105749253 }}</ref> 
[[File:Synthesis of nacnacal(i) generic.png|none|thumb|607x607px|A synthetic route to aluminum (I) complexes supported by β-diketiminate ligands.<ref>{{Cite journal|last1=Cui|first1=Chunming|last2=Roesky|first2=Herbert W.|last3=Schmidt|first3=Hans‐Georg|last4=Noltemeyer|first4=Mathias|last5=Hao|first5=Haijun|last6=Cimpoesu|first6=Fanica|date=2000-12-01|title=Synthesis and Structure of a Monomeric Aluminum(I) Compound [{HC(CMeNAr)2}Al] (Ar=2,6–iPr2C6H3): A Stable Aluminum Analogue of a Carbene|journal=Angewandte Chemie International Edition|language=en|volume=39|issue=23|pages=4274–4276|doi=10.1002/1521-3773(20001201)39:23<4274::AID-ANIE4274>3.0.CO;2-K|pmid=29711904 |issn=1521-3773}}</ref>]]

==== [1+2] Cycloadditions ====
Al(I) compounds exhibit behavior analogous to that of singlet [[carbenes]].<ref name="Liu 2018" /> Like carbenes, they undergo [1+2] cycloadditions with alkynes and azides to afford three membered ring derivatives such as dialuminacyclohexadiene.<ref name="Tacke 1989" /><ref name="Liu 2018" />

Similarly to the nucleophilic carbon center in the carbene, the lone pair on the aluminium center binds to the first azide equivalent. Nitrogen gas is liberated. With the second equivalent of azide, a five-member ring is formed.
[[File:Carbene versus aluminum (I).png|none|thumb|708x708px|Aluminium (I) systems undergo reactions with azides in the same fashion as carbenes.<ref name="Liu 2018" />]]<nowiki/>

==== Reactions with other small molecules ====
Such aluminium (I) complexes can activate water as well as elemental phosphorus, oxygen, and sulfur to yield bridged dimers. This occurs via partial reduction of the elemental small molecule.<ref name="Liu 2018" />
[[File:P4 and nacnacal(I).png|none|thumb|467x467px|Activation of elemental phosphorus (P<sub>4</sub>) by an aluminium (I) complex]]
[[File:(Pentamethylcyclopentadienyl)aluminium(I).png|alt=|thumb|AlCp* ligand.]]

=== AlCp* ===
{{Main|(Pentamethylcyclopentadienyl)aluminium(I)}}

AlCp*, consisting of aluminium (I) bonded with the [[Pentamethylcyclopentadiene|pentamethylcyclopentadiene anion]] ((CCH3)<sub>5</sub><sup>−</sup>), was first synthesized in 1991 by Dohmier et al.<ref name="Dohmeier 1991">{{Cite journal|last1=Dohmeier|first1=Carsten|last2=Robl|first2=Christian|last3=Tacke|first3=Matthias|last4=Schnöckel|first4=Hansgeorg|title=The Tetrameric Aluminum(I) Compound[{Al(η5-C5Me5)}4]|journal=Angewandte Chemie International Edition in English|language=en|volume=30|issue=5|pages=564–565|doi=10.1002/anie.199105641|issn=0570-0833|year=1991}}</ref> (AlCp*)<sub>4,</sub> a yellow crystalline solid, is first produced from the combination of AlCl and MgCp*<sub>2.</sub><ref name="Schnockel 2000" /> When vaporized, the long Al-Al bonds (276.9 pm)<ref name="Dohmeier 1991" /> split, and monomeric molecules of [AlCp*] are created.

As revealed through Schnockel's work, [AlCp*] reacts by inserting itself into other bonds. Reaction with Al<sub>2</sub>I<sub>6</sub> yields subvalent halide species; reaction with As<sub>4</sub>tBu<sub>4</sub> yields As-Al bonds.<ref name="Schnockel 2000" /> When reacted with transition metal-cyclopentadienyl complexes such as NiCp<sub>2</sub>, it offers a straightforward pathway to compounds containing aluminium-transition metal bonds, which has great potential for important catalytic reactions.<ref name="Dohmeier 1996" />
[[File:Nicp2 and alcp* form a butterfly complex.png|none|thumb|430x430px|NiCp<sub>2</sub> and AlCp* react to form a butterfly structure.]]

[[File:Carbon-monoxide-LUMO-phase-3D-balls.png|left|thumb|The LUMO orbitals of CO are shown here. These orbitals are antibonding π orbitals.]]
As with other AlR ligands, [AlCp*] can be regarded as a [[Metal carbonyl|CO]] analogue, as it possesses 2 empty π orbitals and engages in similar coordination modes (terminal and bridging).<ref name="Schnockel 2000" /> This similarity implies the possibility of [[pi backbonding]] interactions between AlCp*and metals it complexes to.

== Metalloidal clusters ==
Work in aluminium clusters has been done by Linti and Schnockel. These metalloidal clusters can be formed from Al(I) compounds, namely aluminium monohalides. These clusters are termed "metalloidal clusters" because the number of unbridged metal-metal bonds is greater than the number of localized metal-ligand bonds. On the way to metal formation, intermediates are trapped in the presence of the bulky ligands which substitute the halide atoms.<ref name="Schnockel 2000" /><ref name="Tacke 1989" /> As a result, metal-rich clusters such as Al<sub>77</sub>R<sub>20</sub> are possible and offer insight into solid bulk metal formation.<ref name="Schnockel 2000" />

Tetrahedral aluminium is available from the reaction between aluminium(I) species and organometallic species.<ref name="Schnockel 2000" /> These clusters can be made through combinations such as AlCp* and LiR, AlBr and Li(THF)<sub>3</sub>(SiMe<sub>3</sub>)<sub>3,</sub> and AlI and NaSiBu<sub>3</sub>.<ref name="Schnockel 2000" />
[[File:Alcptet.png|thumb|Formation of tetrahedral aluminium.|alt=|none|479x479px]]
[[File:Aluminium Octahedral.png|thumb|236x236px|[Al<sub>6</sub>(<sup>t</sup>Bu)<sub>6</sub><nowiki>]</nowiki><sup>•-</sup> cluster, as synthesized by Linti et al.<ref name="Schnockel 2000" /> This is the first octahedral aluminium cluster synthesized.  ]]

This method of cluster formation created the only known incidence of an octahedral aluminium cluster, [Al<sub>6</sub>(tBu)<sub>6</sub>]<sup>−</sup>, which was formed by reaction between AlCl and <sup>t</sup>BuLi.<ref name="Schnockel 2000" /> Similarly, AlCl and LiN(SiMe<sub>3</sub>)<sub>2</sub> react to form the first known example of a cluster where two M<sub>4</sub> tetrahedra are connected by a common center.<ref name="Schnockel 2000" />

== Natural occurrence ==
Aluminium is rarely found in its +1 oxidation state in nature due to the immense stability of the +3 oxidation state.

Rotational transitions of AlF and AlCl have been detected in [[Circumstellar envelope|circumstellar shells]] near [[CW Leonis|IRC +10216]].<ref name="Dyke 1984" /><ref name="Ziurys 1994">{{Cite journal|last1=Ziurys|first1=L. M.|author1-link= Lucy Ziurys |last2=Apponi|first2=A. J.|last3=Phillips|first3=T. G.|date=1994-04-07|title=Exotic fluoride molecules in IRC +10216: Confirmation of AlF and searches for MgF and CaF|journal=The Astrophysical Journal|language=en|volume=433|pages=729–732|doi=10.1086/174682|issn=0004-637X|bibcode=1994ApJ...433..729Z}}</ref> The presence of AlF suggests that fluorine is produced in helium shell flashes instead of [[Nucleosynthesis|explosive nucleosynthesis]].<ref name="Ziurys 1994" />

==See also==
*[[Aluminylene]]

== References ==
{{Reflist}}
<references group=""></references>

{{Chemical compounds by element}}

[[Category:Aluminium(I) compounds| ]]
[[Category:Chemical compounds by element]]