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In chemistry, aluminum (I) refers to monovalent aluminum (+1 oxidation state) in both ionic and covalent bonds. Along with aluminum (II), it is an extremely unstable form of aluminum.

While late Group XIII elements such as thallium prefer the +1 oxidation state, aluminum (I) is rare. Aluminum does not experience the inert pair effect that late Group XIII elements do, where the valence s electrons are extremely core-like and thus do not participate in bonding. Aluminum (I) compounds are both prone to disproportionation and difficult to prepare. At standard conditions, they readily oxidize to the aluminum (III) form.

Characteristics
The Al(I) ion appears to be red, as solutions of AlBr and AlCl in organic solvents are both red.

The geometry of compounds can be determined by analysis of the fine structure of the electronic spectra. Matrix isolation spectroscopy prevents disproportionation of aluminum monohalides and thus allows for the measuring of transitional vibrations as well as reactivity with molecules such as O2.

Analysis by 27Al NMR spectroscopy of AlCl, AlBr, and AlI 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 (steadily upfield with increasing mass).

Monohalides
The aluminum (I) cation reacts with hydrogen halides to form aluminum monohalides (AlF, AlCl, AlBr, AlI). These compounds are only thermodynamically stable at high temperatures and low pressures in the singlet ground state. However, decomposition can be prevented by making disproportionation kinetically unfavorable.

AlCl is synthesized by reaction of liquid aluminum with gaseous hydrochloric acid at 1200 K and 0.2 mbar to yield gaseous AlCl and hydrogen gas. At 77K, 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. It participates in [2+1] cycloadditions with alkynes to yield dialuminacyclohexadiene. AlBr, a red oil, is prepared similarly from liquid aluminum metal and gaseous hydrobromic acid.

Due to the nature of HF, aluminum monofluoride is synthesized instead by the comproportionation of Al and AlF3 which are pressed and mixed into pellets. The pellets are then loaded into a graphite furnace and heated to 1050 K.

Stability increases with mass: while AlCl decomposes at 77 K or above, AlBr remains stable up to 253 K. Remarkably, it has been discovered that at any given temperature, the vapor pressure of AlF is lower than that of other aluminum monohalides.

AlCp*
(AlCp*)4 is produced from the combination of AlCl and MgCp*2. When vaporized, the long Al-Al bonds split, and monomeric molecules of [AlCp*] are created.

[AlCp*] reacts by inserting itself into other bonds. Reaction with Al2I6 yields subvalent halide species; reaction with As4tBu4 yields As-Al bonds. When reacted with transition metal-cyclopentadienyl complexes such as NiCp2, it offers a straightforward pathway to compounds containing aluminum-transition metal bonds, which has great potential for important catalytic reactions.

As with other AlR ligands, [AlCp*] can be regarded as a CO analogue, as it posses 2 empty π orbitals and engages in similar coordination modes (terminal and bridging).

Metalloidal Clusters
Aluminum clusters can be formed from Al(I) compounds, namely aluminum 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. As a result, metal-rich clusters such as Al77R20 are possible and offer insight into solid bulk metal formation.

Tetrahedral aluminum is available from the reaction between aluminum(I) species and organometallic species. These clusters can be made through combinations such as AlCp* and LiR, AlBr and Li(THF)3(SiMe3)3, and AlI and NaSiBu3.

This method of cluster formation created the only known incidence of an octahedral aluminum cluster, [Al6(tBu)6]−, which was formed by reaction between AlCl and tBuLi. Similarly, AlCl and LiN(SiMe3)2 react to form the first known example of a cluster where two M4 tetrahedra are connected by a common center.

Chemistry
Al(I) compounds exhibit behavior analogous to that of singlet carbenes. Like carbenes, they undergo [1+2] cycloadditions with alkynes to afford three membered ring derivatives.

At room temperature, AlX compounds tend to disproportionate to Al and AlX3. The exception is AlBr, which is stable enough at temperatures under -30 C that it conproportionates to AlBr2 in the presence of AlBr3.

Occurrence
Aluminum 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 shells near IRC +10216. The presence of AlF suggests that fluorine is produced in helium shell flashes instead of explosive nucleosynthesis.