Chemistry 242 - Inorganic Chemistry II
Chapter 13 - Aluminium, Gallium, Indium and Thallium
- Aluminium is th most common metallic element in the crust of the earth, but the common minerals, for example, felspars and micas are rather difficult to process. Aluminum is obtained from bauxite, (Al2O3.nH2O) and cryolite, Na3AlF6 by electrolysis.
- Gallium and Indium occur in traces in bauxite and all three are found in certain sulphite ores of other metals.
- While aluminium is obviously th emost important, gallium is used in gallium arsenide semiconductors.
- The elements are all much more metallic than boron, but there are a number of borderline covalent compounds.
- All are trivalent, but for thallium the univalent state (Tl+) becomes the dominant state as covalent bond strenghts diminish down the group. Some thallium III compounds are thermodynamically unstable:
TlX3 TlX + X2
- The MX3 compounds (halides and organometallic) are Lewis bases like boron. The strengths vary in the sequence:
B > Al > Ga > (In ~ Tl)
- The trihalides are not monomers like BX3 but are more or less associated e.g. Al2Cl6 and (AlF3)n.
- All give aqua ions [M(H2O)6]3+ in their salts obtained from aqueous solution.
Occurence, Extraction and Properities of the Elements
- To obtain aluminum, bauxite is dissolved in sodium hydroxide to give sodium aluminate, NaAl(OH)4. The insolubles which include hydrated iron oxide are filtered off, and the pH adjusted (with CO2)to reprecipitate the aluminum as Al(OH)3.3H2O which is then dehydrated to Al2O3 and dissolved in molten cryolite, Na3AlF6 fro electrolysis.
- The other (less reactive) metals can be obtained by electrolysis of aqueous solutions.
- The metals are all soft and quite reactive.
- Aluminum is "passivated" by a film of oxide which prevents it reacting with oxygen, water and even dilute nitric acid. (If the surface is amalgamated, reaction with water can occur.)
- The metals dissolve in non-oxidizing dilute acids. Aluminum and gallium are amphoteric and will dissolve in sodium hydroxide.
- They react with the halogens and sulphur.
- Thallium reacts slowly because the Tl+ salts which are formed are often insoluble and coat and passivate the metal surface.
- The most important are g-alumina, used as a stationay phase in liquid-solid chromatography and a-alumina, used as a catalyst in petroleum cracking.
- The gemstone, ruby is Al2O3 contaminated with traces of Cr3+ in place of the Al3+ and sapphire has Fe2+, Fe3+ and Ti4+ replacing some of the Al3+.
The Aqua Ions, Oxo salts and Aqueous Chemistry
The aqua ions all undergo hydrolysis:
[M(H2O)6]3+ [M(H2O)5(OH)]2+ + H+(aq)
Salts of weak acids cannot exist in solution because the anions would be protonated and the hydroxides would precipitate.
The "hydroxides" of aluminum and gallium are amphoteric:
M(OH)3(s) M3+ + 3OH-
M(OH)3(s) MO2- + H+ + H2O
Depending on the conditions, bridging hydroxide is also common:
2[M(H2O)5(OH)]2+ [(H2O)5MOM(H2O)5]4+ + H2O (etc)
The real hydroxides, by extension of the above, are complicated structures involving bridging OH-, terminal H2O and perhaps [M(OH)4]- for some metals.
These are the compounds for which aluminum was originally named. They are double salts of formula MM'(SO4)2.12H2O where M+ is usually an alkali metal ion (not Li+) and M'3+ is Al3+ or another trivalent ion. For example, plain "alum" or "potash alum" is the potassium/aluminum salt and "chrome alum" is the potassium/chromium(III) salt. These compounds are characterized by easily grown octahedral crystals. Each metal ion is 6-coordinated by water.
Examples are: [Al(H2O)6]3+, [AlF6]3-, Cl3Al(N(CH3)3)2, [Al(ox)3]3- and Al(8-hydroxyquinolinate)3
The metal hydrides are not very stable except "AlH3" which is an air-sensitive polymeric material. The tetrahydroaluminate ion AlH4- is an important reducing agent and hydride source which usually comes as lithium aluminum hydride. The analogous gallium compound exists. The compounds are very sensitive to hydrolysis which is very exothermic and can be explosive.
The is a series of MH3 Lewis adducts with donor molecules which are generally more stable to, for example, hydrolysis than the parent hydrides.
Lower Valent Compounds
This section is mainly about Tl+ which resembles K+ and Ag+ in its chemistry. This section was not covered in depth in lectures. Skip it.
Summary of the Periodic Trends for the Elements of Group 13
- Forms no simple B3+ cation.
- Forms covalent compounds almost exclusively, and polyatomic ions are internally covalently bonded.
- Has a maximum covalence of 4 corresponding to an octet.
- The trivalent compounds are usually strong Lewis acids.
- Its oxide and "hydroxide" are acidic.
- Forms many polyatomic borates.
- The trihalides are easily hydrolysed.
- Forms many hydrides and hydride anions which are polyhedral clusters: the boranes, carboranes and the borane anions. The simplest BH4- is a very important synthetic reagent
- Readily forms the Al3+ ion which is usually coordinated.
- Much more metallic than boron and forms many ionic compounds.
- Forms molecular compounds and ionic lattices with cordination numbers from 4 up to 6 and higher.
- Forms oxides which are chemically and thermally fairly inert.
- Forms a mainly basic but quite amphoteric hydroxide.
- Forms partially hydrolysable halides.
- Forms a polymeric hydride and the AlH4- ion. The latter is important.
- Gallium, Indium, and Thallium.
- Readily form M3+ aquo species and have a rich coordination chemistry.
- Form increasingly stable M+ compounds especially thallium. Covalent bonds successively weaken down the group enhancing this trend.
- Halides are increasingly aggregated with the increasing size of the metals.
- Hydrides and hydride ions are not very important or stable.