Chemistry 242 - Inorganic Chemistry II
Chapter 13 - Aluminium, Gallium, Indium and Thallium

Introduction

Occurence, Extraction and Properities of the Elements

The Oxides

The Halides

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)

Element Ka
Al 1.12x10-5
Ga 2.5x10-3
In 2x10-4
Tl ~7x10-2

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)

Hydroxides

The real hydroxides, by extension of the above, are complicated structures involving bridging OH-, terminal H2O and perhaps [M(OH)4]- for some metals.

Alums

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.

Coordination Compounds

Examples are: [Al(H2O)6]3+, [AlF6]3-, Cl3Al(N(CH3)3)2, [Al(ox)3]3- and Al(8-hydroxyquinolinate)3

Hydrides

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

  1. Boron

    1. Forms no simple B3+ cation.

    2. Forms covalent compounds almost exclusively, and polyatomic ions are internally covalently bonded.

    3. Has a maximum covalence of 4 corresponding to an octet.

    4. The trivalent compounds are usually strong Lewis acids.

    5. Its oxide and "hydroxide" are acidic.

    6. Forms many polyatomic borates.

    7. The trihalides are easily hydrolysed.

    8. 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

  2. Aluminum

    1. Readily forms the Al3+ ion which is usually coordinated.

    2. Much more metallic than boron and forms many ionic compounds.

    3. Forms molecular compounds and ionic lattices with cordination numbers from 4 up to 6 and higher.

    4. Forms oxides which are chemically and thermally fairly inert.

    5. Forms a mainly basic but quite amphoteric hydroxide.

    6. Forms partially hydrolysable halides.

    7. Forms a polymeric hydride and the AlH4- ion. The latter is important.

  3. Gallium, Indium, and Thallium.

    1. Readily form M3+ aquo species and have a rich coordination chemistry.

    2. Form increasingly stable M+ compounds especially thallium. Covalent bonds successively weaken down the group enhancing this trend.

    3. Halides are increasingly aggregated with the increasing size of the metals.

    4. Hydrides and hydride ions are not very important or stable.