Ionization Energies of Group 13 Elements (kJ/mol)

Element IE 1 IE 2 IE 3 Boron Group Compounds B 800.6 2427 3660 Group 13 (3A, III, IIIA) Al 577.5 1817 2745 Ga 578.8 1979 2963 B, Al, Ga, In, Tl In 558.3 1821 2704 Valence electron configuration: ns 2np 1 Tl 589.4 1971 2878

2nd and 3 rd ionization energies would be expected to continually decrease moving down the group

Oxidation States Boron Compounds • The lighter group 13 elements (B, Al) exist in compounds in a +3 oxidation state. • Gallium is also found in a +3 oxidation state, but is sometimes Borax H2O2 found in a +1 oxidation state • Indium is more commonly found in the +1 oxidation state, ‘washing soda’ Boric acid while thallium is only found in this state (e.g. TlBr) - cleaning agents • This behavior is also seen in other p-block groupings, and is explained by the inert pair effect (results from the ionization -insecticides energies of the 2 nd and 3 rd electrons in period 4 and heavier p- block elements being higher than expected). -antiseptics ns 2np 1 Inert pair effect : the apparent stability of the valence s electrons in heavier p-block elements which stabilizes an oxidation number that is 2 less than the element’s (old) group Copyright © 2011 Pearson Canada Inc. number General Chemistry: Chapter 21

Hydrides of Group 13 Elements Electron -Deficient Boron Cluster Compounds • Boron hydride (and carborane) compounds represent a • Having three valence electrons, group 13 elements might be fascinating class of cage structures that were once 2 expected to form EH 3-type hydrides ( sp -hybridized) viewed as viable fuel sources • In fact, there is ample evidence for oligomerization of many • Electron-deficient borane clusters are formed because of these hydrides, to yield bridged compounds boron compounds possess fewer valence electrons than are required for a localized bonding scheme 3c-2e - bond H (bridging) terminal 2c, 2e - bond H H hydrogen (terminal) B B H H H the bridging H atom is labeled µµµ-H • Because of its electron-deficient nature, heavier boron hydrides are extensively bridged structures (need to be in bridging order to obtain octets of electrons) hydrogen

1 Electron -Deficient Boron Cluster Compounds Naming Cluster Compounds

• The name used to identify these electron-deficient cluster • There are several basic cluster types : e.g. closo -, compounds involves several components. The proper name nido -, and arachno - • indicates its shape (relative to a reference shape) • indicates the number of boron and hydrogen atoms • Successively smaller clusters are named • indicates the charge on anions according to structures that are formed as successive vertices of the closed structures are removed ( nido-, arachno-, hypho-). six hydrogens anion with charge = -2 closo - structures are closed shapes closo -hexahydrohexaborate(2-) “ate” indicates anion six borons 6-boron atoms in parent, closed shape

Naming Boron Hydrides Hydrides of Group 13 Elements

Contrast the name of the anionic [B H ]2- with that for a • Aluminum hydride exists as a three-dimensional network 6 6 - neutral boron hydride of octahedral aluminum centers (3c-2e Al-H-Al bonds)
Number of hydrogens is instead indicated by a number at the end of the name (brackets) H H H H H H
Name ends in “-ane” Al H Al H Al H H H H H H

• Ga 2H6 has recently been characterized; structurally nido -pentaborane(9) similar to B 2H6 • Hydrides of In and Tl have not been characterized

Halides of Group 13 Elements Hydrides of Group 13 Elements • Boron trihalides are monomeric under ordinary conditions • Studies on the thermodynamics of formation of boron  Group 13 hydrides are susceptible to attack by trihalide-Lewis base adducts indicate that stability increases Lewis bases in the order: L•BF < L •BCl < L •BBr  The size of the Lewis base can impose different 3 3 3 reaction pathways X X + X B N B X X N X

+ -rel. small base rel. large base H NH3 H H H • How does this trend agree with electronegativities of X? H NH H H NPh3 H E E 3 E E 2 E H H H NH3 H H H H NPh • In the trigonal planar BX 3 structure, πππ-bonding exists, most 3 X X important for smaller elements B E = B or Ga X Asymmetric Symmetric Rem: correct Lewis F F F F F F B-X distance structure shows all B B B X = F: 131 pm Cleavage X = Cl: 174 pm Cleavage bonds and electrons X = Br: 189 pm F F F X = I: 210 pm

2 Boron Nitrides Boron Nitrides

• B-N unit is isoelectronic with C-C bond • Borazine, (HBNH) 3 is structurally similar to (same # of electrons) benzene H H H N H H H benzene is aromatic • The reactivity of these units, however, is B B and requires special N N conditions for chemical H B H very different as a result of different H H reaction electronegativities of these elements (borazine) H H P • However, they react very differently • χχχ B = 2.0 P • Benzene is quite stable • χχχ N = 3.0 B-N bond: polar • (HBNH) is (comparatively) susceptible to • χχχP = 2.6 C-C bond: non-polar 3 C nucleophilic and electrophilic attack

Boron Nitrides • Hexagonal boron nitride exists as layers of fused rings, similar in structure to graphite • In these assemblies, boron atoms lie directly over nitrogen atoms

Structurally, hexagonal boron nitride looks similar to graphite

• London forces operate between planes (weak interactions) and weak dipole-dipole forces • This material is a solid lubricant (like graphite), but a poor conductor (unlike graphite)

Aluminum Electrolysis cell for aluminum production by the Hall-Hérault process Third most abundant element, 8.3% by mass of crust. Lightweight alloys. 5 Mtonne/yr production Easily oxidized to Al 3+

Principal raw source is bauxite Al 2O3

Production of aluminum metal by electrolysis :

2- - oxidation 3××× {C( s) + 2 O CO 2(g) + 4 e }

reduction 3+ - 4××× {Al + 3 e Al( l)} High energy consumption, 15 kWh/kg Al. (cf Na about 5 kWh/kg) 3+ 2- 3 C( s) + 4 Al + 6 O 4 Al( l) + 3 CO 2(g) It takes only 5% of the energy to recycle compared to production from bauxite.

3 Aluminum Aluminum Oxide and Hydroxide Easily oxidized to Al 3+ →→→ ΔΔΔ Anodized aluminum 2 Al(s) + 3/2 O 2(g) Al 2O3(s) H = -1676 kJ The Thermite reaction (used in on-site welding of large objects): →→→ + - 2 Al(s) + 3 H 2O(l) Al 2O3(s) + 6 H + 6 e

Drinking cups made of anodized aluminum.

→→→ 2 Al(s) + Fe 2O3(s) Al 2O3(s) + Fe(s)

Aluminum Halides Friedel Crafts Alkylation

Bonding in Al 2Cl 6

+ - C2H5Cl + AlCl 3 [C 2H5] + [AlCl 4]

Aluminum Sulfate and Alums Amphoteric hydroxide

Al 2SO 4•18H 2O 1 Mtonne/yr production + →→→ 3+ Al(OH) 3(s) + 3 H 3O (aq) 2 [Al(H 2O) 6] (aq) Water purification systems Al(OH) (s) + OH -(aq) →→→ 2 [Al(OH) ]-(aq) 3 4 Sizing deposition agent for paper Alum production – mixed salts with various uses

4 Uses of Other Group 13 Metals Gallium Dopant in semiconductors Indium Makes low melting alloys. Low-temperature transistors and photoconductors. Thallium Extremely toxic. Few industrial uses.

Tl 2Ba 2Ca 2Cu 3O8+ x exhibits superconductivity up to 125K.

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