13

The Group 13 Elements

The elements of Group 13—boron, aluminium, gallium, indium, and thallium—have diverse physical and chemical properties. The first member of the group, boron, is essentially nonmetallic, whereas the heavier members of the group are distinctly metallic.

5B
13Al
31Ga
49In
81Tl

Part A: The Essentials

13.1 The Elements

Key Point: Boron is the only nonmetal in the group. Aluminium is the most abundant Group 13 element.

The elements of Group 13 show a wide variation in abundance in crustal rocks, the oceans, and the atmosphere. Aluminium is abundant but the low cosmic and terrestrial abundance of boron, like that of lithium and beryllium, reflects how the light elements are sidestepped in nucleosynthesis.

There is an increase in metallic character from B to Tl: B is a nonmetal; Al is essentially metallic, although it is often classed as a metalloid on account of its amphoteric character; and Ga, In, and Tl are metals.

B
Boron
Nonmetal
mp: 2300°C
χ: 2.0
Al
Aluminium
Metalloid/Metal
mp: 660°C
χ: 1.6
Ga
Gallium
Metal
mp: 30°C
χ: 1.8
In
Indium
Metal
mp: 157°C
χ: 1.8
Tl
Thallium
Metal
mp: 304°C
χ: 2.0

Diagonal Relationship: Boron and Silicon

B does have a pronounced diagonal relationship with Si in Group 14:

Selected Properties of the Elements

Property B Al Ga In Tl
Covalent radius/pm 80 125 125 150 155
Ionic radius, r(M3+)/pm 27 53 62 80 89
Melting point/°C 2300 660 30 157 304
Boiling point/°C 3930 2470 2403 2000 1460
First ionization energy/kJ mol−1 799 577 577 556 590
Pauling electronegativity 2.0 1.6 1.8 1.8 2.0
E°(M3+,M)/V −0.89 −1.68 −0.53 −0.34 +0.72
First Ionization Energy Trend (kJ mol−1)
799
B
577
Al
577
Ga
556
In
590
Tl
The Inert-Pair Effect:

The heavier elements of the group form compounds with the metal in the +1 oxidation state and this state increases in stability down the group. In fact, the most common oxidation state of Tl is Tl(I). This trend is a consequence of the inert-pair effect.

13.2 Compounds

Key Point: All of the elements form hydrides, oxides, and halides in the +3 oxidation state. The +1 oxidation state becomes more stable down the group and is the most stable oxidation state for compounds of thallium.

A most striking feature of the lighter Group 13 elements is their ns2np1 electron configuration, which contributes up to a maximum of six electrons in the valence shell when three covalent bonds are formed by electron sharing. As a result, many of their compounds have an incomplete octet and act as Lewis acids.

Note on Good Practice

Be careful to distinguish electron deficiency from the possession of an incomplete octet. The former refers to the lack of sufficient electrons to account for the connections between atoms as normal covalent bonds; the latter is the possession of less than eight electrons in a valence shell.

Diborane: The Simplest Borane

Structure of Diborane (B2H6)

B B H H H H H H

The structure contains 2c,2e terminal bonds and 3c,2e bridging bonds (shown as dashed lines)

The binary hydrogen compounds of B are called boranes. The simplest member of the series, diborane, B2H6, is electron-deficient and its structure is commonly described in terms of 2c,2e and 3c,2e bonds: bridging 3c,2e bonds are a recurring theme in borane chemistry.

BOX 13.1: Group 13 Elements in Hydrogen Storage

Hydrogen fuel cells are seen as an alternative to carbon-based fuel. The boron and aluminium hydrides are attractive compounds with high mass percentage hydrogen content:

  • LiBH4: ~18 mass %
  • NaBH4: ~11 mass %
  • LiAlH4: ~11 mass %
  • AlH3: ~10 mass %

Sodium tetrahydridoborate reacts with water to generate hydrogen:

NaBH4(aq) + 4 H2O(l) → 4 H2(g) + NaB(OH)4(aq) ΔH° = −230 kJ mol−1

Boron Trihalides

Boron trihalides consist of trigonal-planar BX3 molecules. Unlike the halides of the other elements in the group, they are monomeric in the gas, liquid, and solid states. The order of Lewis acidity is:

BF3 < BCl3 ≤ BBr3

This order is contrary to what might be expected from electronegativity. The electron deficiency is partially removed by X→B π bonding between the halogen atoms and the B atom.

Property BF3 BCl3 BBr3 BI3
Melting point/°C −127 −107 −46 50
Boiling point/°C −100 13 91 210
Bond length/pm 130 175 187 210
ΔfG°/kJ mol−1 −1112 −339 −232 +21

13.3 Boron Clusters

Key Point: Boron forms an extensive range of polymeric, cage-like compounds which include the borohydrides, metallaboranes, and carboranes.

In addition to the simple hydrides, B forms several series of neutral and anionic polymeric cage-like boron–hydrogen compounds. Borohydrides are formed with up to 12 B atoms and fall into three classes called closo, nido, and arachno.

Wade's Rules

A correlation between the number of electrons, the formula, and the shape of the molecule was established by Kenneth Wade in the 1970s.

Closo

[BnHn]2−

"Cage" - Greek origin

(n+1) skeletal electron pairs

Closed deltahedron

Nido

BnHn+4

"Nest" - Latin origin

(n+2) skeletal electron pairs

One vertex missing

Arachno

BnHn+6

"Spider" - Greek origin

(n+3) skeletal electron pairs

Two vertices missing

Example 13.5: Using Wade's Rules

Problem: Infer the structure of [B6H6]2− from its formula and electron count.

Answer: The formula [BnHn]2− is characteristic of a closo species. Alternatively, counting skeletal electrons: (6 × 2) + 2 = 14, or seven electron pairs, which is (n+1) with n=6. Therefore, the cluster is based on an octahedron with no missing vertices—a closo cluster.

(a) How many framework electron pairs are present in B4H10 and to what structural category does it belong? (b) Predict the structure of [B5H8]−.

Borohydride Structures

[B5H5]2−
Trigonal Bipyramidal
closo structure
[B6H6]2−
Octahedral
closo structure
[B12H12]2−
Icosahedral
closo structure
Type Formula Examples Skeletal Electron Pairs
Closo [BnHn]2− [B5H5]2− to [B12H12]2− n + 1
Nido BnHn+4 B2H6, B5H9, B6H10 n + 2
Arachno BnHn+6 B4H10, B5H11 n + 3

13.6 Simple Hydrides of Boron

(a) Boranes - Synthesis and Properties
Key Point: Diborane can be synthesized by metathesis between a boron halide and a hydride source; many of the higher boranes can be prepared by the partial pyrolysis of diborane; all the boron hydrides are flammable, sometimes explosively, and many of them are susceptible to hydrolysis.

Diborane, B2H6, can be prepared in the laboratory by metathesis of a boron halide with either LiAlH4 or LiBH4 in ether:

3 LiBH4(et) + 4 BF3(et) → 3 LiBF4(et) + 2 B2H6(g)

All the boranes are colourless and diamagnetic. They range from gases (B2H6 and B4H8), through volatile liquids (B5H9 and B6H10), to the sublimable solid B10H14.

All the boron hydrides are flammable, and several of the lighter ones react spontaneously with air:

B2H6(g) + 3 O2(g) → 2 B(OH)3(s)

Boranes are readily hydrolysed by water to give boric acid and hydrogen:

B2H6(g) + 6 H2O(l) → 2 B(OH)3(aq) + 6 H2(g)
(b) Lewis Acidity of Boranes
Key Point: Soft and bulky Lewis bases cleave diborane symmetrically; more compact and hard Lewis bases cleave the hydrogen bridge unsymmetrically; although it reacts with many hard Lewis bases, diborane is best regarded as a soft Lewis acid.

Diborane and many other light boron hydrides act as Lewis acids and are cleaved by reaction with Lewis bases. Two different cleavage patterns have been observed:

Symmetrical Cleavage

B2H6 is broken symmetrically into two BH3 fragments:

B2H6 + 2 NMe3 → 2 H3B−NMe3

Unsymmetrical Cleavage

Cleavage leading to an ionic product:

B2H6 + 2 NH3 → [H2B(NH3)2]+[BH4]−
(c) Hydroboration
Key Point: Hydroboration, the reaction of diborane with alkenes in ether solvent, produces organoboranes that are useful intermediates in synthetic organic chemistry.

An important component of a synthetic chemist's repertoire is hydroboration, the addition of H−B across a multiple bond:

H3B−OR2 + H2C=CH2 → CH3CH2BH2 + R2O

The C−B bond in the primary product is an intermediate stage in the stereospecific formation of C−H or C−OH bonds.

(d) The Tetrahydridoborate Ion
Key Point: The tetrahydridoborate ion is a useful intermediate for the preparation of metal hydride complexes and borane adducts.

Diborane reacts with alkali metal hydrides to produce salts containing the tetrahydridoborate ion, BH4−:

B2H6(polyet) + 2 LiH(polyet) → 2 LiBH4(polyet)

The BH4− ion is isoelectronic with CH4 and NH4+:

Species BH4− CH4 NH4+
Character Hydridic — Protic
Example 13.1: Using NMR to Identify Reaction Products

Problem: Explain how 11B-NMR could be used to determine whether cleavage of diborane with an NMR inactive Lewis base is symmetrical or unsymmetrical.

Answer: Symmetrical cleavage yields BH3L + BH3L and unsymmetrical cleavage yields [BH2L2]+ and BH4−. In the former, 11B is coupled to three equivalent 1H nuclei, producing a quartet. In unsymmetrical cleavage, the first product has 11B coupled to two equivalent 1H nuclei (triplet), and the second has 11B coupled to four equivalent nuclei (quintuplet).

13.7 Boron Trihalides

Key Point: Boron trihalides are useful Lewis acids, with BCl3 stronger than BF3, and important electrophiles for the formation of boron–element bonds; subhalides with B–B bonds, such as B2Cl4, are also known.

All the boron trihalides except BI3 may be prepared by direct reaction between the elements. However, the preferred method for BF3 is the reaction:

B2O3(s) + 3 CaF2(s) + 6 H2SO4(l) → 2 BF3(g) + 3 [H3O][HSO4](soln) + 3 CaSO4(s)

All the boron trihalides form simple Lewis complexes with suitable bases:

BF3(g) + :NH3(g) → F3B−NH3(s)
Example 13.3: Predicting Reactions of Boron Trihalides

Problem: Predict the products of: (a) BF3 and excess NaF in acidic aqueous solution, (b) BCl3 and excess NaCl in acidic aqueous solution, (c) BBr3 and excess NH(CH3)2 in a hydrocarbon solvent.

Answers:

(a) BF3(g) + F−(aq) → BF4−(aq) [Fluoride is hard and strong base; BF3 is hard and strong Lewis acid]

(b) BCl3(g) + 3 H2O(l) → B(OH)3(aq) + 3 HCl(aq) [B−Cl bonds hydrolyse vigorously]

(c) BBr3(g) + 6 NH(CH3)2 → B[N(CH3)2]3 + 3 [NH2(CH3)2]Br [Protolysis with B−N bond formation]

Subhalides with B−B Bonds

Boron halides containing B−B bonds have been prepared. The best known have the formula B2X4 (X = F, Cl, or Br) and the tetrahedral cluster compound B4Cl4.

The thermal stability of B2X4 derivatives increases with increasing tendency of the X group to form a π bond with B:

B2Cl4 < B2F4 < B2(OR)4 < B2(NR2)4

13.8 Boron–Oxygen Compounds

Key Point: Boron forms boric acid, B2O3, polyborates, and borosilicate glasses.

Boric acid, B(OH)3, is a very weak Brønsted acid in aqueous solution. However, boric acid is in fact primarily a weak Lewis acid:

B(OH)3(aq) + 2H2O(l) ⇌ H3O+(aq) + [B(OH)4]−(aq) pKa = 9.2

In concentrated neutral or basic solution, polynuclear anions form:

3B(OH)3(aq) ⇌ [B3O3(OH)4]−(aq) + H+(aq) + 2H2O(l) pKa = 0.85

Boron Oxide

The most important oxide of B, B2O3, is prepared by dehydration of boric acid:

4 B(OH)3(s) → 2 B2O3(s) + 6 H2O(l)

Boron oxide and silica are the main constituents of borosilicate glass, which, because of the low thermal expansivity due to the strong B−O bonds, is used to make heat-resistant laboratory glassware (such as Pyrex®).

13.9 Compounds of Boron with Nitrogen

Key Point: Compounds containing BN, which is isoelectronic with CC, include the ethane analogue ammonia borane, H3NBH3, the benzene analogue, H3N3B3H3, and BN analogues of graphite and diamond.

The simplest compound of B and N, boron nitride, BN, is easily synthesized by heating boron oxide with a nitrogen compound:

B2O3(l) + 2 NH3(g) → 2 BN(s) + 3 H2O(g) (1200°C)

Borazine: "Inorganic Benzene"

Structure of Borazine (B3N3H6)

B B B N N N

Borazine is isoelectronic and isostructural with benzene (b.p. 55°C)

BOX 13.2: Applications of Boron Nitride
  • Hexagonal BN: thermal insulator, mould release, lubricant (stable in O2 up to 900°C)
  • Cubic BN: hard abrasive for high-temperature applications (where diamond would form carbides)
  • BN nanotubes: suitable for high-temperature conditions, potential hydrogen storage (2.6 wt% H2)
  • Cosmetics: pearlescent sheen in nail polishes, lipsticks; light-reflective properties hide wrinkles

13.11 Higher Boranes and Borohydrides

Key Point: The bonding in boron hydrides and polyhedral borohydride ions can be approximated by conventional 2c,2e bonds together with 3c,2e bonds.

The Origin of Wade's Rules

Wade's rules have been justified by molecular orbital calculations. A B−H bond uses one electron and one orbital of the B atom, leaving three orbitals and two electrons for skeletal bonding:

For [B6H6]2−, there are seven orbitals with net bonding character delocalized over the skeleton, separated by a considerable gap from the remaining 11 largely antibonding orbitals. Seven electron pairs can fill these seven bonding orbitals, giving rise to a stable structure in accord with the (n+1) rule.

Synthesis of Higher Boranes

The controlled pyrolysis of B2H6 in the gas phase provides a route to most higher boranes. The mechanism for tetraborane(10) formation:

B2H6 → BH3 + BH3
B2H6 + BH3 → B3H7 + H2
BH3 + B3H7 → B4H10

Characteristic Reactions

Deprotonation of Large Boron Hydrides

Deprotonation occurs readily with the large borane B10H14:

B10H14 + N(CH3)3 → [HN(CH3)3]+[B10H13]−

Deprotonation occurs from a 3c,2e BHB bridge, leaving the electron count unchanged. The Brønsted acidity of boron hydrides increases with size:

B4H10 < B5H9 < B10H14
Cluster-Building Reactions

The cluster-building reaction between a borane and a borohydride provides a convenient route to higher borohydride ions:

5 K[B9H14] + 2 B5H9 → 5 K[B11H14] + 9 H2
Example 13.6: Elucidating Borane Stoichiometry

Problem: Hydrolysis of a mole of a borohydride yields 11 mol of H2 and 4 mol of B(OH)3. Determine its stoichiometry.

Answer: The hydrolysis reaction is:

BnHm + 3n H2O → n B(OH)3 + [(3n+m)/2] H2

So n = 4 and (3n+m)/2 = 11, which gives m = 10. The compound is B4H10.

13.12 Metallaboranes and Carboranes

Key Point: Main-group and d-block metals may be incorporated into boron hydrides through BHM bridges or more robust B−M bonds. When CH is introduced in place of BH in a polyhedral boron hydride, the charge of the resulting carboranes is one unit more positive; carborane anions are useful precursors of boron-containing organometallic compounds.

Carboranes

Closely related to the polyhedral boranes and borohydrides are the carboranes (carbaboranes), a large family of clusters that contain both B and C atoms.

BH− is isoelectronic and isolobal with CH, so we can expect polyhedral borohydrides and carboranes to be related. For example, C2B3H5 has (5 × 2) electrons from each B−H or C−H bond and an additional electron from each C, giving 12 cluster electrons (six pairs). The (n+1) rule predicts a trigonal bipyramidal structure.

Example 13.8: Using Wade's Rules for Carboranes

Problem: Predict the structure of C2B5H7.

Answer: The number of skeletal electrons is (7 × 2) + 2 = 16, or 8 skeletal electron pairs. The (n+1) rule predicts that the shape is based on a seven-vertex polyhedron, a pentagonal bipyramid. As there are seven vertex atoms, this is a closo structure.

Synthesis of Carboranes

The conversion of decaborane(14) to closo-1,2-B10C2H12:

B10H14 + 2 SEt2 → B10H12(SEt2)2 + H2
B10H12(SEt2)2 + C2H2 → B10C2H12 + SEt2 + H2

At 500°C in an inert atmosphere, 1,2-B10C2H12 isomerizes to 1,7-B10C2H12, which in turn isomerizes at 700°C to the 1,12-isomer.

BOX 13.5: Boron Compounds for Cancer Treatment

Boron Neutron-Capture Therapy (BNCT) involves irradiating 10B-labelled boron compounds with low-energy neutrons. The 10B undergoes nuclear fission:

10B + 1n → 4He + 7Li + 2.4 MeV

The most promising boron-containing compounds are polyhedral borohydrides like Na2B12H11SH. Boron carbide nanoparticles can be introduced into T-cells which then deliver them to tumours.

13.13-13.16 Aluminium, Gallium, Indium, and Thallium

13.13 Hydrides of Aluminium and Gallium

Key Point: LiAlH4 and LiGaH4 are useful precursors of MH3L2 complexes; LiAlH4 is also used as a source of H− ions in the preparation of metalloid hydrides, such as SiH4.

The metathesis of halides with LiH leads to lithium tetrahydridoaluminate or the analogous LiGaH4:

4 LiH + ECl3 → LiEH4 + 3 LiCl (E = Al, Ga)

With the halides of many nonmetallic elements, AlH4− serves as a hydride source in metathesis reactions:

LiAlH4 + SiCl4 → LiAlCl4 + SiH4

13.14 Trihalides of Al, Ga, In, and Tl

Key Point: Aluminium, gallium, and indium all favour the +3 oxidation state, and their trihalides are Lewis acids. Thallium trihalides are less stable than those of its congeners.

The Lewis acidities reflect the relative chemical hardness of the Group 13 elements:

Transmetallation is important for preparing main-group organometallic compounds:

AlCl3(sol) + 3 LiR(sol) → AlR3(sol) + 3 LiCl(s)

13.15 Low-Oxidation-State Halides

Key Point: The +1 oxidation state becomes progressively more stable from aluminium to thallium.

All the AlX compounds, GaF, and InF are unstable, gaseous species that disproportionate in the solid phase:

2 AlX(s) → 2 Al(s) + AlX3(s)

The other monohalides of Ga, In, and Tl are more stable. Gallium(I) and In(I) halides both disproportionate when dissolved in water:

3 MX(s) → 2 M(s) + M3+(aq) + 3 X−(aq) (M = Ga, In)

Thallium(I) is stable with respect to disproportionation in water because Tl3+ is difficult to achieve.

13.16 Oxo Compounds

The most stable form of Al2O3, α-alumina, is a very hard, refractory, and amphoteric material. In its mineral form it is known as corundum and as a gemstone it is sapphire or ruby:

Indium tin oxide (ITO) is In2O3 doped with 10% SnO2, forming an n-type semiconductor that is transparent and electrically conducting. Uses include LCD displays, touch panels, solar cells, and OLEDs.

13.20 Organometallic Compounds

(a) Organoboron Compounds

Key Point: Organoboron compounds are electron-deficient and act as Lewis acids; tetraphenylborate is an important ion.

Organoboranes of the type BR3 can be prepared by hydroboration of an alkene with diborane:

B2H6 + 6 CH2=CH2 → 2 B(CH2CH3)3

An important anion is the tetraphenylborate ion, [B(C6H5)4]− (BPh4−):

BPh3 + NaPh → Na+[BPh4]−

The Na salt is soluble in water but salts of most large, monopositive ions are insoluble. It is useful as a precipitating agent in gravimetric analysis.

(b) Organoaluminium Compounds

Key Point: Methyl- and ethylaluminium are dimers; bulky alkyl groups result in monomeric species.

Alkylaluminium compounds can be prepared by transmetallation:

2 Al + 3 Hg(CH3)2 → Al2(CH3)6 + 3 Hg

Triethylaluminium, Al(C2H5)3, is an organometallic complex of major industrial importance, used in the Ziegler–Natta polymerization catalyst.

In alkylaluminium dimers, the Al−C−Al bonds are longer than terminal Al−C bonds, suggesting they are 3c,2e bonds, analogous to the bonding in diborane.

(c) Organometallic Compounds of Ga, In, and Tl

Key Point: The only +1 oxidation state organocompounds are formed with the cyclopentadienide ligand.

The trigonal planar Ga(III), In(III), and Tl(III) organocompounds, R3Tl, are reactive, air-sensitive compounds. The R3Tl compounds are useful in carbon–carbon bond formation:

R3Tl + R′COCl → R2TlCl + R′COR

The only stable Ga(I), In(I), and Tl(I) organocompounds are those with the cyclopentadienyl ligand, C5H5−, which are useful sources of the cyclopentadienyl ligand in organometallic synthesis.

Summary & Key Trends

Metallic Character

Increases down the group: B (nonmetal) → Al (metalloid) → Ga, In, Tl (metals)

Oxidation States

+3 favored for lighter elements; +1 becomes more stable down group (inert-pair effect)

Lewis Acidity

Incomplete octets lead to Lewis acid behavior; halides and hydrides act as electron acceptors

Cluster Chemistry

Boron forms extensive clusters (boranes, carboranes) with 3c,2e bonding; Wade's rules predict structures

Selected Exercises

13.1 Give a balanced chemical equation and conditions for the recovery of boron.
13.2 Describe the bonding in (a) BF3, (b) AlCl3, (c) B2H6.
13.3 Arrange the following in order of increasing Lewis acidity: BF3, BCl3, AlCl3.
13.14 How many skeletal electrons are present in B5H9?
13.16 (a) From its formula, classify B10H14 as closo, nido, or arachno. (b) Use Wade's rules to determine the number of framework electron pairs for decaborane(14).