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Group IIIA, (Group 13)
Boron, Aluminum, Gallium, Indium, Thallium
GROUP 3A
The elements: B, Al, Ga, In, Tl; all metals except boron
valence electron configuration, ns2 np1 – so the +3 oxidation state is expected to be most stable
Gallium:- unusually low melting point, 303 K
long liquid range; 303 – 2477 K
used in transistors and high T thermometers
The lower oxidation states become more
important down the group
-- ionic Tl exist primarily in the +1 oxidation
state
Related to the inert pair effect – the fact that the
heavier elements lose their np electrons but not
their ns electrons
Lower oxidation state oxides are more basic
BORON
Boron shows nonmetallic properties – all boron
compounds are covalent
There are no ionic B3+ compounds, c.f. Mg2+ and Li+.
Sources
Ulexite硼钠钙石: NaCa[B5O6(OH)6].5H2O
Borax硼砂: Na2[B4O5(OH)4].8H2O
Colmanite: Ca2[B3O4(OH)3].2H2O
Kenite: Na2[B4O5(OH)4].2H2O
Structures of Elements
Al-Tl are all metals, as expected from their R/I values.
Boron would like to be covalent, but it ‘wants’ to make 5 bonds to achieve a Lewis octet.
It only has 4 orbitals though (1x2s+3x2p)
B gets around its problem by forming delocalised polyhedra.
Very important in B chemistry.
Halides
the boron halides are highly reactive
They are volatile, covalent compounds
2B + 3X2 2BX3 X = F, Cl, Br, I
BF3 and BCl3 (gases); BBr3 (liquid); BI3 (low
melting point solid)
BX3 acts as Lewis acids
Boron halides are highly reactive, volatile compounds. The gas BF3 acts as a Lewis acid with ammonia.
Vacant P orbital
General Properties
The compounds which are coordinatively unsaturated (e.g.BCl3) are very strong Lewis acids.
Tetrahedral adducts and anions are common, for example:
BF3:O(C2H5)2 BF4- B(C6H5)4-
Boron Subhalides
BX3 are the most common halides.
Extensive sub-halide chemistry though, e.g. B4Cl4 and B8F12.
B10F12 structurally characterised.
Metal Borides
10B absorbs neutrons strongly. 40 tonnes of boron carbide were dropped on Chernobyl.
Boron carbides also used in light-weight protective armour and bullet-proof clothing.
Metal borides can be prepared by direct combination of elements:
Ca + 6B CaB6
Metal-rich borides (e.g. TiB2) can have higher conductivities than parent metals.
Also much more inert and harder.
B-Rich borides contain B-clusters
Hydrides
boron compounds of hydrogen are
called boranes
Molecular, volatile compounds; BnHm
Simplest borane – diborane, B2H6
Bonding in Diborane
The B-H-B unit is held together by 2e.
This is called a 3 centre - 2 electron bond (3c2e).
The orbital basis can be made up of two sp3 hybrids of the B atoms and two H(1s) orbitals.
The remaining boron orbitals form normal 2c2e bonds to the terminal H’s.
B
B
H
H
3c2e Bonds in Diborane
The two electrons occupy the fully bonding combination, so that the overall bond order between the B and the bridging H is 1/2.
3c2e Bonds
3c2e bonds are occasionally shown in structural diagrams like this:
Bond Energies:
BH 381 kJ/mol
BHB 441 kJ/mol
1.19
1.32
Electron Deficiency
All boranes are electron deficient.
The need to form 3c2e bonds (BHB and BBB) causes the molecules to ‘curl-in’ on themselves.
The more electron deficient the more ‘spherical’ a molecule becomes.
For example [B6H6]2- is more electron deficient than B4H10
Reactions of Boranes
Boranes undergo a wide variety of reactions.
Oxidation to produce oxides
Pyrolysis to produce more complex boranes
Attack by nucleophiles and electrophiles.
Reduction to borane anions.
Reactions with bases.
Often the reactions of boranes can be classified in terms of the reaction of terminal or bridging hydrogens.
Pyrolysis of Diborane
B2H6
B4H10
B5H11
B5H9
B10H14
200oC
100oC
H2/100oC
190oC
120oC
60oC
H2/100oC
Many complex boranes are known and all contain B-H-B bonds.
closo B7H72-
nido B6H10
arachno B5H11
闭(笼)式
开(巢)式
网式
cage
nest
spider’s web
Mechanism of Pyrolysis
Key step is dissociation of B2H6 into highly reactive BH3:
For example, synthesis of B4H10:
B2H6 2BH3
B2H6 + BH3 B3H7 + H2
BH3 + B3H7 B4H10
Handling Boranes
Boranes are TOXIC and combust EXPLOSIVELY in air.
Lots of shielding, gloves & face protection are necessary.
Only use small quantities, so if there is an accident it can be contained.
Use vacuum techniques for handling and transferring.
General Reactions of Diborane
Reaction with oxygen (extremely exothermic)
B2H6 + 3O2 B2O3 + H2O H = -2160 kJ/mole
Reaction with water
B2H6 + 6H2O 2B(OH)3 + 6H2
B2H6 + 6ROH 2B(OR)3 + 6H2
Reaction with weak acids
2B2H6 + 2HCl 2B2H5Cl + H2
Reaction with HCl
Terminal H are replaced
B2H6 + Cl2 2BCl3 + 6HCl
Reaction with Cl2
Reactions of Diborane with Amines
B2H6 + 2NH3 [H2B(NH3)2]+[BH4]-
This reaction is the result of the electron deficient nature of the 3c-2e bridge which are sites for nucleophillic attack. Small amines can give unsymmetrical cleavage of diborane.
[H2B(NH3)2]+ + 2PR3 [H2B(PR3)2]+ + 2NH3
Boronium ion products are tetrahedral and undergo substitution by bases.
If large amines are used like (CH3)3N and pyridine a symmetric cleavage of diborane occurs.
B2H6 + 2N(CH3)3 2H3B N(CH3)3
These products obtained from the symmetrically cleaved diborane are Lewis Acid adducts.
Reactions of Boranes with Reducing Agents.
2B2H6 + 2Na NaBH4 + NaB3H8
B2H6 + NaBH4 NaB3H8 + H2
Reduction of diborane with sodium borohydride also a route to polyhedral anionic borane.
Diborane Summary:
Unique bonding and versatile chemical reactions.
This lead to the a Nobel Prize.
carborane 碳硼烷
Carboranes or carbaboranes are compounds having as the basic structural unit a number of C and B atoms arranged on the vertices of a triangulated polyhedron. Their structure are closely related to those of the isoelectronic boranes.
metallocarborane
Borazine
Inorganic Benzene 无机苯
Inorganic Benzene 无机苯
Formal Charge
Completely different charge sign from formal charge !
Unlike benzene, borazine undergoes addition reactions:
Notice where the H + and the Cl - end up: This illustrates how unrealistic the formal charges on the boron and nitrogen atoms really are!
Like benzene, borazine can form -complexes with transition metals:
As strong acid in H2SO4
Increase acidity by 104 times
Boric Acid
Other Boron Compounds
Boron nitride BN
BN layer structure
Comparison with graphite
Isoelectronic with C-C
CO(NH2)2 + B(OH)3
BCl3 + NH3
Higher T & P can convert
BN to diamond structure,
harder than diamond.
Borax bead reactions
Boron resembles silicon more than aluminum in some ways:
B2O3 and B(OH)3 are acidic rather like SiO2 and Si(OH)4 whereas the Al compounds are weakly amphoteric.
The borates have some features in common with the silicates.
The halide compounds of B and Si are readily hydrolyzed (except BF3) whereas the halide compounds of Al are only partly hydrolyzed.
The B and silicon hydrides are volatile molecular compounds, which inflame in air, while AlH3 is an involatile solid.
Aluminum, Gallium,
Indium and Thallium
Aluminum: Al
KAl(SO4)2 12H2O
Identified in 1807 it was not isolated until 1828.
W hler
AlCl3 + 3K Al + 3KCl
um or ium
It is the most abundant metal iin the earth’s crust.
Aluminum as a precious metal
He didn’t use silverware.
That 9-inch aluminum pyramid, which completes the top of the structure as it narrows to a point, is 100-ounces of solid aluminum, part of the monument's lightning protection system. In the 1880s, aluminum was a rare metal, selling for $1.10 per ounce and used primarily for jewelry. The pyramid was the largest piece of aluminum of its day and was such a novelty that it was displayed at Tiffany's jewelry store in New York before it was placed at the top.
Hall-Heroult Process
Cryolite Na3AlF6
2Al2O3 + 3C 4Al(l) + 3CO2(g)
He was only 23!!
Gallium
Gallium Arsenide
M.P. = 29.78oC
Semiconductor Applications
Electrolysis of GaO and potash.
Now a byproduct of Al production.
liquid 29.78oC - 2403oC
Indium and Thallium
Indium and Thallium were discovered by spectroscopy.
In is rare, toxic, and $$$.
Why is it so important
(16) anion vacancies
(48) lattice anions
(8) b site cations
(24) d site cations
Cubic Bixbyite Structure of InO
oxygen
inequivalent In centers
vacant oxygen (n-type conductivity)
Sn replaces In in a ratio of 5-9% for ITO
Transparent electrode
Aluminum
Aluminum is a silvery white metal. The surface of aluminum metal is covered with a thin layer of oxide that helps protect the metal from attack by air. So, normally, aluminum metal does not react with air. If the oxide layer is damaged, the aluminum metal is exposed to attack. even by water.
Gallium
Semiconductor industry: GaAs, GaP
Diode laser
Indium
Semiconductor industry
Glass sealing alloys
Dental alloys
Thallium
Electronic components: thallium activated NaI crystal in photomultiplier tube
Glass seal
High temperature superconductor
Tl compounds are toxic.
Reaction of Al
Alizarin-S (or sodium alizarin sulphonate) reagent
red precipitate or lake in basic solution, which is fairly stable to dilute
acetic acid.
Alumina, Al2O3
Great hardness ( Mohs scale 9), high mp, chemical inertness
Crystal form containing metal ion impurities:
ruby (Cr III, red)
sapphire (Fe II/III, Ti IV, blue)
oriental emerald (Cr III/V III, green)
Compounds of Aluminum
- form
红宝石(Cr3+) 蓝宝石(Fe3+,Cr3+) 黄玉/黄晶(Fe3+)
Alumina fiber
Very high strength, high resistance to heat <1400 oC, in diameter of 1/25 of human hair, flexible.
Reactive with acids and Bases, used as Absorbent
Absorbent for column chromatography 柱层析 thin layer chromatography TLC
- Al2O3
it finds many applications in abrasives (including toothpaste), refractories, and ceramics,
crystalline alumina, more chemically resistant than amorphous Al2O3.
Existed as [Al(OH)4]- or [Al(OH)6]3-
in basic solutions
Abbreviated as
Hydroxide：Al(OH)3
Amphoteric： Al(OH)3+ 3H+ Al3+ + 3H2O
Al(OH)3+ OH- [Al(OH)4]-
Halides of Al, Ga, and In
Preparation.
M + 3HX(aq) MX3 + 3H3O+
“AlF3” is different.
Al2O3 + 6HF 2AlF3 + 3H2O 700oC
THE ANHYDROUS HALIDES DO NOT EXIST AS “MX3”
Upon melting “AlCl3” a dimer is obtained.
At very high temperature AlCl3 is produced.
2.21
2.33
115o
87o
93o
This is a common structure for all Al halides and MX3 where M = Ga, In and X = Cl, Br, I.
Synthesis of AlCl3：
Halides
Ionic crystal molecular crystal
Molecular crystal：low mp，volatile，soluble in organic solvents，dimerization。
Violent hydrolysis：
AlF3 AlCl3 AlBr3 AlI3
ionic covalent
Trihalides of Aluminum

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