Main group III elements

1
Main group III elements

• Chapter 12,13

2
The Periodic Table of the Elements
Most Probable Oxidation State
1
0
2
3
_4
- 3
- 2
- 1
H
He
Li
Be
B
C
F
O
N
Ne
3
4
2
1
5
Na
Mg
Al
Si
Cl
S
P
Ar
K
Sc
Ca
Ga
Ti
Ge
Br
Se
As
Zn
Cu
V
Kr
Cr
Mn
Fe
Co
Ni
Rb
Sr
Zr
I
Ag
Nb
Xe
Mo
Tc
Ru
Rh
Pd
Y
In
Sn
Te
Sb
Cd
Ba
Hg
Au
Rn
W
Re
Os
Ir
Pt
Cs
La
Tl
Hf
Pb
At
Po
Bi
Ta
Fr
Ra
Ac
Rf
Du
Sg
Bo
Ha
Me
3
Ce
Pr
Nd
Pm
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
Th
Pa
U
Np
Pu
Am
Cm
Bk
Cf
Es
Fm
Md
No
Lr
3
3
Boron

• Boron
• In nature it is found as Borates
• Ulexite NaCaB5O6(OH)6.5 H20
• Borax Na2B405(OH)4. 8 H 20
• Colemanite Ca2B304(OH)32.2 H 20)
• Kernite Na2B4O5(OH)4.2 H20
• Borates do have complex structures, but common to
  all is that Boron is contained as trigonal BO3 or
  tetragonal BO4 units.

4
Boron

• The cations in these minerals are typically
  alkali or alkaline earth cations.
• The largest source of Boron is in the form of
  Borax found in the mojave desert in california
• No ionic compounds involving simple B3 cations
  are formed because the ionization enthalpies for
  boron are so high that lattice energies or
  hydration enthalpies cannot offset the energy
  required for formation of a cation.

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Boron

• Boron is sp2 hybridized in trigonal planes.
• All BX3 planes compounds are strong lewis acids
• interaction with Lewis bases (molecules or ions)
  gives tetrahedral adducts such as BF3.O(C2H5)2
  ,BF4-, and B(C6H5)-4. The formation of such Lewis
  acid-base adducts requires a change to Sp3
  hybridization for boron.

6
Boron

• Isolation of the element
• Boron is made in 95-98 purity as an amorphous
  powder by reduction of the oxide B203 with Mg
• Or Zn

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Uses of Boron

• Borosilicate glass-pyrex
• Detergents
• Flame retardants
• Ceramics

• Pyrotechnics
• Used in production of impact resistant steels
• Control rods in nuclear reactors

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Common Bonds in Boranes

• 2c-2e- B-H
• 3c-2e- B-H-B
• 2c-2e- B-B
• 3c-2e- B-B-B

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Diborane B2H6
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Tetraborane B4H10
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Dodecaborane B12H122-
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Elemental Forms of Boron
b-rhombohedral, B12(B12)12, (B12)(B12)(B60)
a- rhombohdral
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Boron Deltahedra Parent Clusters
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Boron

• The structure of Boranes

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Boron

• The hydrides of Boron
• Diborane
• Lab quantities
• Industrial Quantities

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Boron
17
Boron

• Reactions of Boranes

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Boron

• Deca boranes

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Boron

• Borohydrides of many metals have been made and
  some representative syntheses are

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Boron Hydrides
21
Huheey, J. E. Keiter, E. A. Keiter, R. L.
Inorganic Chemistry principles of structure and
reactivity, 4th ed. New York HarperCollins
College Publishing, 1993. 799.
22
Wades Rules

• n number of B atoms in parent closo-deltahedron
• Always n1 bonding e- pairs and n1 bonding MOs
• nido has n-1 vertices
• arachno has n-2 vertices
• hypho has n-3 vertices

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Using Wades Rules

• Find total available bonding e-s
• Each B-H unit gives 2 e-s
• Each additional H gives 1 e-
• Overall charge
• Find parent closo-deltahedron
• n1 bonding e- pairs
• Is it closo, nido, arachno, hypho?
• Lose highest connectivity B first then lose
  adjacent sites
• Determine number of remaining hydrogen atoms
• Each vertex has a H
• sew up hole with H atoms
• Bridging H atoms
• Low connectivity B atoms can get another 2c-2e-
  B-H bond
• Try to keep it as symmetrical as possible

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Boron Neutron Capture Therapy (BNCT)

• 10B has large cross-section for neutron capture
• 10B ? a 7Li
• Products can kill cells
• Cancer treatment
• Cages - need high 10B in cell

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Boron

• The main resemblances to silicon and differences
  from the more metallic aluminum are as follows
• 1. The oxide B20 3 and B(OHh are acidic. The
  compound Al(OH)3 is a basic hydroxide, although
  it shows weak amphoteric properties by dissolving
  in strong NaOH.
• 2. Borates and silicates are built on similar
  structural principles with sharing of oxygen
  atoms so that complicated chain, ring, or other
  structures result .

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Boron

• 3. The halides of Band Si (except BF3) are
  readily hydrolyzed. The AI halides are solids and
  only partly hydrolyzed by water. All act as Lewis
  acids.
• 4. The hydrides of B and Si are volatile,
  spontaneously flammable, and readily hydrolyzed.
  Aluminum hydride is a polymer, (AlH3)n

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Boron

• Crystalline boron is very inert and is attacked
  only by hot concentrated oxidizing agents.
  Amorphous boron is more reactive. With ammonia
  for instance, amorphous boron at white heat gives
  (BN)x a slippery white solid with a layer
  structure resembling that of graphite, but with
  hexagonal rings of alternating B and N atoms.

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Boron

• Hydrated borates contain polyoxo anions in the
  crystal, with the following important structural
  features
• 1. Both B03 and tetrahedral B04 groups are
  present, the number of B04 units being equal to
  the charge on the anion.
• 2. Anions that do not have B04 groups, such as
  metaborate, B3063-, or metaboric acid, B303(OH)3,
  hydrate rapidly and lose their original
  structures.
• 3. Certain discrete as well as chain-polymer
  borate anions can be formed by the linking of two
  or more rings by shared tetrahedral boron atoms.

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Boron

• Boric acid
• The acid B(OH)3 can be obtained as white needles
  either from borates, or by hydrolysis of boron
  trihalides.
• When heated, boric acid loses water stepwise to
  form one of three forms of metaboric acid, HB02.
  If B(OH)3 is heated below 130C, the so-called
  form-III is obtained, which has a layer structure
  in which B303 rings are joined by hydrogen
  bonding. On continued heating of form-III of
  HB02, between 130 and 150C, HB02-II is formed.

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Boron
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Boron

• Halides
• Boron trihalide is a gas (bp -101 deg C)
• Boron trihalides are the strongest lewis acids.
• They react with Lewis bases
• B-X bonds are somewhat shorter than is expected
  from the sum of the single-bond covalent radii.
  This suggests a delocalized p-bond system

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Al, Ga, In, Tl

• Al is the most common of the elements
• It is produced in pure form by electrolysis, and
  is the most dirty of the industrial processes.
• Costs a lot of energy.
• Main source is Bauxite, a hydrous Al oxide
• Al is attacked by diluted acids, but passivated
  by strong acids.
• Al oxides are used to protect metals (anodized)

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Ga,In,Tl

• They are made from their salts by electrolysis.
• Ga is used mainly in semiconductors with Group V
  elements. (GaAs).
• Tl is a trace element and is very toxic.
• Main use to get rid of spies.

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Oxides

• Al has only one oxide formed Al2O3
• There is an alpha and a gamma oxide.
• Difference is the process and the temperature to
  get alpha or gamma oxide.
• Mixed Al oxides are ruby (Cr3)and sapphire
• (Fe2,Fe3, Ti4)

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Halides

• Halides are formed of all elements, the only one
  that is special is TlI3.
• Tl and I2 form rather a Tl1 and I3- compound
• All halides readily dissolve in benzene

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Aqua ions
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Hydroxides
39

• The most important hydride is LiAlH4
• It is a strong reducing agent and is mainly used
  in organic chemistry
• It is used e.g. to hydrate double bonds

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Summary of group IIIa trends

• 1. Boron
• (a) Forms no simple B3 cation.
• (b) Forms covalent compounds almost exclusively,
  and all polyatomic ions have covalent bonds.
• (c) Obeys the octet rule, the maximum covalence
  being four.
• (d) Forms trivalent compounds that readily serve
  as Lewis acids.

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Summary of group IIIa trends

• (e) Frequently forms polyhedral structures
  boranes and borates.
• (f) Forms an oxide, B203, and a hydroxide, B(OH)3
  both of which are acidic.
• (g) Forms covalent halides that are readily
  hydrolyzed.
• (h) Forms numerous covalent hydrides, all of
  which are volatile, flammable, and readily
  hydrolyzed.
• (i) Forms a stable and important hydride anion,
  BH4-.

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Summary of group IIIa trends

• 2. Aluminum
• (a) Readily forms an important 3 ion, because it
  is electropositive.
• (b) Is much more metallic than boron, and forms a
  greater number and variety of ionic substances.
• (c) Forms both molecular and ionic substances,
  with coordination numbers of six and higher.
• (d) Forms two oxides, only one of which is
  acidic.
• (e) Forms a hydroxide that is weakly amphoteric,
  although mostly basic.
• (f) Forms solid halides that are only partially
  hydrolyzable.
• (g) Forms a polymeric hydride.
• (h) Forms an anionic hydride (AlR-) that is more
  reactive than BH4-.

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Summary of Group IIIa trends

• 3. Gallium, Indium, and Thallium
• (a) Readily give the M3 ion in solution, and
  have a rich coordination chemistry typical of
  metals.
• (b) Form increasingly stable lower valent
  compounds, especially TI.
• (c) Increasinglyformweakercovalent
  bondsondeseentofthegroup,enhancing the formation
  of monovalent compounds.
• (d) Form MX3 halides that are increasingly
  aggregated in the solid state (through halide ion
  bridges) to give coordination numbers of four,
  six, and higher.
• (e) Do not form important EH4- anions, except
  perhaps GaH4-.