Transcript Main group III elements

Chapter 12,13
The Periodic Table of the Elements
Most Probable Oxidation State
+1
0
+3 +_4 - 3
H +2
Li Be
B C N
+1 + 2 Al Si P
Na Mg +3 +4 +5
K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As
Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb
Cs Ba La Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi
Fr Ra Ac Rf Du Sg Bo Ha Me
+3
+3
-2 -1
He
O F Ne
S Cl Ar
Se Br Kr
Te I Xe
Po At Rn
Ce Pr Nd PmSm Eu Gd Tb Dy Ho Er TmYb Lu
Th Pa U Np Pu AmCmBk Cf Es FmMd No Lr
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Boron:
In nature it is found as Borates:
Ulexite : {NaCa[B5O6(OH)6].5 H20}
 Borax : {Na2[B405(OH)4]. 8 H 20}
 Colemanite: {Ca2[B304(OH)3]2.2 H 20)}
 Kernite: {Na2[B4O5(OH)4].2 H20}
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Borates do have complex structures, but
common to all is that Boron is contained as
trigonal BO3 or tetragonal BO4 units.
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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 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.
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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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Borosilicate glasspyrex
Detergents
Flame retardants
Ceramics
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Pyrotechnics
Used in production of
impact resistant steels
Control rods in
nuclear reactors
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2c-2e3c-2e2c-2e3c-2e-
B-H
B-H-B
B-B
B-B-B
Huheey, J. E.; Keiter, E. A.; Keiter, R. L.
Inorganic Chemistry: principles of
structure and reactivity, 4th ed. New
York: HarperCollins College Publisher,
1993. 790.
Huheey, J. E.; Keiter, E. A.; Keiter, R. L.
Inorganic Chemistry: principles of
structure and reactivity, 4th ed. New
York: HarperCollins College Publisher,
1993. 794.
a- rhombohdral
b-rhombohedral,
B12(B12)12, (B12)(B12)(B60)
Housecroft, C. E.; Sharpe, A. G.
inorganic Chemistry. New York:
Pearson Education Limited, 2001. 251-2.
Housecroft, C. E.; Sharpe, A. G.
inorganic Chemistry. New York:
Pearson Education Limited, 2001. 275.
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The structure of Boranes:
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The hydrides of Boron:
Diborane:
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Lab quantities:
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Industrial Quantities:
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Reactions of Boranes:
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Deca boranes:
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Borohydrides of many metals have been made
and some representative syntheses are:
Housecroft, C. E.; Sharpe, A. G.
inorganic Chemistry. New York:
Pearson Education Limited, 2001. 272.
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.
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n = number of B atoms in parent closodeltahedron
Always n+1 bonding e- pairs and n+1 bonding
MOs
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nido has n-1 vertices
arachno has n-2 vertices
hypho has n-3 vertices
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Find total available bonding e-s:
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Find parent closo-deltahedron
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Each B-H unit gives 2 e-s
Each additional H gives 1 eOverall charge
n+1 bonding e- pairs
Is it closo, nido, arachno, hypho?
Lose highest connectivity B first then lose adjacent sites
Determine number of remaining hydrogen atoms
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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
Huheey, J. E.; Keiter, E. A.; Keiter, R. L.
Inorganic Chemistry: principles of
structure and reactivity, 4th ed. New
York: HarperCollins College Publisher,
1993. 798.
 10B
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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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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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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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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.
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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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 130°C, the so-called formIII 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 150°C, HB02-II is formed.
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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 πbond system
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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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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.
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Main use to get rid of spies.
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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 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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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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1. Boron
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(a) Forms no simple B3 +cation.
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(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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(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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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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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-.