Download 6.5 Main Group

6.4 Group 13: B, Al, Ga, In, Tl
READING: Chapter 12, Intro & Sections 12.3 – 12.6, 12.11 – 12.18; Chapter 4, Section 4.3.-4.9
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Trivalent: B, Al, Ga.
Mono- and trivalent: In, Tl.
Inert Pair effect: Although the ionization energy decreases down the group with increasing
atomic radius (heavier elements form cations more readily), the heavier elements also show
greater stability of M+ (ns2np0). One possible explanation is that the ns2 electrons are harder to
remove due to a relativistic contraction experienced by heavier elements (RECALL!!)
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The first element, boron is unique and metalloid; all others are metals.
This is an example of the diagonal relationship: The first element of a group often shows
properties/reactivity patterns similar to the second element of the next group to the right – this
particularly important for the first 4 groups:
H
Li
Be
B
C
Na
Mg
Al
Si
Explanation: The charge densities (defined as the ratio charge/radius) of the corresponding
cations (Li+/Mg2+, Be2+/Al3+, B3+/Si4+) are comparable.
→ These pairs show a similar balance of covalent/electrostatic bonding to more electronegative
partners and therefore similar reactivity patterns.
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Electronegativity: B > Al < Ga > In > Tl
The anomaly in the general trend to lower EN down the group is called the alternation effect
and results from increased effective nuclear charges of the 4p elements due to the presence of
their poorly shielding 3d electrons. (The alternation effect is also seen in Group 14 where Ge is
more electronegative than Si.)
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Lewis acidity: Neutral compounds of Group 13 elements tend to be Lewis acids.
Boron:
Typical compounds:
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Boranes: B2H6 (diborane is a dimer of BH3);
B10H14, B6H10, B4H10, etc… a large variety
of cage-like structures). Flammable!
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Tetrahydroborate ion: NaBH4 and LiBH4 (drying agents, like LiAlH4; reducing agents)
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Trihalides: BF3 (gas), BCl3 (liquid), BI3 (solid). Monomeric. Lewis acids. Trigonal planar.
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Oxygen compounds: B2O3 (a glass), B(OH)3 (boric
acid), Na2(B4O7)•10 H2O (Borax)
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Nitrogen compounds: (BN)n; H3BNH3, B3N3H6
Lewis acid: an electron pair acceptor.
Lewis base: an electron pair donor.
I.e., has an energetically low lying LUMO.
I.e., has an energetically high lying HOMO.
Adduct: Reaction of a Lewis acid with a Lewis base results in a coordination compound.
Def.: A coordination compound (or complex) is a chemical entity that consists of two or more
components that can also exist “by themselves”.
For an adduct to form, the symmetries of the HOMO (base) and the LUMO (acid) must match!
Note: The shapes and thus the symmetries of the acid and base components may change from
their “free” state in order to meet this condition.
An example: BH3-CO
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During the reaction, the symmetry of diborane (D2h) changes to that of two C3v BH3
fragments, i.e. analogous to NH3
BUT, unlike NH3, the 3a1 orbital is empty forming the LUMO of the acid.
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The MOs of CO and NH3 are found in your lecture notes.
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The two fragments combine to give the only known main group carbonyl coordination
compound.
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Just as AO's of individual atoms can be combined to give MO's, the MO's of the fragments
BH3(C3v) and CO(C v) can be combined to give the adduct H3B-CO:
∞
Source: Purcell + Kotz, Inorganic Chemistry, 1977, Holt-Saunders
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The important interaction is the new σ-bond between
the LUMO of the acid BH3 (a1 in C3v) and
the HOMO of the base CO (Σ+ = a1 in C v).
∞
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There is also a much weaker secondary retrodative π-interaction between
the HOMO of the acid (e symmetry) and
the LUMO of the base (also e symmetry)
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The last point leads to a phenomenon called synergistic bonding (give a little, take a little),
which is extremely important in transition metal complexes (→ CHEM 365).
HOMEWORK: Sketch the MOs and construct an MO energy diagram for the formation of the
BH3NH3 adduct. To which hydrocarbon is this analogous?
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Boron Nitride – inorganic graphite (replace all atoms with carbon and you get graphite).
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High temperature lubricant – used in rocket engines and
nuclear reactors.
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Insulator (graphite is a two-dimensional conductor).
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Also exists in a diamond-like cubic allotrope that is
formed at high pressure (> 50000 bar).
Def.: Allotropes are compounds or elements of the same bulk compositions, but different
molecular structure and hence chemical and physical properties.
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The structure of the cubic diamond-like allotrope
of (BN)n: all atoms have a Td environment.
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Replace all atoms with carbon and you get
diamond, which is thus isoelectronic and
isostructural to cubic boron nitride.
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Both materials are very hard, in fact diamond is the hardest material known – a direct
consequence of the molecular structure.
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The most important application of (synthetic) diamond is in drill bits for technical and
geological applications, e.g. this one …
… was used here
… for the US-DOE geothermal energy program.
Diamonds are a drill’s best friend!
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Isoelectronic and Isostructural Compounds:
Def.: Two molcules are isoelectronic, if they possess the same number and types of bonds and
the same total electron count, even though they consist of different atoms.
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By necessity, isoelectronic molecules are also isostructural.
Not surprisingly, isoelectronic compounds are often (but not always!) similar in their
reactivity patterns and bulk material properties.
Examples:
• The graphite- and diamond-like modifications of boron nitride are isoelectronic to carbon
based graphite and diamond, respectively – see above.
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Borazine and benzene:
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Methane and aluminum tetrahydride:
Aluminum, Gallium, Indium, Thallium:
Typical compounds:
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Halides: mono- and tri-valent halogenides AlX3, GaX3, TlX.
AlX3 is dimeric:
Lewis acidity reflects hardness. Acidity toward hard bases: BCl3 > AlCl3 > GaCl3
Acidity toward soft bases: GaX3 > AlX3 > BX3
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Oxides: Al2O3 (alumina), K2SO4Al2(SO4)3 = potash alum
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The elements:
• Aluminum: use in the automotive and aerospace industry as DURALUMINIUM alloyed with
Mg and Cu; in ship building as HYDRONALIUM, alloyed with 3-12 % Mg – with disastrous
consequences in the BC SeaCat Ferry building program and the Falkland War:
Al/Mg + n O2(g) → Al2O3 + MgO + lots of heat !
in water:
Al/Mg + n H2O(l) → Al2O3 + MgO + n H2(g) !
…, i.e. Mg and Al burn underwater once ignited !
This reaction has a high activation energy, because the metal passivates itself with an oxide layer
a few µm thick – a very common phenomenon with reactive metals.
Aluminum forms an amalgam with mercury. Although mercury will
not react with the aluminum oxide layer, even a small scratch
exposing the metal aluminum can start the formation of the amalgam.
For this reason, mercury is banned from transport on aircrafts.
Thermite is a solid mixture of metal and metal oxide, often
aluminum metal and iron oxide. The aluminum metal reduces
the iron oxide in an extremely exothermic reaction used for
railway welding.
Fe2O3 + 2Al → 2Fe + Al2O3 + heat
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Gallium has the largest liquid range of all elements. It melts at
29.76 °C and boils at 2204 °C. It is therefore used in hightemperature thermometers.
Thallium is extremely toxic causing liver and kidney failure, deafness and blindness. Tl2SO4
has been used as rat poison in the past. (See: thallium poisoning of ex-KGB spy Alexander
Litvinenko.)
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Amphoterism and Isopolyoxo Ions
Amphoterism: Being able to do two opposite things at the same time.
In chemistry: Acid/Base amphoterism, Redox amphoterism
Definitions:
Acidic oxide: Generates an acidic species upon dissolution in water:
Based on these equilibria, what could be a potential effect of an increased CO2 concentration in
atmosphere on the world's oceans?
Basic oxide: Generates a basic species upon dissolution in water:
… if you combine the last three reactions with this one
CaCO3 + heat → CaO + CO2(g)
… you can build castles, bridges, houses, etc. - why ?
Amphoteric Oxide: Can react with both acids and bases, acting either as an acidic or basic
oxide, depending on conditions:
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Amphoteric behaviour of oxides is primarily observed for BeO, Al2O3, Ga2O3, SnO2, and
Sb2O3.
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In general the elements on the borderline metallic/non-metallic show amphoterism:
Q. Why do some elements form acidic oxides, some basic oxides and some amphoteric oxides?
Element oxides and their reaction in water
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For all elements except fluorine (why not?) and the noble gases (with the exception of xenon)
the following reactions are possible:
1)
x E + y/2 O2(g) → ExOy
2)
ExOy + y H2O → E(OH)2y
3)
E(OH)2y → E(O)(OH)2y-2 + H2O
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But for which elements do the hydroxides react as bases and for which as acids?
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Are there any trends and can we rationalized them?
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Periodic trends in oxo-acids and oxo-bases:
Metals
Semi-metals
Non-metals
weak acid
B(OH)3
NaOH
Mg(OH)2
Al(OH)3
Si(OH)4
PO(OH)3
SO2(OH)2
ClO3(OH)
Ga(OH)3
In(OH)3
Tl(OH)3
strong base
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weak base
strong acid
The more electro-positive the oxygen binding partner, the more ionic the element-oxygen
bond and the easier OH- ions are formed → basic reaction in water.
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The more electronegative the oxygen binding partner, the more covalent the element-oxygen
bond and the easier H3O+ ions are formed → acidic reaction in water.
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With increasing valence, the more OH functional groups can be accommodated at the central
atom…however the tendency to eliminate water also increases.
E(OH)n → E(O)(OH)n-1 + H2O
This introduces E=O bonds, which due to their electron withdrawing effect increase the
polarization of the E --O-H + bond system further → acidic reaction in water.
δ
δ
A. Correlates with the nature of the element oxygen bond (E-O):
Basic oxides:
Ionic bond
Amphoteric oxides:
Both, ionic and covalent contributions
Acidic oxides:
Covalent bond
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For the transition metals (cf. CHEM 365) the basicity/acidity of their oxide is a function of
their oxidation states, which are much more variable than for the main group elements.
Source: Shriver & Atkins, Inorganic Chemistry, 3rd Edition, Freeman.
HOMEWORK: Exercises 4.10 – 4.18; 12.2 – 12.8, 12.14
Isopolyoxo Ions: Al as an example
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In strongly acidic solution at pH < 3 the hexaaqua species [Al(OH2)6]3+ is stable.
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With increasing pH > 3 condensation reactions take place (liberating water and a proton)
yielding higher aggregates until precipitation occurs:
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In the higher aggregate each Al atom is surrounded by six oxygens in an octahedral
environment.
Exception: The central Al atom in [Al13O14(OH)24(H2O)12]7+ is tetrahedral.
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The average charge of a single Al atom decreases with aggregation.
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At pH values > 10 the isopolyoxo ions re-dissolve
due the formation of Al(OH)4-.
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TAKE HOME MESSAGE: Aluminum equipment
will dissolve in your acid bath and in your base bath!
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Similar phenomena occur with B2O3, SiO2, and
P2O5, BUT in these cases the isopolyoxo ions are
anions, as these compounds are acids.
Source: Shriver & Atkins, Inorganic Chemisty, 3rd
Edition, Freeman.
6.5 Group 14: C, Si, Ge, Sn, Pb
READING: Chapter 13 Intro & Sections 13.5, 13.8 - 13.10, 13.13; Chapter 3, Section 3.18
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Tetravalent (or divalent, e.g., carbenes R2C: ). PbIV is very oxidizing (inert pair effect).
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Carbon is non-metallic and easily forms carbon-carbon bonds → diamond, graphite, bucky
balls, (bio)organic & polymer chemistry.
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Silicon, germanium and the α-modification of tin have a diamond structure and are semiconductors.
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β-tin and lead are metallic and conduct electricity.
Typical compounds:
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Hydrides : CnH2n+2 (many short- and long- chain hydrocarbons)
SiH4 (silanes up to Si4H10)
EH4 (E = Ge, Sn, Pb less stable; no catenation of hydrides)
Def.: Catenation is the formation of chains of like atoms. (-E-E-E-)
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Organyls: Many catenation compounds of Si, Ge, and Sn are
known with substituent CH3 or C2H5 groups, including chains,
rings, cages, etc.
SnxRy comounds are considered organometallic. Tin organyls (e.g.
Bu3SnH) are important radical reagents in organic chemistry and
also find use as pesticides (e.g. R3SnX), and stabilizers for PVC
(e.g. R2SnX2.)
Pb(C2H5)5 was used as an anti-knock agent for Otto engines. It
prevents early ignition by acting as a radical scavenger.
Like all lead organyls it is very toxic. It has been replaced by MTBE
(methyl-t-butyl ether) in most of the first world nations.
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Oxides:
CO2 (carbon dioxide), etc.;
(SiO2)n (quartz, silica gels, glass, ceramics, rocks),
Aluminosilicates (rocks, zeolites) e.g., type A zeolite Na12(AlO2)12(SiO2)12·xH2O
Silicones (condensation polymers of di-/mono-alkyl-silane-diols and -triols)
PbIVO2 (The primary use of Pb today is in lead-acid batteries - rechargeable car,
truck, submarine). PbO2(s) + HSO4-(aq) + 3H+(aq) + 2e- → PbSO4(s) + 2H2O(l)
Source: Inorganic Chemistry: An Industrial and Environmental Perspective; T. W. Swaddle,
Academic Press, 1997.
Some interesting trivia
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From antiquity up to ca.1910, “lead white” was used as a white pigment (lead white = PbCO3
· x Pb(OH)2 ). Rembrandt used lead white in his paintings. The typical dark brown
appearance of the old Dutch masters is an artifact of the formation of brown lead sulfide PbS:
PbCO3 • x Pb(OH)2 + (x +1) H2S → (x+1) PbS + CO2 + (x+2) H2O
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Through the use of lead piping the Romans found out that Pb(OAc)2 tastes very sweet and
used to add it to their wine. Some historians claim that this habit contributed to the collapse
of their empire – they were all a little dumber than they should have been …
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Semiconductors
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Silicon and germanium find extensive use in the electronics industry as semiconductors.
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Doping of Si or Ge with other elements can lead to extrinsic semiconductors with specific
properties (i.e., “tuned” band gaps).
p-type semiconductors: “hole doped” semiconductors
If one atom of Ge is replaced by an atom of Ga, but the structure of the solid stays the same, the
valence band will be one electron short…there will be a “hole” in the valence band!
n-type semiconductors: “electron doped” semiconductors
If one atom of Ge is replaced by an atom of As, the conduction band will have one electron!
HOMEWORK: Exercises 13.4 – 13.6, 13.8, 13.11 – 13.13
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