Lewis Acids & Bases
• Lewis acid =
electron pair
acceptor (BF3)
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Lewis Acids & Bases
• Lewis acid =
electron pair
acceptor (BF3)
• Lewis base =
electron pair donor
(NH3)
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2
Lewis Acids & Bases
A Lewis acid and base can interact by
sharing an electron pair.
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3
Lewis Acids & Bases
A Lewis acid and base can interact by
sharing an electron pair.
Formation of hydronium ion is an excellent
example.
H
+
ACID
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•• ••
O—H
H
BASE
•• +
H O—H
H
4
Lewis Acids & Bases
Other good examples involve metal ions.
Co
2+
ACID
•• ••
O—H
H
BASE
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Co
2+
•• ••
O—H
H
5
Lewis Acids & Bases
Other good examples involve metal ions.
Co
2+
ACID
•• ••
O—H
H
BASE
Co
2+
•• ••
O—H
H
Such bonds as the H2O ---> Co bond are often
called COORDINATE COVALENT BONDS
because both electrons are supplied by one
of the atoms of the bond.
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6
Lewis Acids & Bases
7
The combination of metal
ions (Lewis acids) with
Lewis bases such as
H2O and NH3
------>
COMPLEX IONS
All metal ions form
complex ions with water
—and are of the type
[M(H2O)x]n+ where x = 4
and 6.
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[Cu(NH3)4]2+
Lewis Acids & Bases
Add NH3 to light blue [Cu(H2O)4]2+ ------>
light blue Cu(OH)2 and then deep blue
[Cu(NH3)4]2+
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9
Lewis Acids & Bases
[Ni(H2O)6]2+ + 6 NH3 ---> [Ni(NH3)6]2+
+ DMG
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10
Lewis Acids & Bases
The Fe2+ in heme can interact with O2 or
CO in a Lewis acid-base reaction.
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11
Lewis Acids & Bases
Many complex ions containing water
undergo HYDROLYSIS to give acidic
solutions.
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12
Lewis Acids & Bases
Many complex ions containing water undergo
HYDROLYSIS to give acidic solutions.
[Cu(H2O)4]2+ + H2O ---> [Cu(H2O)3(OH)]+ + H3O+
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13
Lewis Acids & Bases
Many complex ions containing water
undergo HYDROLYSIS to give acidic
solutions.
This explains why water solutions of Fe3+,
Al3+, Cu2+, Pb2+, etc. are acidic.
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14
Lewis Acids & Bases
This explains AMPHOTERIC nature of
some metal hydroxides.
Al(OH)3(s) + 3 H+ --> Al3+ + 3 H2O
Here Al(OH)3 is a Brønsted base.
Al(OH)3(s) + OH- --> Al(OH)4Here Al(OH)3 is a Lewis acid.
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15
Lewis Acids & Bases
This explains AMPHOTERIC nature of
some metal hydroxides.
Al(OH)3(s) + 3 H+ --> Al3+ + 3 H2O
Here Al(OH)3 is a Brønsted base.
Al(OH)3(s) + OH- --> Al(OH)4Here Al(OH)3 is a Lewis acid.
••
O—H••
••
Al
3+
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16
Amphoterism of Al(OH)3
Al(OH)3 on right
Add NaOH
See Kotz/Treichel, page 830
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Add HCl
17
Neutral Lewis Acid
Carbon dioxide is a neutral Lewis acid.
-0.75
+1.5
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-0.75
18
Lewis Acids & Bases
Many complex ions are very
stable.
Cu2+ + 4 NH3
[Cu(NH3)4]2+
K for the reaction is called
Kformation
or a “formation constant”
Here K = 6.8 x 1012. Reaction
is strongly product-favored.
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19
Lewis Acids & Bases
Formation of complex ions explains why you
can dissolve a ppt. by forming a complex
ion.
AgCl(s) + 2 NH3
Ag(NH3)2+ + Cl-
AgCl(s)
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20
Lewis Acids & Bases
Formation of complex ions explains why you
can dissolve a ppt. by forming a complex
ion.
AgCl(s)
10-10
Ag+ + Cl-
Ag+ + 2 NH3 --> Ag(NH3)2+
Ksp = 1.8 x
Kform = 1.6 x 107
-------------------------------------
AgCl(s) + 2 NH3
Ag(NH3)2+ + Cl-
Knet = Ksp • Kform = 2.9 x 10-3
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