Extramaterial till A1

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The diversity of chemical bonding
Danny Fredrickson Inorganic Chemistry danny_at_inorg
.su.se
Kemisk bindning (KE3770)
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A1. Introduction the diversity of chemical
bonding
What kinds of compounds are around? What kinds of
chemical bonds creating them? What is happening
in these bonds?
Kemisk bindning (KE3770)
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The diversity of chemical bonding

• Chemical systems
• Types of bonding in chemical systems
• Bond strengths
• The total energy of a chemical system
• 5. Atomic interactions

Kemisk bindning (KE3770)
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1. Chemical systems
In chemistry, we look at what nature can build
out of atoms and their valence
electrons.
Starting with electrons and nuclei, atoms and
ions are created
Atoms bind to create molecules, clusters,
covalent crystals, metals
?
-
-
X, Xn, Xn-
Z
-
-
Ions bind to form ionic crystals
graphite
NaCl
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1. Chemical systems
In chemistry, we look at what nature can build
out of atoms and their valence
electrons.
Molecules further bind to form molecular
crystals, van der Waals complexes,
charge-transfer complexes, and liquids
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Chemical systems an amazing variety

• MgO perfect insulator YBa2Cu3O7
  superconductor
• Na metal soft, low melting W metal hard,
  high melting
• Ca metal bad conductor Cu metal
  fantastic conductor
• C millions of compounds Si few
  thousands compounds
• CO2 gaseous molecule SiO2 like glass

Diversity of structure and bonding
Diversity of properties
?
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Structural and Physical Diversity, Theoretical
Schemes
2c-2e bonding
Valence compounds 2c-2e bonding
B2H6
bcc
VSEPR
3c-4e bonding
Boron clusters
Te64
ligand field theory
colours, magnetism
Isolobal analogy
Nb6I11 MoCl2
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Inorganic, Organic, and Organometallic Chemistry
Where are the borders?
The coordination environment of Fe in
myoglobin
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Inorganic, Organic, and Organometallic Chemistry
Where are the borders?
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Molecular and Solid State Chemistry
Where are the borders?
Molecular
Solid state
Rh coordination compound
Rh carbonyl cluster
Rh metal
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Molecular and Solid State Chemistry
Where are the borders?
Molecular
Solid state
BF4 anion
B12H122 cluster compound
Elemental B structure
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Chemical bonding three extremes
Ionic
Metallic
Covalent
van Arkel-Ketelaar bond triangle
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Electronegativity (EN) our guide to bonding
character
For a bond between elements A and B How ionic
ENA-ENB ?EN How covalent vs. metallic

?EN
EN
ENAENB

EN
2
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Regions in the bond triangle
?EN
EN
Metallic-metallic or metallic-metalloid
Covalent-covalent or covalent-metalloid
clearly metallic
clearly covalent
metalloid
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Aspects of covalent bonding
Composition elements involved in bond, A and
B, have high and similar electronegativites. Nat
ure of bonds electron pairs shared between two
or more atoms. Structures large variety of
systems, from small molecules to crystals
often small number of bonds 1 electron pair for
each bond geometries determined electron
count (VSEPR concept) Examples Systems
following the octet rule, also known as the (8-N)
rule.
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Aspects of ionic bonding
Composition large electronegativity difference
between elements A and
B. Nature of bonds electrostatic interactions
between A and B. Structures solid state
structures based on maximizing AB contacts,
while minimizing AA and BB
contacts. Examples
CsCl
NaCl
ZnS
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Aspects of ionic bonding
Radius ratio rules B tends to be much bigger
than A. The AB contacts are then
restrained by the B ions bumping into each
other. For the AB compound to form in a given
structure the A ion should be big enough to
fill the spaces left by the B ions. This
leads to expected radius ratios (rA/rB) for each
ionic crystal structure.
rA/rB
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Aspects of metallic bonding
Composition Elements have low
electronegativity, (electrons should
be loosely held). Nature of bonding Very
difficult to describe. Images for this type of
bond range from a gas of electrons bathing a
lattice of cations, to very delocalized covalent
bonds. Structures simple or complex
structures, with many short contacts (bonds) per
electron.
Mg
Al
Mg
Mg
Mg17Al12
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Kinds of covalent bonds
The 2 center-2 electron bonds (2c-2e)
Non-polar bonds homonuclear, or similar
electronegativity Polar bonds elements of
quite different electronegativity Dative
(donor-acceptor) bonds one atom brings both
electons
C (diamond)
CH4
Graphite fluoride
CF4
NH3BF3
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Kinds of covalent bonds
Multiple bonds
Double bonds
3-
Cl
Cl
Cl
Cl
Cl
Triple bonds
W
W
Cl
Cl
Cl
Cl
Quadruple bonds
Delocalized ?-bonding
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Kinds of covalent bonds
The 3 center-2 electron bonds (3c-2e)
Did you know that you never really see BH3?
Other examples
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Intermolecular interactions (van der Waals
interactions)
Interactions from weak electrostatic forces
between atoms and molecules, resulting
from their permanent or temporary electric
fields.
These can cause atoms or molecules to form
liquids, or
Molecular crystals
Secondary structures of polymers, such as
proteins.
Dannys opinion generally, these are given too
much creditthey are used to hide interactions
that are not understood yet.
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Intermolecular interactions (van der Waals
interactions)
Moving down the periodic table, van der Waals
interactions become stronger, blurring the
distinction between real covalent bonds and
weaker interactions.
Chain packing with increasingly Strong interchain
contacts going From S to Se to Te
Covalently bound Chain in S, Se and Te
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Intermolecular interactions (hydrogen bonding)
?
?-

XH Y
X Electronegative element, such as O or N
Much stronger than van der Waals interactions.
Electrostatics not the whole story.
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Summary of bonding ideas
Weaker interactions
Three extreme types of bonds
I
van der Waals
C
M
?
?-

XH Y
Hydrogen bond
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The diversity of chemical bonding

• Chemical systems
• Types of bonding in chemical systems
• Bond strengths
• The total energy of a chemical system
• 5. Atomic interactions

Kemisk bindning (KE3770)
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The strength of a bond
Bond energy ? difference in the total energy
between (1) a chemical system, and (2) all
its noninteracting components
tot
Energy
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The strength of a bond
molecules, covalent crystals, and metals
Molecules. Ebond for a molecule is called its
dissociation energy, the energy of the reaction
which breaks a molecule into its component
atoms CH4(g) ? C(g) 4 H(g)
DCH4 (EC 4EH) - ECH4
1660 kJ/mol This measures the
strength of all four C-H bonds. Dividing by the
number Of bonds gives the energy per bond, the
bond dissociation energy DC-H DCH4
/4 1660 kJ/mol 415 kJ/mol Covalent
crystals and metals. The Ebond is given in
energy per formula unit, and is called the
atomization energy C(s, diamond) ? C(g)
Ebond 720 kJ/mol Na(s) ?
Na(g) Ebond 105 kJ/mol
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Atomization energies of the elements
Ebond
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Dissociation energies of the diatomic molecules


Note connection to of multiple bonds
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Bond strengths in salt-like crystals
Ebond for salts the lattice energy U, the
energetic cost for separating the
salt into its individual ions.
Ebond UNaCl (ENaECl-) ENaCl 785 kJ/mol
This lattice energy can be mainly
electrostatic in origin, depending on where
exactly we are on the bond triangle.
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Bond strengths in molecular crystals, liquids
Ebond is again the lattice energy U, and is the
reaction energy for separating the individual
molecules from each other (the result of the
reaction is a gas, so this is a sublimination
energy).
Example
Sb(C5H5)3(g)
Sb(C5H5)3(s)
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The diversity of chemical bonding

• Chemical systems
• Types of bonding in chemical systems
• Bond strengths
• The total energy of a chemical system
• 5. Atomic interactions

Kemisk bindning (KE3770)
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The total energy of a system
At the most basic level, the energy of a chemical
system is given by adding the total kinetic and
potential energy of all the electrons and nuclei
involved.
all electrons, j all nuclei, k
all pairs of nuclei k, n
all pairs of electrons j,m
0
all electrons
all nuclei
Zkee
ZkeZne
e e
?½mevj2 ?½mkvk2 ?? ??
??
E
4??0Rjk
4??0Rkn
4??0Rjm
j
k
j
k
k
n
j
m
Born-Oppenheimer Approximation
A more informative way to express the energy is
to express it as a sum of attractive and
repulsive parts E Eattract Erep
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Two ways of breaking down the total energy
E Ekinetic Epotential Eattract Erep

• How are these two divisions related?
• Potential energy is easy to divide into
• attractive and repulsive parts.
• 2. The kinetic energy is more tricky
• For quantum phenomena,such as
• covalent bonding, the role of the
• kinetic energy is complicated. For
• more classical phenomena (ionic
• or intermolecular interactions), the
• kinetic energy role is small.

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The diversity of chemical bonding

• Chemical systems
• Types of bonding in chemical systems
• Bond strengths
• The total energy of a chemical system
• 5. Atomic interactions

Kemisk bindning (KE3770)
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Classification of atomic interactions
2 basic types closed-shell and shared-electron
interactions
closed-shell interactions
Interactions between ions, noble gas atoms,
molecules, or anything without dangling
electrons. Very short-range repulsive from
Pauli exclusion principle. Longer-range mainly
electrostatic in nature. Throughout these
interactions, the interacting species keep their
identity.
shared-electron interactions
Interactions involve sharing of electrons, which
alters the underlying quantum mechanical
structure of the species involved. Quantum
mechanics is a must here.
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Classical description of chemical bonding

• Approximation
• Only electrostatic interactions (no Ekin for e-)
• Large particles keeping there individuality.
• Ions - ionic bonding
• Molecules - van der Waal
• Noble gases - van der Waal
• Ebind ?Etot ?V
• Molecular mechanics (geometry of molecules) ?Etot
  ?Epot

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Closed-shell interactions 1 Ionic bonding
ZAeZBe
Electrostatic attraction, Coulomb energy
Eattr Born model for repulsion Erep
4??0RAB
c
n,c empirical constant numbers that work
Rn
Now, we just add up these terms for every pair of
ions in the salt crystal. For this, well
need to look at the structure...
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Closed-shell interactions 1 Ionic bonding
Consider NaCl each ion has a shells of
neighbors, each with contributions to
Eattr and Erep
Madelung constant
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Closed-shell interactions 1 Ionic bonding
Madelung constant
Each ionic crystal structure has its own
characteristic Madelung constant
M(CsCl) 1.763 M(NaCl) 1.748 M(ZnS) 1.641
These terms for each ion, plus the repulsive Born
portion combine to give the total lattice energy.
The calculated lattice energy can be in nice
agreement with experiment for very ionic
compounds. NaCl theory 776
kJ/mol, experimental 782 kJ/mol
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Closed-shell interactions 2 van der Waals
Several physical origins for these interactions
Interactions between molecular dipole moments
Interaction Energy? 1/R3
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Closed-shell interactions 2 van der Waals
Several physical origins for these interactions
Interaction between a dipole moment and an
induced dipole moment
Interaction Energy? 1/R6
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Closed-shell interactions 2 van der Waals
Several physical origins for these interactions
Interaction between a temporary dipole, and
another temporary dipole induced
by it.
Interaction Energy? 1/R8
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Closed-shell interactions 2 van der Waals
The Lennard-Jones Potential for modeling these
interactions
?, ? empirical parameters