Week 2 F11 F - Bonding
Model
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Unformatted text preview: Bonding
Model
 Valence
bond
theory
 •  This
method
describes
bonding
using
hybrid
orbitals
 and
electron
pairs,
as
an
extension
of
the
electron‐dot
 and
hybrid
orbital
methods
used
for
simpler
molecules
 (CH4).

 •  Valence
Bond
(VB)
theory
treats
the
formaBon
of
a
 molecule
as
arising
from
the
bringing
together
of
 complete
atoms
which,
when
they
interact,
to
a
large
 extent
retain
their
original
character.
 •  a
covalent
bond
is
formed
between
the
two
atoms
by
 the
overlap
of
half
filled
valence
atomic
orbitals
of
 each
atom
containing
one
unpaired
electron.
 Valence
Bond
theory
for
Methane
 (CH4)
 valence
bond
theory:
 hybridizaBon
 nomenclature
 •  (s,
pz):
linear;
(s,
px,
py)
 trigonal
planar;
(s,
px,
py,
pz)
 tetrahedral;
(s,
px,
py,
dx ‐y )
 square
planar;
(s,
px,
py,
pz,
 dz );
(s,
px,
py,
pz,
dx ‐y )
 square‐based
pyramidal;
(s,
 px,
py,
pz,
dz ,
dx ‐y )
 octahedral.


 2 2 2 2 2 2 2 2 VB
theory
 •  In
[Cr(NH3)6]3+,
Octahedral
complexes
of
Cr(III)
(d3)
 3d
 4p
 4s
 •  With
the
electrons
from
the
ligands
included
and
a
 hybridizaBon
scheme
applied
for
an
octahedral
 complex
the
diagram
becomes
 3d
 d2sp3
 •  This
diagram
is
appropriate
for
all
octahedral
Cr(III)
 complexes
because
the
three
3d
electrons
always
 singly
occupy
different
orbitals.
 limitaBons
of
VB
theory
 •  Octahedral
Fe(III)
complexes
(d5),
existence
of
both
 HS
and
LS.
 free
Fe3+
ion
 3d
 LS
Fe3+

 e.g.
[Fe(CN)6]3‐
 3d
 4s
 4p
 d2sp3
 HS
Fe3+

 e.g.
[FeF6]3‐
 3d
 sp3d2
 must
use
4d
set
 unrealis)c
because
4d
orbitals
much
higher
in
energy
 Molecular
Orbital
(MO)
theory
 •  VB
theory:
electron
pairs
are
localized
between
two
 specific
atoms
in
a
molecule

 •  but
in
MO
theory
:
electrons
are
distributed
in
sets
 of
molecular
orbitals
which
can
extend
over
the
 enBre
molecule.
 •  Molecular
Orbital
(MO)
theory
allocates
electrons
 to
molecular
orbitals
formed
by
the
overlap
 (interacBon)
of
atomic
orbitals.
 MO
theory
 Important
rules
 •  Molecular
Orbital
arises
form
interacBons
 between
atomic
orbitals
and
such
interacBons
 are:
 –  Allowed
if
the
symmetries
of
the
atomic
orbitals
 are
compaBble
with
one
another.
 –  Efficient
if
the
region
of
overlap
between
the
two
 atomic
orbitals
is
significant.
 –  Efficient
if
the
atomic
orbitals
are
relaBvely
close
 in
energy.
 MO
theory
 •  Important
ground
rule
of
MO
theory
 –  The
number
of
MOs
that
can
be
formed
must
 equal
the
number
of
atomic
orbitals
of
the
 consBtuent
atoms.
 •  To
fill
MOs,
au\au
principle,
lowest
energy.
 MOs
in
H2

 •  Linear
combinaBon
of
atomic
orbital
(LCAOs).
 SchemaBc
representaBon
of
MO
for
H2
 Nomenclature

 •  σ
label
means
that
rotaBon
around
of
the
orbital
 about
the
internuclear
axis
generates
no
phase
 change.
 •  *
label
means
that
there
is
a
nodal
plane
between
 the
nuclei,
orthogonal
to
the
internuclear
axis.
 •  σg(1s)
:
g
stands
for
gerade.
 •  σu(1s)*
:
u
stands
for
ungerade.

 •  g
and
u
labels
depending
on
the
sign
change
of
the
 MO
wavefuncBon



 MO
theory
H2
 •  Each
H
atom
contributes
one
electron.
 •  ΨMO
is
stabilized
while
ΨMO*
is
destabilized.
 •  In
MO
diagram,
construct
the
diagram
first
and
then
 put
in
the
electron
according
to
the
au\au
principle
 (lowest
energy
first).
 •  The
bonding
and
anBbonding
MOs
in
H2
are
labeled
 σ
and
σ*
respecBvely.
 MOs
in
H2

 •  H2
is
diamagneBc
(electrons
are
paired)
 Bond
order
 •  Bond
order
=
1/2[(number
of
bonding
 electrons)‐(number
of
anBbonding
electrons).
 •  Bond
order
vs.
bond
length:
the
lower
the
 bond
order,
the
larger
the
internuclear
 separaBon,
the
longer
the
bond.
 Bonding
in
He2
and
Li2
 •  •  •  •  Bond
order
in
He2??
 Ignore
1s‐2s
overlap
for
Li2.
 Bond
order
in
Li2
(just
look
at
valence
orbitals).

 What
about
Be2?
 Overlap
of
p
orbitals
 By
convenBon
z
axis
is
the
internuclear
axis
 Overlap
of
p
orbitals
 InteracBons
of
p
orbitals
 Nomenclature

 •  π
label
means
that
rotaBon
around
the
orbital
 about
the
internuclear
axis
generates
a
phase
 change.
 •  *
label
means
that
there
is
a
nodal
plane
between
 the
nuclei,
orthogonal
to
the
internuclear
axis.
 •  πu(px)
and
πu(py)
are
ungerade.
 •  πg*(pz)
is
gerade.

 •  g
and
u
labels
depending
on
the
sign
change
of
the
 MO
wavefuncBon



 Overlap
between
AO
not
always
 allowed
by
symmetry
 MO
diagram
for
X2

 X
having
only
2s
and
2p
orbitals
 • 
If
energy
separaBon
of
2s
and
2p
in
F2
is
sufficiently
great
that
only
2s‐2s
and
2p‐2p
orbital
 interacBons
occur.
 • 
difference
in
overlap
between
pz‐pz
orbitals
and
px‐px
or
py‐py
orbitals.

 
What
MO
 diagram
for
F2
 and
O2?
 DiamagneBc/ paramagneBc?
 Experimental
evidence
 Some
of
the
Key
successes
of
MO
theory
 •  Predicts/explains
that
O2
is
paramagneBc.
 •  
Predicts/explains
that
B2
is
paramagneBc
and
the
σ‐ π
crossover.
 paramagneBc:

 one
or
more
unpaired
electrons
 diamagneBc:

 all
electrons
are
paired
(no
unpaired)
 What
happens
if
s‐p
separaBon
small?
 Orbital
mixing
 σ‐π
crossover
between
N2
and
O2
 Effect
of
the
s‐p
mixing
 How
to
deal
with
s‐p
mixing
 •  The
more
stable
orbital

becomes
even
more
 bonding
as
a
result
of
s‐p
mixing.
 •  The
less
stable
bonding
orbital
becomes
 relaBvely
nonbonding,
whereas
in
the
absence
 of
s‐p
mixing
it
was
bonding.
 •  For
the
two
σu*,
upon
s‐p
mixing,
the
lower
 orbital
changes
from
anBbonding
to
relaBvely
 non‐bonding
and
the
upper
orbital
becomes
 even
more
anBbonding.


 Photon
electron
microscopy
 •  UV‐light
or
X‐rays
remove
electrons
from
molecules
 •  one
of
the
more
direct
methods
to
determine
the
 orbital
energies.
 Oxygen
 Nitrogen
 interacBon
of
the
electronic
energy
with
the
vibraBonal
energy
of
the
molecule
 ...
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This note was uploaded on 10/03/2011 for the course CHEM 113A taught by Professor Professornotknown during the Spring '09 term at San Jose State University .

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