What is electronic spectroscopy?
Absorption of radiation leading to electronic transitions within a molecule or
complex
Absorption
Absorption
[Ru(bpy)3]2+
104
[Ni(H2O)6]2+
10
200
400
UV
~14 000
700
25 000
visible
visible
50 000
UV
n / cm-1 (frequency)
l / nm (wavelength)
UV
=
higher energy transitions
- between ligand orbitals
visible
=
lower energy transitions - between d-orbitals of transition metals
- between metal and ligand orbitals
Absorption maxima in a visible spectrum have three
important characteristics
1.
number (how many there are)
This depends on the electron configuration of the metal centre
2.
position (what wavelength/energy)
This depends on the ligand field splitting parameter, Doct or Dtet and on the
degree of inter-electron repulsion
3.
intensity
This depends on the "allowedness" of the transitions which is described by
two selection rules
Absorption of light
[Ti(OH2)6]3+ = d1 ion, octahedral complex
white light
400-800 nm
3+
blue: 400-490 nm
Ti
yellow-green: 490-580 nm
red: 580-700 nm
A
This complex is has a light purple
colour in solution because it absorbs
green light
l / nm
lmax = 510 nm
The energy of the absorption by [Ti(OH2)6]3+ is the ligand-field splitting, Do
ES
ES
eg
hn
eg
Do
GS
t2g
complex in electronic
Ground State (GS)
[Ti(OH2)6]3+
GS
t2g
complex in electronic
excited state (ES)
d-d transition
lmax = 510 nm
Do is 
243 kJ mol-1
20 300 cm-1
An electron changes orbital; the ion changes energy state
d2 ion
Electron-electron repulsion
x2-y2
z2
eg
z2
x2-y2
t2g
xy
xz
yz
eg
t2g
xy
xz
yz
xy + z2
z
xz + z2
z
y
x
y
x
lobes overlap, large electron repulsion lobes far apart, small electron repulsion
These two electron configurations do not have the same energy
Selection Rules
Transition
e
complexes
Spin forbidden
Laporte forbidden
10-3 – 1
Many d5 Oh complexes
[Mn(OH2)6]2+
1 – 10
Many Oh complexes
[Ni(OH2)6]2+
10 – 100
Some square planar complexes
[PdCl4]2-
100 – 1000
6-coordinate complexes of low symmetry,
many square planar complexes particularly with
organic ligands
102 – 103
Some MLCT bands in cxs with unsaturated
102 – 104
Acentric complexes with ligands such as acac,
with P donor atoms
103 – 106
Many CT bands, transitions in organic species
Spin allowed
Laporte forbidden
Spin allowed
ligands
Laporte allowed
or
Tanabe-Sugano diagram for d2 ions
10
e
[V(H2O)6]3+: Three spin allowed transitions
5
E/B
30 000
20 000
n1 = 17 800 cm-1
visible
n2 = 25 700 cm-1
visible
10 000
n / cm-1
n3 = obscured by CT transition in
UV
25 700 =
1.44
D/B
17 800
n3 = 2.1n1 = 2.1 x 17 800
D/B = 32
 n3 = 37 000 cm-1
=
32
Magnetism
macroscopic world
« traditional, classical » magnets
N
S
macroscopic world
A pioneering experiment
by M. Faraday
« Farady lines of forces »
about magnetic flux
N
S
macroscopic world
« traditional » magnets
N
N
S
S
attraction
N
S
N
S
macroscopic world
« traditional » magnets
S
N
S
N
repulsion
N
N
S
S
macroscopic world
looking closer to the magnetic domains
N

S


many
sets of
domains

many
sets of
atomic
magnetic
moments
The magnetic moments order at Curie temperature
A set of molecules / atoms :
T
C
kT ≈ J
Magnetic Order
Temperature
Solid, Magnetically Ordered
or Curie
thermal agitation (kT) weaker
Temperature
than the interaction (J)
between molecules
kT << J
… Paramagnetic solid : thermal
agitation (kT) larger than the
interaction (J) between
molecules
kT >> J
Magnetic Order :
ferro-, antiferro- and ferri-magnetism
Ferromagnetism :
Magnetic moments
are identical
and parallel
+
=
Ferrimagnetism (Néel) :
Magnetic moments
are different
and anti parallel
Antiferromagnetism :
Magnetic moments
are identical
and anti parallel
+
=
0
+
=
Origin of Magnetism
… the electron
I am an electron
• rest mass me,
• charge e-,
• magnetic moment µB
everything, tiny, elementary
Origin of Magnetism
« Orbital » magnetic moment
« Intrinsic » magnetic moment
µorbital
due to the spin
s = ± 1/2
eµorbital = gl x µB x
µspin
µspin = gs x µB x s ≈ µB
µtotal = µorbital + µspin
Dirac Equation
The Principles of Quantum Mechanics, 1930
Nobel Prize 1933
1
e 
eh
p
eh
eh
(E'+e   
p + A  
•  A•
•  p ]
2m
c
2mc
8mc 8m  c
4m  c
1905
1928
http://www-history.mcs.st-and.ac.uk/history/PictDisplay/Dirac.html
Electron : particle and wave
Wave function or « orbital » n, l, ml …
l =
0
1
s
2
3
p
x
y
z
y,z,x
x,y,z
d
z
y
x
angular representation
y
x
Electron : also an energy level
Orbitals
Energy
Empty
Singly occupied
Doubly occupied
Electron : also a spin !
Up
Down
Singly
occupied
« Paramagnetic »
S = ± 1/2
Doubly
occupied
« Diamagnetic »
S = 0
Molecules
are most often regarded
as isolated, non magnetic
Dihydrogen
u 2

1
g
diamagnetic
Spin S = 0

1
2
the dioxygen
that we continuously breath
is a magnetic molecule
OA
E
px
O-O
OB
orthogonal π
molecular
orbitals
p y pz
paramagnetic, spin S =1
Two of its electrons have parallel magnetic moments that shapes
aerobic life and allows our existence as human beings
Transition Elements
5 d orbitals
E
Unpaired Electrons
Partial Ocupancy
Paramagnetism
Conductivity
z
y
x
x2-y2
z2
xz
yz
xy
Mononuclear complex
ML6
Splitting of the
energy levels
L
L
L
L
M
L
L
z
y
x
E
How large is the splitting ?
z
y
eg
x
?
Weak Field
High spin
L = H2O
[C2O4]2-
² oct
y
z
x
Intermediate Field Strong Field
Temperature
Dependent
Spin Cross-Over
z2
x2-y2
Low spin
L = CN-
xy
z
x
xz
y
yz
t2g
Spin Cross-Over
Room Temperature
3
M T / cm3 mol-1
Red
TC
TC
White
T / K
0
250
300
350
The system « remembers » its thermal past !
O. Kahn, C. Jay and ICMC Bordeaux
to get magnetic compounds …
Understanding …
why the spins of two neighbouring electrons
(S = 1/2) become :
antiparallel ?
S=O
or parallel ?
S=1
Interaction Models between Localized Electrons
Sa
1
2
A
B
Sb
^
^ S
^
H = - J S
1 2
Energy levels
J =
2 k
>0
if S = 0
Orthogonality
+
4ßS
<0
if S≠0;|ßS|>>k
Overlap
O2
H2
Hund
Aufbau
Exchange interactions can be very weak …
Energy
Exchange interactions
order of magnitude :
cm-1 or Kelvins …
≈
« Chemical » bonds
Robust !
order of magnitude :
>> 150 kJ mol-1 …
Cu(II)
Cu(II)
≈ 5 Å
Negligible Interaction !
Problem :
How to create the interaction … ?
Cu(II)
Ligand
Cu(II)
≈ 5 Å
Orbital Interaction …
Solution :
The ligand !
A
Ligand
B
Examples with the ligand
• Cyanide
CNCyanide Ligand
Friendly ligand : small, dissymetric, forms stable complexes
Warning : dangerous, in acid medium gives HCN, lethal
Dinuclear µ-cyano
homometallic complexes
“Models” Compounds Cu(II)-CN-Cu(II)
J/cm-1
Compounds
exp
[Cu2(tren)2CN]3+
-160
[Cu2(tmpa)2CN]3+
-100
Overlap : antiferromatic coupling …
Rodríguez-Fortea et al. Inorg. Chem. 2001, 40, 5868