The Period 4 transition metals
Colors of representative compounds of the Period 4 transition metals
nickel(II) nitrate
hexahydrate
sodium chromate
titanium oxide
scandium oxide
vanadyl sulfate
dihydrate
potassium
ferricyanide
manganese(II)
chloride
tetrahydrate
cobalt(II)
chloride
hexahydrate
zinc sulfate
heptahydrate
copper(II)
sulfate
pentahydrate
Aqueous oxoanions of transition elements
One of the most characteristic
chemical properties of these
elements is the occurrence of
multiple oxidation states.
Mn(II)
Mn(VI)
Mn(VII)
Mn(VII)
Cr(VI)
V(V)
Effects of the metal oxidation state and of ligand identity on color
[V(H2O)6]3+
[V(H2O)6]2+
[Cr(NH3)6]3+
[Cr(NH3)5Cl ]2+
Linkage isomers
An artist’s wheel
The five d-orbitals in an octahedral field of ligands
Splitting of d-orbital energies by an octahedral field of ligands
D is the splitting energy
The effect of ligand on splitting energy
Electronic Spectroscopy of Transition Metal Complexes
Chemistry 412 Experiment 1
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
700
visible
l / nm (wavelength)
~14 000
25 000
visible
50 000
UV
n / cm-1 (frequency)
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
Energy of transitions
Excited State
molecular rotations
lower energy
(0.01 - 1 kJ mol-1)
microwave radiation
electron transitions
higher energy
(100 - 104 kJ mol-1)
visible and UV radiation
Ground State
molecular vibrations
medium energy
(1 - 120 kJ mol-1)
IR radiation
During an electronic transition
the complex absorbs energy
electrons change orbital
the complex changes energy state
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
y
x
x
lobes overlap, large electron repulsion
lobes far apart, small electron repulsion
These two electron configurations do not have the same energy
Which is the Ground State?
3P
States of the same
spin multiplicity
DE
3F
D E = 15 B
B is the Racah parameter and is a measure of inter-electron repulsion
within the whole ion
Relative strength of coupling interactions:
MS = S ms
>
ML = S ml
>
ML - MS
Effect of a crystal field on the free ion term of a d1 complex
d1  d6
tetrahedral field
free ion
octahedral field
2E
g
2T
2
6 Dq
2D
4 Dq
2E
2T
2g
Energy level diagram for d1 ions in an Oh field
2E
Energy
g
D
2D
2T
2g
ligand field strength, Doct
For d6 ions in an Oh field, the splitting is the same, but the multiplicity of the states is 5,
ie 5Eg and 5T2g
d1 oct
2E
g
[Ti(OH2)6]3+
A
2E
g
 2T2g
D
2D
2T
2g
10 000
20 000
30 000
n- / cm-1
Orgel diagram for d1, d4, d6, d9
E
Eg or E
T2g or T2
D
D
T2g or T2
Eg or E
D
d1, d6 tetrahedral
d4, d9 octahedral
0
LF strength
d1, d6 octahedral
d4, d9 tetrahedral
D
The Jahn-Teller Distortion: Any non-linear molecule in a degenerate electronic state
will undergo distortion to lower it's symmetry and lift the degeneracy
Degenerate electronic ground state:
T or E
Non-degenerate ground state:
A
2E
g
2B
1g
d3
d5 (high spin)
d6 (low spin)
d8
4A
2g
6A
1g
1A
1g
3A
2g
A
[Ti(H2O)6]3+, d1
2A
1g
2T
2g
10 000
20 000
30 000
n- / cm-1
Racah Parameters
Free ion [Co2+]: B = 971 cm-1
[Co(H2O)6]2+
[CoCl4]2-
d7 octahedral complex
d7 tetrahedral complex
15 B' = 13 800 cm-1
15 B' = 10 900 cm-1
B' = 920 cm-1
B' = 727 cm-1
B' = 0.95
B
B' = 0.75
B
Nephelauxetic ratio, b
b is a measure of the decrease in electron-electron repulsion on complexation
The Nephelauxetic Effect
cloud expanding
- some covalency in M-L bonds – M and L share electrons
-effective size of metal orbitals increases
-electron-electron repulsion decreases
Nephelauxetic series of ligands
F- < H2O < NH3 < en < [oxalate]2- < [NCS]- < Cl- < Br- < INephelauxetic series of metal ions
Mn(II) < Ni(II) Co(II) < Mo(II) > Re (IV) < Fe(III) < Ir(III) < Co(III) < Mn(IV)
Selection Rules
Transition
e
complexes
Spin forbidden
Laporte forbidden
10-3 – 1
Many d5 Oh cxs
[Mn(OH2)6]2+
1 – 10
Many Oh cxs
[Ni(OH2)6]2+
10 – 100
Some square planar cxs
[PdCl4]2-
100 – 1000
6-coordinate complexes of low symmetry,
many square planar cxs particularly with
organic ligands
102 – 103
Some MLCT bands in cxs with unsaturated ligands
102 – 104
Acentric complexes with ligands such as acac, or
with P donor atoms
103 – 106
Many CT bands, transitions in organic species
Spin allowed
Laporte forbidden
Spin allowed
Laporte allowed
The Spectrochemical Series
eg
D
eg
I- < Br- < S2- < SCN- < Cl-< NO3- < F- < OH- < ox2-
D
t 2g
< H2O <
NCS- <
CH3CN < NH3 < en < bpy
< phen < NO2- < phosph < CN- < CO
t 2g
weak field ligands
strong field ligands
e.g. H2O
e.g. CN-
high spin complexes
low spin complexes
The Spin Transition
d5
Tanabe-Sugano diagrams
4T
2g
2A
1g
E/B
4T
1g
4E
g
4T
2g
4A
4
1g, E
2A
1g
2T
1g
All terms included
2T
2g
2E
g
High-spin and low-spin configurations
4A
2
2g, T1g
Ground state assigned to E = 0
Higher levels drawn relative to GS
Energy in terms of B
Critical value of D
4T
2g
6A
1g
4T
1g
2T
2g
WEAK FIELD
D/B
STRONG FIELD
10
Tanabe-Sugano diagram for d2 ions
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
E/B
n1 = 17 800 cm-1
n2 = 25 700 cm-1
n2
E/B = 43 cm-1
n1
E/B = 30 cm-1
E/B = 43 cm-1
E = 25 700 cm-1
B
=
600 cm-1
Do / B
=
32
Do
=
19 200 cm-1
D/B = 32
Tanabe-Sugano diagram for d3 ions
n1 = 17 400 cm-1
visible
[Cr(H2O)6]3+: Three spin allowed transitions
n2 = 24 500 cm-1
visible
n3 = obscured by CT transition
E/B
24 500 =
1.41
17 400
D/B
=
24
n3 = 2.1n1 = 2.1 x 17 400
 n3 = 36 500 cm-1
D/B = 24
Calculating n3
n1 = 17 400 cm-1
n2 = 24 500 cm-1
E/B
When
n1 = E =17 400 cm-1
E/B = 24
so
B = 725 cm-1
When
n2 = E =24 500 cm-1
E/B = 34
E/B = 34
so
cm-1
E/B = 24 cm-1
B = 725 cm-1
If D/B = 24
D = 24 x 725 = 17 400 cm-1
D/B = 24
d0 and d10 ions
Zn2+
d0 and d10 ion have no d-d transitions
d10 ion white
TiF4
d0 ion
white
TiCl4
d0 ion
white
TiBr4
d0 ion
orange
TiI4
d0 ion
dark brown
[MnO4]-
Mn(VII)
d0 ion extremely purple
[Cr2O7]-
Cr(VI)
d0 ion
bright orange
[Cu(MeCN)4]+
Cu(I)
d10 ion colourless
[Cu(phen)2]+
Cu(I)
d10 ion dark orange
Charge Transfer Transitions
Metal-to-ligand charge transfer
Ligand-to-metal charge transfer
MLCT transitions
LMCT transitions
Charge Transfer Transitions
d-d transitions
eg*
Lp*
t2g*
Md
Lp
Ls