kuliah spec 3 tanabe spectroscopi
TRANSCRIPT
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The Period 4 transition metals
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Colors of representative compounds of the Period 4 transition metals
titanium oxide
sodium chromate
potassium ferricyanide
nickel(II) nitrate hexahydrate
zinc sulfate heptahydrate
scandium oxide
vanadyl sulfate dihydrate
manganese(II) chloride
tetrahydrate cobalt(II) chloride
hexahydrate
copper(II) sulfate
pentahydrate
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Aqueous oxoanions of transition elements
Mn(II) Mn(VI) Mn(VII)
V(V)Cr(VI)
Mn(VII)
One of the most characteristic chemical properties of these elements is the occurrence of multiple oxidation states.
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Linkage isomers
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An artist’s wheel
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The five d-orbitals in an octahedral field of ligands
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Splitting of d-orbital energies by an octahedral field of ligands
Δ is the splitting energy
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The effect of ligand on splitting energy
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The color of [Ti(H2O)6]3+
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Effects of the metal oxidation state and of ligand identity on color
[V(H2O)6]2+ [V(H2O)6]3+
[Cr(NH3)6]3+ [Cr(NH3)5Cl ]2+
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The spectrochemical series
•For a given ligand, the color depends on the oxidation state of the metal ion.
•For a given metal ion, the color depends on the ligand.
I- < Cl- < F- < OH- < H2O < SCN- < NH3 < en < NO2- < CN- < CO
WEAKER FIELD STRONGER FIELD
LARGER ΔSMALLER Δ
LONGER λ SHORTER λ
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High-spin and low-spin complex ions of Mn2+
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Orbital occupancy for high- and low-spin complexes of d4 through d7 metal ions
high spin: weak-field
ligand
low spin: strong-field
ligand
high spin: weak-field
ligand
low spin: strong-field
ligand
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What is electronic spectroscopy?
Absorption
Absorption of radiation leading to electronic transitions within a molecule or complex
UV = higher energy transitions - between ligand orbitals
visible = lower energy transitions - between d-orbitals of transition metals
- between metal and ligand orbitals
UV
400
λ / nm (wavelength)
200 700
visible
Absorption
~14 000 50 00025 000
UVvisible
ν / cm-1 (frequency)−
[Ru(bpy)3]2+ [Ni(H2O)6]2+
10104
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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, Δoct or Δtet 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
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[Ti(OH2)6]3+ = d1 ion, octahedral complex
white light400-800 nm
blue: 400-490 nm
yellow-green: 490-580 nm
red: 580-700 nm
3+
Ti
A
λ / nm
This complex is has a light purple colour
in solution because it absorbs green light
λmax = 510 nm
Absorption of light
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eg
t2g
Δo
hν
d-d transition
[Ti(OH2)6]3+ λmax = 510 nm Δo is ∴ 243 kJ mol-1
20 300 cm-1
The energy of the absorption by [Ti(OH2)6]3+ is the ligand-field splitting, Δo
An electron changes orbital; the ion changes energy state
complex in electronic Ground State (GS)
complex in electronic excited state (ES)
GS
ES
GS
ES
eg
t2g
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Limitations of ligand field theory
LFT assumes there is no inter-electron repulsion
[Ni(OH2)6]2+ = d8 ion
2+
Ni
A
3 absorption bands
eg
t2g
Repulsion between electrons in d-orbitals has an effect on the energy of the whole ion
15 00025 000ν / cm-1−
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Electron-electron repulsiond2 ion
eg
t2g
xy xz yz
z2 x2-y2 eg
t2g
xy xz yz
z2 x2-y2
xz + z2
lobes overlap, large electron repulsion
x
z
xy + z2
lobes far apart, small electron repulsion
x
z
y y
These two electron configurations do not have the same energy
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A
ν / cm-1-30 00020 00010 000
[Ti(H2O)6]3+, d1
2T2g
2Eg
2B1g
2A1g
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
d3 4A2gd5 (high spin) 6A1gd6 (low spin) 1A1gd8 3A2g
Degenerate electronic ground state: T or E
Non-degenerate ground state: A
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- 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- < I-
Nephelauxetic series of metal ions
Mn(II) < Ni(II) Co(II) < Mo(II) > Re (IV) < Fe(III) < Ir(III) < Co(III) < Mn(IV)
cloud expandingThe Nephelauxetic Effect
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Selection Rules
Transition ε complexes
Spin forbidden 10-3 – 1 Many d5 Oh cxsLaporte forbidden [Mn(OH2)6]2+
Spin allowedLaporte forbidden 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
Spin allowed 102 – 103 Some MLCT bands in cxs with unsaturated ligandsLaporte allowed
102 – 104 Acentric complexes with ligands such as acac, or with P donor atoms
103 – 106 Many CT bands, transitions in organic species
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Tanabe-Sugano diagram for d2 ions
E/B
Δ/B
[V(H2O)6]3+: Three spin allowed transitions
ν1 = 17 800 cm-1 visible
ν2 = 25 700 cm-1 visible
ν3 = obscured by CT transition in UV
10 000
ε
30 000ν / cm-1−
10
20 000
5
25 700 = 1.4417 800
Δ/B = 32
ν3 = 2.1ν1 = 2.1 x 17 800
∴ ν3 = 37 000 cm-1
= 32
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E/B
Δ/B = 32
ν1 = 17 800 cm-1
ν2 = 25 700 cm-1
ν1
ν2E/B = 43 cm-1
E/B = 30 cm-1
E/B = 43 cm-1 E = 25 700 cm-1
B = 600 cm-1
Δo / B = 32
Δo = 19 200 cm-1
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Tanabe-Sugano diagram for d3 ions
E/B
Δ/B
[Cr(H2O)6]3+: Three spin allowed transitionsν1 = 17 400 cm-1 visible
ν2 = 24 500 cm-1 visible
ν3 = obscured by CT transition
24 500 = 1.4117 400
Δ/B = 24
ν3 = 2.1ν1 = 2.1 x 17 400
∴ ν3 = 36 500 cm-1
= 24
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Calculating ν3
E/B
Δ/B
ν1 = 17 400 cm-1
ν2 = 24 500 cm-1
= 24
E/B = 34 cm-1
E/B = 24 cm-1
When ν1 = E =17 400 cm-1
E/B = 24
so B = 725 cm-1
When ν2 = E =24 500 cm-1
E/B = 34
so B = 725 cm-1
If Δ/B = 24
Δ = 24 x 725 = 17 400 cm-1
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Tanabe-Sugano diagrams
E/B
Δ/B
2T2g
4A1g, 4E
4T2g
4T1g
4T2g
4T1g
2A1g
4T2g
2T2g
6A1g
2Eg
4A2g, 2T1g
2T1g
2A1g
4EgAll terms includedGround state assigned to E = 0Higher levels drawn relative to GSEnergy in terms of BHigh-spin and low-spin configurations
Critical value of Δ
d5
WEAK FIELD STRONG FIELD
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TiF4 d0 ion
TiCl4 d0 ion
TiBr4 d0 ion
TiI4 d0 ion
d0 and d10 ion have no d-d transitions
[MnO4]- Mn(VII) d0 ion
[Cr2O7]- Cr(VI) d0 ion
[Cu(MeCN)4]+ Cu(I) d10 ion
[Cu(phen)2]+ Cu(I) d10 ion colourless
dark orange
Zn2+ d10 ion white
extremely purplebright orange
d0 and d10 ions
white
whiteorange
dark brown
Charge Transfer Transitions
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Charge Transfer Transitions
Ligand-to-metal charge transferLMCT transitions
Metal-to-ligand charge transferMLCT transitions
MdLπ
Lσ
Lπ∗
t2g*
eg*
d-d transitions
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Energy of transitions
molecular rotationslower energy (0.01 - 1 kJ mol-1)microwave radiation
electron transitionshigher energy (100 - 104 kJ mol-1)visible and UV radiation
molecular vibrationsmedium energy (1 - 120 kJ mol-1)IR radiation
Ground State
Excited State
During an electronic transition
the complex absorbs energy
electrons change orbital
the complex changes energy state