Transcript Slide 1

Lecture 29
Electronic Spectra of Coordination Compounds MLx (x = 4,6)
1) Electron repulsion B’ and Do parameters for d3 & d8 Oh species
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The d-electron-d-electron repulsions present in dX metal complexes with x > 1 make the
relationship between the d-orbital splitting parameter D and absorption frequencies complex.
The following equations relate D, the electron repulsion parameter B’ and transition
frequencies n1, n2, n3 (n1 < n2 < n3) for d3 and d8 octahedral complexes:
Energy d3, d8 O
h
d2, d7 Td
n1 = D
n2 = 7.5B’ + 1.5D - 0.5 [225 B’2+D2-18B’D]1/2
n3 = 7.5B’ + 1.5D + 0.5 [225 B’2+D2-18B’D]1/2
d2, d7 Oh
d3, d8 Td
A2
T1(P)
T1g(P)
T1(F)
T2g
T2
T1g(F)
A2
D
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0
D
For example, for d3 chromium(III) complex CrF63- the absorptions are at 14900, 22700 and
34400 cm-1.
From the formulas above D = 14900 cm-1, n2 + n3 = 15B’ + 3D = 57100
Therefore, 15B’ = 12400 and B’ ≈ 827 cm-1.
2) Tanabe-Sugano diagrams. d2 octahedral complexes
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Tanabe-Sugano diagrams have been calculated for
the cases of all ligand field strengths for all
octahedral dn-metal complex configurations.
The diagrams allow for estimation of a complete set
of parameters included in the calculations: i) the
(apparent) electron repulsion parameter B’, ii) D, iii)
transition frequencies.
To calculate these parameters we need to have at
least two experimentally found transition
frequencies.
Consider an example of a d2 metal complex and the
appropriate Tanabe-Sugano diagram.
From 3 bands expected for V(H2O)63+ (d2) in its UV
spectrum we have one at n1 = 17200 (3T1g(F) 
3T ) and another at n = 25700 (3T (F)  3T (P))
2g
2
1g
1g
-1
3
cm . The third expected transition n3 ( T1g(F) 
3A ) is far in the UV region and is masked by other
2g
absorptions. We can calculate the n3.
For n2/n1 = 1.49 on diagram we find n1/B ≈ 25; n2/B=
37; n3 / B = 52; D/(10B) = 2.7. Then B ≈ 691cm-1; D =
18660 cm-1; n3 ≈ 35930 cm-1.
d
2
52
37
25
2.7
3
1
D/(10B)
3) Terms of d5 metal complexes
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Terms of free d5 metal ions are 6S, 4G, 4F, 4D, 4P, 2I, 2H, 2G, 2G, 2F, 2F, 2D, 2D, 2D, 2P, 2S
(16 terms, 252 microstates). The lowest energy term is 6S.
In the octahedral ligand field the 6S term will NOT be split. It gives rise to a single 6A1g
term.
The 6A1g term is the ground state term at weak ligand fields. NO terms of the same
multiplicity exists and thus NO spin-allowed e-e transition is possible.
At strong ligand fields spin pairing occurs (t23e2  t25). As a result, the ground state term
and the multiplicity change from 6A1g to 2T2g(I).
free ion
4
4
w eak field
strong field
P
G
6
S
4
T
4 1
E
4
T2
4
T1
6
4
A2
4
(t2)4(e)1
E
4
2
3
T
4 2 (t2) (e)
T1
5
(t
)
2
T2
(t2)1(e)4
octahedraland tetrahedrald5
A1
2
4) Weak ligand fields, all dn configurations: the number and
intensity of bands
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The number of the spin-allowed electronic transitions for all dn metal configurations for
the case of weak ligand fields is given in table below.
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The intensity of absorption bands can be evaluated on the basis of the selection rules.
The symmetry forbidden d-d transitions for d1-d4, d6-d9 complexes below have e in the
range of 1 – 103 L/(mol cm). Both symmetry and spin forbidden transitions for d5
complexes have e of 0.001 – 1 L/(mol cm).
Configuration (example)
Ground state
Excited states w/same S
# Abs.bands
d1 oct (Ti(H2O)63+), d9 tetr.
2T
2
2E
1
d2 oct (V(H2O)63+), d8 tetr.
3T (F)
1
d3 oct (Cr(H2O)63+), d7 tetr.
4A
2
d4 oct (Cr(H2O)62+), d6 tetr.
5E
d5 oct (Mn(H2O)62+) or tetr.
6A
d6 oct (Fe(H2O)62+), d4 tetr.
5T
2
d7 oct (Co(H2O)62+), d3 tetr.
4T
1
(P), 3A2
3
(F), 4T1 (P)
3
2
5T
2
1
1
none
0
5E
1
(F)
d8 oct (Ni(H2O)62+), d2 tetr.
3A
2
d9 oct (Cu(NH3)62+), d1 tetr.
2E
2
3T , 3T
2
1
2
4T , 4T
2
1
4T , 4T
2
1
3T , 3T
2
1
2
(P), 4A2
3
(F), 3T1 (P)
3
2T
2
1
5) Line width in electronic spectra
A regular absorption band in electronic spectra is of several
thousands cm-1 wide. The width is associated with:
a) combination of vibrational and electronic (vibronic)
transitions;
E
3rd Vibrational
(excited) state
...
2.3
2.2
2.1
width:
thousands of cm-1
2nd Vibrational
(excited) state
1st Vibrational
(ground) state
2
2 Electronic
(excited state) term
nd
n
b) distortion of the complex geometry (ligands shift) with or
without change of initial symmetry.
The related change of D and/or term splitting pattern affect
the position of the “immediate” absorption maximum.
In rare cases bands are as narrow as of few hundreds of cm-1.
This is the case when the energy gap between the ground
and excited states is not a function of D.
...
1.3
1.2
1.1
2nd Vibrational
(excited) state
1st Vibrational
(ground) state
1
1 Electronic
(ground state) term
st
6) Line width in electronic spectra
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Terms which are not affected by metal complex
vibrations appear as horizontal lines on TanabeSugano diagrams.
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For high-spin d5 complexes these are in particular
the terms 4A1g, 4Eg (G). The electronic transitions
to these levels produce narrow absorption lines.
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Narrow lines corresponding to these transitions
can be seen in a spectrum of d5 high spin MnF2
(MnII ion has an octahedral environment in the
crystal lattice):
4A (G)
1g
4E (G)
g
4T (G)
1g
4T (G)
2g