Transcript Slide 1

Lecture 27
Electronic Spectra of Coordination Compounds MLx (x = 4,6)
1) Octahedral complexes of d1 configuration
Absorption of a TMC in the UV and visible regions results from transitions of
electrons between the energy levels available in the metal complex.
• We will be interested in those transitions which occur within the metal valence
shell (d-subshell). Of our interest will be:
A b so rp tio n
1) The number of the energy levels (terms) and their ordering;
2) The number of absorption bands and their intensity;
3) Absorption band width
1. Consider terms of an isolated d1 metal ion (Ti3+)
1 2 5 00
The number of microstates possible for dX configuration is given by formula
25000
 , cm -1
N!
X ! ( N  X )!
d1 case corresponds to X = 1 and N = 10 (maximum occupancy of the d-level). The
number of microstates is then 10 which means that any of the five degenerate
d-orbitals may be occupied by an electron with a spin of ½ or - ½.
The orbital angular momentum for Ti3+, L = 2, the spin S = 1/2 and the term is 2D.
2) Terms and absorption of Oh d1 metal complexes. Selection rules
•
•
•
The wavefunction for a D term is of the same symmetry as
for a d-orbital. As a result, D terms behave as d-orbitals do.
In particular, when d1 metal ion is exposed to an octahedral
ligand field, the five-time degenerate 2D term will be split
into 2Eg (doubly degenerate) and 2T2g (triply degenerate)
terms (recall how d-orbital split in octahedral complexes in
crystal field theory!).
Oh
d1
2
2
Eg
D
D
Since we have two terms only, one can expect a single
band in the spectrum of d1 metal complexes due the only
possible T2g  Eg transition within the metal d-shell. This
band is weak [e of 1 – 103 L/(mol cm)].
2
M
T 2g
M L6
Selection rules help rationalize the observed intensity of
electronic transitions.
•
The Laporte rule states that symmetry allowed are the transitions with the change of
parity, g  u or u  g.
•
It means that T2g  Eg transitions are symmetry forbidden. Nevertheless, due to the
asymmetric vibrations there is always a small fraction of molecules of a reduced
symmetry so that such a vibronic transition is allowed though with a low intensity.
•
Another selection rule states that spin allowed are the transitions with DS = 0 (both states
should be of the same multiplicity). This holds true for the 2T2g  2Eg transition.
3) Estimating Do from electronic absorption spectra of d1 species
•
Values of Do can be readily obtained from absorption spectra of d1 transition metal
complexes.
•
In the case of a d1 metal complex [Ti(H2O)6]3+ (spectrum is shown below) lmax = 500
nm what corresponds to the Do =  = 1/ lmax = 1/(5.00 10-5cm)= 20000 cm-1.
d1
2
Eg
Eg
lmax
D = 
D
2
•
2
T 2g
2
T 2g
In the case of dn (n>1) species the relationship between the measured lmax and Do
may be much more complex.
4) Terms of complexes of dn, d10-n, d5-n and d5+n configurations
•
In more complex cases of dn (n > 1) configurations the number of terms available is
much more significant than for d1 species.
Here are some rules to make analysis of the terms simpler:
•
•
•
d
1
For any given ligand field, term splitting pattern for a metal of d10-n configuration will be
the same as for dn one, but the energy level sequence will be the opposite.
d10-n configuration can be considered as a combination of n positrons (“hole formalism”)
and a d10 metal shell. Because of the opposite charge of positrons, their interaction with
the ligands will be of the opposite sign resulting in the opposite energy level sequence .
Similarly, in the case of high spin complexes, dn, d5-n and d5+n configurations have the
same term splitting pattern. The ordering of the energy levels is the same for dn and
d5+n and the opposite for dn and d5-n.
Oh
d
2
2
D
10- 1
Oh
2
Eg
D
2
D
d
Oh
5+ 1
T 2g
5
5
D
d
D
(n o t a s in g le te rm )
5- 1
Oh
5
Eg
D
5
D
T 2g
D
(not a single term)
2
M
ML 6
2
T 2g
M
ML 6
5
Eg
M
ML 6
5
T 2g
M
ML 6
Eg
5) Tetrahedral vs octahedral complexes of dn configuration
•
Octahedral and tetrahedral ligand fields cause the opposite ordering of split levels for each
of the metal ion terms.
•
•
The term splitting pattern for octahedral and tetrahedral complexes is the same.
The term sequence is the opposite for octahedral and tetrahedral complexes of the same
configuration.
The term sequence is of the same order for dn octahedral and d10-n tetrahedral
complexes.
•
Note: Tetrahedral terms have no center of inversion and thus no labels g or u.
d
1
Oh
Td
d1
2
2
D
2
2
M
ML 6
2
Eg
Do
D
Td
Dt
2
2
T 2g
M
M L4
d 1 0 -1
T2
D
2
Dt
2
E
M
E
M L4
T2