Download Electronic Spectra

8.0 Electronic
Spectra
By Prof Yeap Guan Yeow
8.0 Electronic Spectra
(i)
(ii)
Characteristic of d-block metal complexes:
Colours
Provide information about structure and
bonding.
8.1 Electronic Transition
i.
Transition between metal-centred orbitals with
d-character (d-d transition).
ii.
Transition between metal- and ligand-centred
MOs which transfer charge from metal to
ligand or ligand to metal.

Note:
When Charge transfer (CT) give rise to
intense absorptions, whereas d-d bands are
much weaker. In some spectra, CT mask bands
due to d-d transitions often occurs at higher
energies than d-d absorptions.
MLCT = metal-to-ligand charge transfer
LMCT = ligand-to-metal charge transfer
_
ν=1 =ν
λ
c



400 nm corresponds to 25000 cm-1
Absorption bands in electronic spectra are usually broad
Absorption of a photon of light occurs in ≅ 10-18 s and
molecular vibrations and rotations occur more slowly.
[MnO4]-
528 nm
[CrO4]2-
373 nm
[FeCl4]2-
220 nm
[OsCl6]3-
282 nm
[TcO4]-
286 nm
[FeBr4]2-
244 nm
[OsCl6]2-
370 nm
(ii) Laporte selection rule : Must be a change in
parity;

Allowed transitions:
g ↔u

Forbidden transitions: g ↔g
u↔u
Δl = ± 1

So, allowed transitions are s ↔ p, p ↔ d, d ↔
f;

Forbidden transitions are s ↔ s, p ↔ p, d ↔
d, f ↔ f, s ↔ d, p ↔ f etc.
Explain why an electronic transition for high-spin [Mn(H2O)6]2+ is spinforbidden, but for [Co(H2O)6]2+ is spin-allowed.
8.3

Electronic Spectra Of Octahedral Complexes
General concept
Ions in symmetric ligand fields have different energy
states which result not only from differences in electron
repulsion energy but also from the splitting of orbital
degeneracies by the ligand field.
For [Ti(H2O)6]3+
with d1 electronic configuration
Term symbol for the ground state of Ti3+ (d1, one electron
for which ml = 2, L = 2, S = 1/2) is 2D. In octahedral
field, it is split into 2T2g and 2Eg terms separated by
energy Δoct.




In d9 configuration (e.g.Cu2+) in an octahedral field, the
ground state of the free ion (2D) is again split into 2T2g
and 2Eg but this ion consists of 2Eg term with lower
energy than 2T2g.
The d9 and d1 configurations are related by a positive
hole concept.
For d9 ion in an octahedral field, the splitting diagram is
an inversion of that for the octahedral d1 ion.
Orgel Diagram for d1, d4 (high-spin), d6 (high-spin) and
d9 in octahedral can be illustrated as follows:
Orgel diagram for d2, d3, d7 and d8 ions (high-spin) in
octahedral.
Multiplicities 2S+1
Term Symbol 2S+1LJ
S = 0, 1/2
The electronic spectrum of an aqueous solution of [Ni(en)3]2+
exhibits broad absorptions with λmax = 325, 550 and 900 nm.
(a) Suggest assignments for the electronic transitions.
(b) Which bands are in the visible region?
(a) [Ni(en)3]2+ is a Ni(II), d8 complex.
From the Orgel diagram, the three transitions can be
assigned. Lowest wavelength corresponds to highest
energy transition.
900 nm assigned to 3A2g → 3T2g
550 nm assigned to 3A2g → 3T1g(F)
325 nm assigned to 3A2g → 3T1g(P)
(b) Visible region 400 - 700 nm. Only the 550 nm absorption
falls in this range.
8.4

Tanabe-Sugano diagrams
For more precise interpretation of electronic spectra, the
interelectronic interaction must be taken into account
and parameters besides Δoct is required. They are Racah
parameters, B.
d2 configuration
Aqueous solutions of [V(H2O)6]3+ show absorptions at
17200 and 25600 cm-1 assigned to the 3T1g(F) → 3T2g and
3T (F) → 3T (P) transitions. Estimate values of B and Δ
1g
1g
oct
for [V(H2O)6]3+.
Let the transition energies be E2=25600 cm-1 and E1=17200 cm-1.
Ratio of energies can be obtained as follows:
(E2/B) = E2
(E1/B)
E1
E2 = 25600 = 1.49
E1 17200
When Δoct /B = 20, (E2/B) = 32 = 1.78
(E1/B)
18
When Δoct /B = 30, (E2/B) = 41 = 1.46
(E1/B)
28
Below is the spectrum of the d3 complex [Cr(NH3)6]3+. The two broad absorptions
around 25,000 cm-1 correspond to transitions involving electrons in the d orbitals.
Summary
The Selection Rules governing transitions between electronic
energy levels of transition metal complexes are:
(i) ΔS = 0 The Spin Rule
The first rule says that allowed transitions must involve
the promotion of electrons without a change in their spin.
(ii) ΔL = +/- 1 The Orbital Rule (Laporte)
The second rule says that if the molecule has a centre of
symmetry, transitions within a given set of p or d orbitals
(i.e. those which only involve a redistribution of electrons
within a given subshell) are forbidden.
 Charge transfer, either ligand to metal or metal to ligand.
These are often extremely intense and are generally found
in the UV but they may have a tail into the visible.
 d-d, these can occur in both the UV and visible region but
since they are forbidden transitions have small intensities.
 Relaxation of the Rules can occur through:
a) Spin-Orbit coupling - this gives rise to weak spin forbidden
bands.
b) Vibronic coupling - an octahedral complex may have
allowed vibrations where the molecule is asymmetric.
Absorption of light at that moment is then possible.
c) π-acceptor and π-donor ligands can mix with the d-orbitals
so transitions are no longer purely d-d.