This document discusses the electronic spectra of transition metal complexes and the Orgel diagram. It begins with an introduction to electronic spectra and selection rules for d-d transitions. It then explains the Orgel diagram and how it can be used to understand the electronic spectra of transition metal complexes with d1 to d8 configurations. Specific examples are provided for each electronic configuration to illustrate the allowed d-d transitions and absorption bands.

1. G. H. Raisoni University, Nagpur
Presented By :- Ashwini Pardhikar
Chemistry Department
Electronic Spectra of Transition Metal complexes
And Orgel Diagram
Guided by :- Nikhat Mam
M.Sc. (2nd sem)
2020-21

2. Contents :
1. Electronic Spectra of Transition Metal complexes
A) Selection Rules
a) Spin Selection Rule
b) Leporate Selection Rule
2. Orgel Diagram
A) Application of Orgel diagram to Electronic Spectra of Transition metal complexes
a) 𝒅𝟏
metal ions
b) 𝒅𝟐
metal ions
c) 𝒅𝟑 metal ions
d) 𝒅𝟒
metal ions
e) 𝒅𝟓
metal ions
f) 𝒅𝟔
metal ions
g) 𝒅𝟕 metal ions
h) 𝒅𝟖
metal ions

3. Electronic Spectra of Transition Metal complexes
• The study in which transition metal complex ion absorbs electromagnetic radiation is called the
electron absorption spectroscopy.
• The absorbed radiation promotes electron from lower energy d-orbital to higher energy d-orbital.
• Such electronic transition are of high energy and in addition much lower energy vibrational and
rotational transitions always occur. The vibrational and rotational levels are too close in energy.
Therefore, absorption band due to vibrational and rotational transitions are responsible for
broadening of the electronic absorption bonds in d-d spectra.
• Due to promotion of electron occurs a change in the overall arrangement of electrons in d-orbitals.
The change in the overall arrangement of electron in d-orbital gives rise to electron absorption
spectrum.
• The study of transition metal complex spectrum gives following information
i) Colour of complex
ii) Magnitude of energy Gap between ground state and excited state
iii) Extent of Covalent character in metal-ligand bond.
iv) Geometry of Complex

4. Selection Rules
• When light fall on a certain molecular / atom/ species it will absorb light or not depend on certain rule called
selection rules.
• These rules help us to differential between allowed and forbidden transition.
There are 2 types of Selection Rules :-
1) Spin Selection Rule :
• It states that transition from lower to higher electronic energy level of a molecular energy occur provides
two energy level involved in transition are of same multiplicity.
• While transition in which number of unpaired electron are changing on going from lower energy state to
higher energy state. That transition are spin forbidden or multiplicity forbidden.

5. 2) Leporate Selection Rule :
• This transition which allow change in [ L ] i.e. ∆L not equal to 0 are all leporate transition.
• It mean transition which do not involve change in secondary or subsidiary quantum ∆L not equal
to 0 is forbidden or Leporate forbidden.
• This forbidden transition are called Leporate forbidden transition or are also called Leporate
selection rules.
• E.g. If an electron undergo transition from one d-level to another d-level (d-d) transition than
metal complex are not allowed.
Even (gerate)  odd (ungerate)
Odd (ungerate)  Even (gerate)
g  u Allowed Transition
u  g
Even (gerate)  Even (ungerate)
Odd (ungerate)  Odd (ungerate g  g Forbidden Transition
u  u
>

6. Orgel Diagram :
• Orgel diagrams are correlation diagram which shows the relative energy of electron terms in
transition metal complex, much like Tanabe Sugana Diagram.
• Name orgel belong to creator of this orgel
• Because orgel diagrams are guative, no energy calculation can be performed from their
diagram.
• Trick
d1 = 𝑑9
d2 = 𝑑8
d3 = 𝑑7
d4 = 𝑑6

7. Application of Orgel diagram to Electronic Spectra of Transition metal
complexes
𝒅𝟏 metal ions  The splitting of energy for d ion
Eg. [𝑇1(𝐻2𝑂)6]3+
is shown in the figure
• Free 𝑇1
3+
ion contains only one electron in
d-orbital hence at here only one energy term
denoted as 2D.
• When 6(H2O) molecule approach [𝑇1]3+
ion, this electron enter T2g set ( dxy, dyz and
dzx) called 2T2g energy level of orbitals.
• On absorption of radiation of frequency v1
the electron gets excited to one of eg
orbitals.

8. The absorption band at 20400 𝑐𝑚−1
can be assigned to
2T2g  2Eg
𝒅𝟐 metal ions :
The complex ion [𝑉 𝐻2𝑂 6]3+
absorbs
at 17200 𝑐𝑚−1
and 25699 𝑐𝑚−1
as in fig.

9. The free 𝑉3+ 𝑖𝑜𝑛 ℎ𝑎𝑠 𝑎2 𝑐𝑜𝑛𝑓𝑖𝑔𝑢𝑟𝑎𝑡𝑖𝑜𝑛 which
can be 𝑑𝑥1𝑦, 𝑑𝑥𝑦1, 𝑑𝑧𝑥1 𝑜𝑟 𝑑𝑧𝑥1𝑑𝑥𝑦1 in ground
state represented as fig.
𝒅𝟑
𝒎𝒆𝒕𝒂𝒍 𝒊𝒐𝒏 ∶
The complex ion 𝐶𝑟 𝐻2𝑂 6
3+
shows absorption
band at 1700 𝑐𝑚−1 , 2400 𝑐𝑚−1 and 3700 𝑐𝑚−1
(weak absorption band)

10. In this case three electron transition
𝑉𝑖24𝐴2𝑔 → 4𝑇2𝑔 , 4𝐴2𝑔 →
4𝑇1𝑔 𝐹 𝑎𝑛𝑑 4𝐴2𝑔 →
4𝐴𝑇𝑖𝑔 𝑃 𝑎𝑟𝑒 𝑝𝑜𝑠𝑠𝑖𝑏𝑙𝑒
For 𝑑3
ion in octahedral ligand field
𝒅𝟒 𝒎𝒆𝒕𝒂𝒍 𝒊𝒐𝒏𝒔 :
The complex ion 𝐶𝑟 𝐻2𝑂 6
2+
shows an absorption
band at 1400 𝑐𝑚−1
as shown in fig.

11. 𝒅𝟓 𝒎𝒆𝒕𝒂𝒍 𝒊𝒐𝒏𝒔 :
The high spin
𝑑5 𝑖𝑜𝑛𝑠 𝑀𝑛 𝐻2𝑂 6
2+ 𝑜𝑟 𝑇𝑒 𝐻2𝑂 6
3+ are
almost colourless and have the following features
in their absorption spectra.
• All the five d-orbitals are singly occupied. Any
electronic transition within d level must result
in the spin raising in one of the orbitals. Thus,
The d-d transition are spin forbidden.
In this case electronic transition 5Fg  5T2g
shows an absorption band at 1400 𝑐𝑚−1

12. The Orgel diagram for Octahedral 𝑴𝒏𝟐+:
• The ground state 6S is not split and
transforms to the 6𝐴1𝑔 state which is drawn
along the horizontal axis.
• The terms 4𝐸𝑔 (G), 6𝐴1𝑔, 4𝐸𝑔 (D) and
4𝐴2𝑔(F) are also horizontal as shown in Fig.

13. 𝒅𝟔 𝒎𝒆𝒕𝒂𝒍 𝒊𝒐𝒏𝒔 :
• The complex ion 𝐹𝑒 𝐻2𝑂 6
2+
shows an
absorption band at 1000 𝑐𝑚−1
• The ground state of 𝒅𝟔 ion in 5D which gets splitted
into low energy.
5𝑇2𝑔 𝑙𝑒𝑣𝑒𝑙 𝑎𝑛𝑑 𝐻𝑖𝑔ℎ𝑒𝑟 𝑒𝑛𝑒𝑟𝑔𝑦 5𝐸𝑔.
• Thus, absorption band is due to 5𝑇2𝑔  5𝐸𝑔 as in
fig.

14. 𝒅𝟕 𝒎𝒆𝒕𝒂𝒍 𝒊𝒐𝒏𝒔 :
• The ground state term for a d ion is 4F
and its other term with same multiplicity
is 4P.
• The 4F term split into
4𝑇1𝑔 𝐹 , 4𝑇2𝑔 𝐹 𝑎𝑛𝑑 4𝐴2𝑔 𝑤ℎ𝑒𝑟𝑒 4
P term remain unsplit in presence of
weak octahedral field.

15. 𝒅𝟖 𝒎𝒆𝒕𝒂𝒍 𝒊𝒐𝒏𝒔 :
• The example of this complex is
𝑁𝑖 𝐻2𝑂 6
2+
which shows three absorption
bands at 8700 𝑐𝑚−1 , 14500
𝑐𝑚−1
and 25300 𝑐𝑚−1
as shown in fig.
• The energy terms of the free 𝑁𝑖2+
ion are
the same as that of the metal ion having 𝑑2
configuration.
• Its ground term is 3F and other term with the
same multiplicity is 3P.

16. THANK YOU