Orgel diagrams include D and F orgel diagrams with examples of d1 and d2 cases.

1. ORGEL DIAGRAMS
Prof. Muhammad Ali
INORGANIC SPECTROSCOPY
AAFIA ASLAM

2. Orgel Diagram
Discovery
Orgel diagram is named after Leslie Eleazer Orgel who developed
these diagrams in 1955.
Description
In coordination compounds, when transition metal atoms are placed
in ligand field, show splitting of energy levels. This splitting of
energy levels is expressed by Orgel diagrams. Orgel diagram are the
class of correlation diagrams.
For complexes with D ground terms only one electronic transition is
expected and the transition energy corresponds directly to D.
For complexes with F ground terms, three electronic transitions are
expected and D may not correspond directly to a transition energy.
Explanation
Orgel diagrams follow the spin allowed transitions but not the spin forbidden transitions. Spin
allowed transitions are transitions between states of different inversion symmetry (g → u, u → g).
Orgel diagrams are studied for high spin (weak field) complexes.
Octahedral and tetrahedral transition metals both can be studied for explaining the splitting of
energy levels in D and F term.
They are purely qualitative, and show only the states of highest spin multiplicity 2(S+1).
 Labels on the Orgel diagrams do not include spin multiplicity or the symmetry subscript
labels for the octahedral cases, but have to be specified when we discuss electronic
transitions for the complexes possessing centre of symmetry.
In an Orgel diagram, the parent term (P, D, or F) in the presence of no ligand field is located
in the center of the diagram, with the terms due to that electronic configuration in a ligand
field at each side.
 The splitting for dn
is the same as dn+5
and the opposite of d10-n
is readily seen on an Orgel
diagram, both for octahedral and tetrahedral fields.
 dn
and dn+5
in an octahedral field is reverse to that of dn
and dn+5
for a tetrahedral field.
 dn
of octahedral field is same as that of d10-n
of the tetrahedral field.
 dn
of the tetrahedral field is same as that of d10-n
of the octahedral field.
Hole Formalism: The configurations in which subshells are one or two electrons less than half-
filled and full-filled state.

3. Kinds of Orgel Diagram
There are two Orgel diagrams, one for d1
, d4
, d6
, and d9
configurations and the other is for the d2
,
d3
, d7
, and d8
configurations.
D Orgel Diagram
F/P Orgel Diagram
D Orgel Diagram
The left side contains d1
and d6
tetrahedral and d4
and d9
octahedral complexes. The right side
contains d4
and d9 tetrahedral and d1
and d6
octahedral complexes.
For complexes with D ground terms only one electronic transition is expected and the transition
energy corresponds directly to D.
In an Orgel diagram, energy is represented as the X axis, and the vertical line in the center
represents the gaseous ion in absence of ligand field, ∆ = 0.

4. Note: Gerade/ungerade labels are omitted for "simplicity", but apply to octahedral symmetries.
F/P Orgel Diagram
The left side of diagram is for d3
and d8
configuration in an octahedral field as well as for the d2
and d7
case of the tetrahedral field. The right side comprises of d2
and d7
octahedral as well as d3
and d8
tetrahedral ligand field.
The central vertical line is for the term without any field or free ion terms represented as F as the
ground state free ion term and P as the excited state of the lowest energy.
From the diagram, the lines of T1g(F) and T1g(P) states curve away from each other due to the
quantum mechanical non crossing rule. Thus the terms of same symmetry will never cross and will
repel each other.
Multiplicity of ligand field states is same as the multiplicity of ground state term from which
they arise. Therefore, d2 ion gives 3F ground state that is split into T1g + T2g + A2g.
It can be concluded that possible transitions are from 3
T1g state to three ligand field states of higher
energy.
At the left hand side, depending on the ligand field energy of the complexes, the possible
transitions are
 4 A2g → 4 T2g
 4 A2g → 4 T1g (F)
 4 A2g → 4 T1g (P)

5. For right hand side, depending on the ligand field energy of the complexes, the possible
transitions are
3 T1g → 3 T2g
3 T1g → 3 A2g
3 T1g → 3T1g(P)
The last two transitions can occur in a reverse way at higher ∆ values.
Note: Gerade/ungerade labels are omitted for "simplicity", but apply to octahedral symmetries.
For d1
Octahedral Complexes:
d1
case includes a free-ion D state that is split into a T2 ground state and a E excited state.
The ground state free ion term for this configuration can be calculated as follows.
 Total orbital angular momentum value = L = (m + l) = 2
 Total spin angular momentum value = S = Σms = 1/2
 Spin multiplicity = (2S+1) = (2(1/2)+1) = 2
Thus the free ion ground state term for d1
configuration comes out to be 2D.
Possible Electronic Configurations for d1 Case:
In the presence of an octahedral ligand field this
free ion term for d1
configuration split up to form
new ground state and excited state terms.
Orgel Diagram for d1 Case:

6. For d2
Octahedral Complexes
According to orgel the d2
configuration having ground that term symbol 3
F & various exited state
term symbol like 1
S, 1
D, 1
G, 3
P. Out of which only 3
P term symbol is used during the electronic
transition because the electronic transition from ground state to this excited state being allowed
while the electron transition from ground state to other excited states like 1
S, 1
G, and 1
D being
forbidden.
The ground state term free ion term for this configuration can be calculated as follows.
Total orbital angular momentum value = L = Σml = 2 +1 = 3
Total spin angular momentum value = S = Σms = 1
Spin multiplicity = (2S+1) = (2(1)+1) = 3
Thus the free ion ground state term for d1
configuration comes out to be 3
F.
Electronic Arrangement in ground state and excited state: Orgel Diagram
Diagrammatic Comparison
D Orgel Diagram F/P Orgel Diagram
d1
, d4
, d6
, and d9
configurations d2
, d3
, d7
, and d8
configurations
Left Side
d1
, d6
tetrahedral d2
, d7
tetrahedral

7. d4
, d9
octahedral d3
, d8
octahedral
Right Side
d4
, d9
tetrahedral
d1
, d6
octahedral
d3
, d8
tetrahedral
d2
, d7
octahedral
Electronic Transitions
Complexes of D ground terms shows one
electronic transition
Complexes of F ground terms shows three
electronic transitions
Importance of Orgel Diagram
Different applications can be employed for use of Orgel diagram in the following determinations:
The Orgel diagrams are important in understanding the following:
 Spectral properties like spectrum observations made at bands absorption peaks for
configuration determination.
 Optical properties for colors in their transition states.
 Magnetic properties of the transition metal complexes.
Conclusion
It has been evaluated that Orgel diagram is an effective qualitative method for the determination
of splitting of energy levels in D and F terms. D Orgel diagram shows the d1,
d6
and d4
, d9
configurations containing complexes. F Orgel diagram shows d3
, d8
and d2
, d7
of both octahedral
and tetrahedral terms.
It includes the diagrammatic representations of electronic terms n reference to their relative
energies with each other.
Orgel diagrams are employed for the study of high spin complexes.
.