Coordination Compounds - CHARACTERIZATION AND OVERVIEW, DETECTION OF COMPLEX FORMATION, Formation of precipitate, Conductivity measurements , Change in chemical behaviour, Spectral methods, Magnetic Method, Spectral Calculations, Term symbols, Mulliken term symbols, Orgel diagrams - d1 configuration in Oh environment , d9 configuration in Oh environment, d1 configuration in Oh environment, Limitations Tanabe sugano diagrams, d2 configuration in Oh environment , Advantages over Orgel’s, Racah parameters

1. UNIT IV – COORDINATION COMPOUNDS
CHARACTERIZATION AND OVERVIEW
SP | MSc – I | Jan 08th

2. DETECTION OF COMPLEX FORMATION

3. • When a neutral covalent complex is formed in polar
medium, it gets precipitated. Formation of the precipitate
on the addition of ligand indicates the formation of a
neutral covalent complex.
For e.g.
Ni2+ + 2DMG -----> [Ni (DMG)2] + 2H+
Al3+ + 3CH3-CO-CH2-CO-CH3 + 3OH- ----->
[Al (CH3-CO-CHCO-CH3)3] + 3H2O
I. Formation of precipitate

4. II. Change in chemical behaviour
• The altered reactivity of the metal ions in the presence of ligands is
still a useful indication of complex formation.
For e.g.:
a. Fe3+ is not precipitated as Fe(OH)3 by NH4OH in the presence of
tartaric acid. This indicates the formation of Fe3+- tartarate complex.
b. H2S fails to precipitate Cu3+ ions as CuS in the presence of CN- ions
due to formation of [Cu (CN)4 ]3-
c. Ag+ does not give the precipitate of AgCl in the presence of
[Ag(NH3)2 ]+ complex.

5. III. Spectral method
• A change in the colour of the solution on addition of another reagent
indicates the formation of a new species.
• In many cases, the colour of the uncomplexed metal ion gets altered
or intensified & in some cases a completely new colour is formed.
For e.g.
Fe2+ + 6CN- -----> [Fe(CN)6]4-
green yellow
Cu2+ + 4Cl- -----> [Cu(Cl)4]2-
blue green

6. IV. Conductivity measurements
• If the complex formation reaction involves changes in the number of
ions, then there would be a change in the conductivity of the solution.
For e.g.
Mn+ + xCl- -----> [MClx]n-x
• Since there is a decrease in the number of ions in the solution, its
conductivity decreases due to formation of complex.
• Similarly, if complexation involves liberation or consumption of protons or
hydroxyl ions , then this rise or fall in the no. of ions leads to an abnormal
change in the conductance.

7. V. Magnetic method
• On complexation, magnetic susceptibility χ of the metal ion
is altered. As complexation changes w.r.t d electron
configurations, χ can be diagnostic of complexation.
The following data illustrates this:
Species Fe3+ [Fe(CN)6]4- Co3+ [Co(NH3)6 ]3+
Magnetic moment (BM) 5.9 1.7 4.9 0.0

8. • Many other physical properties of the system depending upon the
nature of species present in the solution can be used to detect
complex formations.
• Some methods include:
a. Chromatography
b. Polarography
c. X-ray analysis
d. Mass spectroscopy
e. NMR spectroscopy
f. IR spectroscopy

9. SPECTRAL CALCULATIONS

10. Term symbols
• Term symbol for a particular atomic state term is written as:
2S+1X
L = 0, 1, 2, 3, 4
X = S, P, D, F
where
• S - total spin quantum number (∑s), 2S+1 is spin multiplicity
• L - total orbital quantum number (∑l )
E.g. For d8 configuration,
∑s =
1
2
+
1
2
= 1 , 2S+1 = 2 x 1 + 1 = 3
∑l = (+2 x 2) + (+1 x 2) + (0 x 2) + (-1 x 1) + (-2 x 1) = 3
Term symbol: 3F
+2 +1 0 -1 -2

11. Electronic
configuration
Term symbols
d1 2D
d2 3F
d3 4F
d4 5D
d5 6S
d6 5D
d7 4F
d8 3F
d9 2D
d10 1S

12. Mulliken’s term symbols
Angular
momentum (L)
Terms IRREDUCIBLE REPRESENTATIONS
Octahedral Oh Tetrahedral Td
0 S A1g A1
1 P T1g T1
2 D Eg, T2g E, T2
3 F A2g, T1g, T2g A2, T1, T2
• singly degenerate – A
• doubly degenerate – E
• triply degenerate – T
• half filled – T1g, Eg, T2g
• fully filled – A2g

14. ORGEL DIAGRAMS

15. • An Orgel diagram is a plot of change in energies of the possible d-orbital
configurations (ordinate) vs. ligand field strength (abscissa).
• It shows splitting of different terms depending on field strength.
• Only two diagrams are sufficient for d1 to d9 configurations for the interpretation
of spin allowed crystal field transitions under the influence of octahedral and
tetrahedral fields.
• Zero ligand field corresponds to the free (gas
phase) ion with all d-orbitals equivalent
(Δo = 0).
• The lowest line represents ground state
energy and other lines represent excited
states.
d1 d6 Td
d4 d9 Oh
d1 d6 Oh
d4 d9 Td

16. 0.6 Δo (E.S.)
0.4 Δo (G.S.)
2
D
E
Δo
• Consider a Ti(III) Oh complex such as [Ti(H2O)6]3-
• The G.S. term for d1 configuration is 2D
• In the G.S. , the electron occupies the lower triply degenerate level t2g and only one
transition is possible to the higher excited doubly degenerate energy eg level.
• The transition occurs between from 2T2g to 2Eg state of energy.
d1 configuration in Oh environment
• d6 configuration in Oh environment and
d4 & d9 in Td environment have similar
ORGEL diagrams.
t2g
eg

17. 0.4 Δo
0.6 Δo
Δo
2
D
E
d9 configuration in Oh environment
• Consider a Cu(III) Oh complex such as [Cu(H2O)6]2+
• G.S. term for d9 configuration is 2D
• In this configuration, there is a single hole in the upper eg level.
• The electronic transition in an Oh field is movement of hole from t2g to eg these two
energy levels.
• The G.S. energy level 2Eg originates from t6
2g e3
g and is doubly degenerate.
• The E.S. energy level 2T2g originates
from t5
2g e4
g and is triply degenerate.
The absorption band is 2Eg  2T2g
• d4 configuration in Oh environment and
d1 & d6 in Td environment have similar
ORGEL diagrams.
t2g
eg

18. Transitions in tetrahedralenvironment
• For a tetrahedral ligand field, the energy level is inverse of that on octahedral field.
• The metal ions under tetrahedral environment experience weak field and the
splitting is about (4/9) times of octahedral field. The suffix ‘g’ is not denoted as Td
does not have a center of symmetry.
• All the above transitions can be
combined into single diagram known as
Orgel diagrams showing splitting of
different terms depending on the field
strength.

19. Limitations
• Orgel diagrams show splitting only in presence of weak field ligands i.e. high spin
interactions
• They show only spin allowed transitions, not forbidden (vide. Laporte selection rule)
• s  s, p  p, f  f, d  d are forbidden transitions.
• They do not have separate diagrams for different dn cases
• They provide less qualitative information.

20. TANABE - SUGANO DIAGRAMS

21. • The Tanabe – Sugano are actually elaborated
versions of the simple correlation diagrams. They
provide an alternate way of describing the
variation of term energies with field strengths.
• These diagrams are obtained by plotting E/B
against Δo/B
Where E = term energies
B = Racah parameters
Δo = L.F.SE.
• The energies of the level of dn system are plotted
as the vertical coordinate in units of inter-
electronic repulsions parameter B; the crystal
field strength is the horizontal coordinate.
• The energy of the lowest term is taken as zero for
drawing this diagram.
• The energies of the other states are plotted
relative to this. Thus, the diagram is
discontinuous (slope varies) when the ligand field
becomes strong enough to produce electron
pairing.
• Some lines are non linear because of the mixing
of terms of same symmetry type. This occurs in
presence of a strong field ligand and is the non-
crossing rule.

22. • The electronic spectra of V(III) is shown alongside.
• On moving up the line from ground term to where
lines from the other terms cross it, we are able to
identify both the spin-forbidden and spin-allowed
transitions.
• It shows three major peaks with frequencies of :
ν1 = 17400 cm-1 , ν2 = 25400 cm-1 , ν3 = 34500 cm-1
• These are assigned to the following spin allowed
transitions:
a. 3T1g  3T2g
b. 3T1g  3T1g
c. 3T1g  3A2g
d2 configuration in Oh environment

24. Advantages over Orgel’s
• Tanabe – Sugano diagrams are more useful than Orgel diagrams because they
include both weak and strong field effect.
• They predict the transition energies for both spin allowed and spin forbidden
transitions.
• The diagrams although simple correlation diagrams provide more qualitative
information about absorption spectra of complexes.
• They have separate diagrams for different dn cases
• The Δ value for a complex can be evaluated by analyzing the observed spectral data
of Tanabe – Sugano diagram of the ion.
• From these diagrams, inter-electronic Racah parameter B can also be calculated.

25. Racah parameters
• Coulombic repulsions exist among electrons in the molecular orbitals of the
complex, therefore different terms of a configuration have different energies.
• The repulsion energy of each term of a configuration is expressed as sums of three
quantities.
• The sums of integrals are then called Racah parameters denoted as A, B, C
• The accurate evaluation of these terms is not possible even from spectral data of
free ions
• For d2 configuration in Oh environment, E(3P) = A + 7B, E(3F) = A – 8B
• Since A parameter is common, the difference in energies between a free ion in
ground state term F and an excited P term of the spin multiplicity is 15B

26. THANK YOU