This document discusses transition metal coordination compounds and their properties. It begins by explaining color and absorption spectra in relation to an artist's color wheel. It then discusses crystal field theory and how ligand bonding splits the d-orbital energies of metal ions. Stronger field ligands create more splitting. This splitting determines the wavelengths absorbed and the observed color. Examples are given of color changes from oxidation state or ligand changes. The spectrochemical series ranks ligands by field strength. Magnetic properties depend on unpaired electrons. Complexes can be high-spin or low-spin depending on the relative energies of pairing and splitting. Examples of splitting patterns and orbital occupancy are shown for octahedral, tetrahedral, and square planar complexes. Hemoglobin and carbon

1. 23-1
Mr.G.M.Dongare
(M.Sc,B.Ed,SET)
Associate Professor
Dept.of Chemistry
Shri Shivaji Colege of Arts,Commerce and
Science College, Akola.
TRANSITION ELEMENTS AND THEIR
COORDINATION COMPOUNDS
Part IV
Academic year 2018-19

2. 23-2
An artist’s wheel.
Each color has a complementary
color; the one opposite it on the
artist’s wheel.
The color an object exhibits depends
on the wavelengths of light that it
absorbs.
An object will have a particular color because
• it reflects light of that color, or
• it absorbs light of the complementary color.

3. 23-3
Relation Between Absorbed and Observed Colors
Absorbed Color λ (nm) Observed Color λ (nm)
Violet 400 Green-yellow 560
Blue 450 Yellow 600
Blue-green 490 Red 620
Yellow-green 570 Violet 410
Yellow 580 Dark blue 430
Orange 600 Blue 450
Red 650 Green 520

4. 23-4
Crystal Field Theory
Crystal field theory explains color and magnetism in
terms of the effect of the ligands on the energies of the
d-orbitals of the metal ion.
The bonding of the ligands to the metal ion cause the
energies of the metal ion d-orbitals to split.
Although the d-orbitals of the unbonded metal ion are equal in
energy, they have different shapes, and therefore different
interactions with the ligands.
The splitting of the d-orbitals depends on the relative
orientation of the ligands.

5. 23-5
The five d-orbitals in an octahedral field of ligands.
The ligands approach along the x, y and z axes. Two of the
orbitals point directly at the ligands, while the other three
point between them.

6. 23-6
Splitting of d-orbital energies in an octahedral
field of ligands.
The d orbitals split into two groups. The difference in energy
between these groups is called the crystal field splitting energy,
symbol Δ.

7. 23-7
The effect of ligands and splitting energy on
orbital occupancy.
Weak field ligands lead to
a smaller splitting energy.
Strong field ligands lead to a
larger splitting energy.

8. 23-8
The color of [Ti(H2O)6]3+.
The hydrated Ti3+ ion is purple.
Green and yellow light are absorbed
while other wavelengths are
transmitted. This gives a purple color.

9. 23-9
The color of [Ti(H2O)6]3+.
When the ion absorbs light, electrons can move from the lower t2g
energy level to the higher eg level. The difference in energy
between the levels (Δ) determines the wavelengths of light
absorbed. The visible color is given by the combination of the
wavelengths transmitted.

10. 23-10
The Colors of Transition Metal Complexes
The color of a coordination compound is determined
by the Δ of its complex ion.
For a given ligand, the color depends on the oxidation
state of the metal ion.
For a given metal ion, the color depends on the ligand.

11. 23-11
Effects of oxidation state and ligand on color.
[V(H2O)6]2+ [V(H2O)6]3+
[Cr(NH3)6]3+ [Cr(NH3)5Cl ]2+
A change in oxidation state
causes a change in color.
Substitution of an NH3 ligand
with a Cl- ligand affects the
color of the complex ion.

12. 23-12
The spectrochemical series.
I- < Cl- < F- < OH- < H2O < SCN- < NH3 < en < NO2
- < CN- < CO
WEAKER FIELD STRONGER FIELD
LARGER D
SMALLER D
LONGER  SHORTER 
As Δ increases, shorter wavelengths (higher energies) of light
must be absorbed to excite electrons. For reference H2O is
considered a weak-field ligand.

13. 23-13
SOLUTION:
Ranking Crystal Field Splitting Energies (Δ)
for Complex Ions of a Metal
PROBLEM: Rank the ions [Ti(H2O)6]3+, [Ti(NH3)6]3+, and [Ti(CN)6]3- in
terms of Δ and of the energy of visible light absorbed.
PLAN: The formulas show that Ti has an oxidation state of +3 in all
three ions. From Figure 23.21, we tank the ligands by crystal
field strength: the stronger the ligand, the greater the
splitting, and the higher the energy of light absorbed.
The ligand field strength is CN- > NH3 > H2O, so the
relative size of Δ and energy of light absorbed will be
[Ti(CN)6]3- > [Ti(NH3)6]3+ > [Ti(H2O)6]3+

14. 23-14
The Magnetic Properties of Transition
Metal Complexes
Magnetic properties are determined by the number of
unpaired electrons in the d orbitals of the metal ion.
Hund’s rule states that e- occupy orbitals of equal energy
one at a time. When all lower energy orbitals are half-
filled:
The number of unpaired e- will depend on the relative
sizes of Epairing and Δ.
- The next e- can enter a half-filled orbital and pair up by
overcoming a repulsive pairing energy, (Epairing).
- The next e- can enter an empty, higher, energy orbital by
overcoming Δ.

15. 23-15
High-spin and low-spin octahedral complex
ions of Mn2+.

16. 23-16
Orbital occupancy for high-spin and low-spin
octahedral complexes of d4 through d7 metal ions.
high spin:
weak-field
ligand
low spin:
strong-field
ligand
high spin:
weak-field
ligand
low spin:
strong-field
ligand

17. 23-17
Identifying High-Spin or Low-Spin Complex
Ions
PROBLEM: Iron (II) forms a complex in hemoglobin. For each of the
two octahedral complex ions [Fe(H2O)6]2+ and [Fe(CN)6]4,
draw an energy diagram showing orbital splitting, predict
the number of unpaired electrons, and identify the ion as
low spin or high spin.
PLAN: Fe2+ electron configuration shows the number of d
electrons, and the spectrochemical series shows the
relative ligand strengths. We draw energy diagrams and
separate the t2g and eg orbital sets more for the strong-field
ligand. Then we add electrons, noting that a weak-field
ligand gives the maximum number of unpaired electrons
and a high-spin complex, whereas the strong-field ligand
will give the minimum number of unpaired electrons and a
low-spin complex.

18. 23-18
SOLUTION:
t2g
eg
Potential
energy
↑
↑↓ ↑
↑ ↑
[Fe(H2O)6]2+
high-spin
t2g
eg
Potential
energy
↑↓ ↑↓ ↑↓
[Fe(CN)6]4-
low-spin

19. 23-19
Splitting of d-orbital energies by a tetrahedral
field of ligands.
The splitting of d-orbital energies is less in a tetrahedral than an
octahedral complex, and the relative d-orbital energies are
reversed. Only high-spin tetrahedral complexes are known
because Δ is small.

20. 23-20
Splitting of d-orbital energies by a square planar
field of ligands.
Square planar complexes are low-spin and usually diamagnetic
because the four pairs of d electrons fill the four lowest-energy orbitals.

21. 23-21
Hemoglobin and the octahedral complex in heme.
Chemical Connections
Hemoglobin consists of four protein chains, each with a bound
heme. In oxyhemoglobin (B), the octahedral complex in heme has
an O2 molecule as the sixth ligand for iron(II).
(Illustration by Irving Geis. Rights owned by Howard Hughes Medical Institute.
Not to be used without permission.)

22. 23-22
Some Transition Metal Trace Elements in Humans

23. 23-23
The tetrahedral Zn2+ complex in carbonic anhydrase.
Chemical Connections