Spectrochemical Series. Crystal Field Splitting. Strong Field Ligands and Weak Field Ligands.

1. Spectrochemical
Series:-
❑ INTRODUCTION
❑ CRYSTAL FIELD SPLITTING
❑ CRYSTAL FIELD SPLITTING PARAMETER
❑ OCTAHEDRAL SPLITTING
❑ TETRAHEDRAL SPLITTING
❑ STRONG AND WEAK LIGANDS

2. Introduction:-
► Spectrochemical series is a list of ligands arranged in order of
their field strength.
► The ligands cyanide and co are considered as strong field
ligands and the halides are weak field ligands. Ligands such
as water (h2o) and ammonia (nh3) are medium field ligand.
► The spectrochemical series places ligands in order of
increasing (Δ). (Δ) is known as Crystal Field Splitting
parameter (CFSP) which tells us about splitting of d orbital.

3. CRYSTAL FIELD SPLITTING:-
► In order to understand the importance of spectrochemical
series , we need to understand Crystal Field Theory which
explains how ligands affect the energy levels of degenerate
orbitals.
► In this theory ligands are considered to be point negative
charges and metal ions are assumed to occupy appropriate
atomic orbitals of the metal.

4. ► It considers two types of interactions as follow:-
❑ Electrostatic attraction between the positive metal ion nucleus
and the negatively charged electrons of the ligands.
❑ Electrostatic repulsion between valence shell of the metal and
the ligand electrons.
Electrostatic attraction will result in overall decrease of
energy and repulsion will result in overall increase in
energy.

5. Crystal Field Splitting Parameter:-
► The size of split is called crystal field splitting parameter or (CFSP)
and is represented by Δ. This is affected by type of ligands as
❖ Strong Field Ligand repel the metal electrons very effectively so in
this case large splits will produce.
❖ Weak Field Ligands only partly repel the electrons result in smaller
splits.
On the other hand size and number of energy splits also depend on
geometry of the complex.
In order to understand splitting we will now study Octahedral splits
and Tetrahedral splits.

6. Octahedral Splits:-
► If we look at the diagram of octahedral complex have six ligands and are
symmetrical and there are 5 d orbitals:-
dxy,dyz,dxz,dx2-y2 and dz2.
(co(f6)).
Co

7. ► Now electrons in octahedral fields will have two possible
energy levels.
► Five different orbitals can be simplified into two set of orbitals:
HIGH ENERGY eg. LOWER ENERGY t2g.
Now ligand strength will effect how electrons fill these orbitals.
Filling OF Orbitals:-
❑ When surrounded by weak field ligands electrons fill the
orbitals as normal: one electron fills each orbital before a
second joins in. The Complexes have high spin.
❑ Strong field ligands push electrons to lower energy orbitals so
that t2g orbitals fill before starting eg. These complexes have
low spin.

8. Tetrahedral splits:-
► In case of tetrahedral there are four ligands attached to central metal atom.
► Two energy levels eg lower and t2g higher energy level here splitting will
be according to diagram:-

9. LIGANDS:-
Strong Field Ligand
► Strong field ligands result in
greater splitting of the crystal field.
► They form complexes with low
spin.
► They are diamagnetic in nature.
► C,N and P are usually strong
ligands.
► Example : Co and cyanide ion.
Weak Field ligand
► Weak field ligands include less
splitting of the crystal fields.
► They form complexes with high
spins.
► They are paramagnetic in nature.
► Halides, Oxygen and Sulphur are
usually weak ligands.
► Example : chloride and fluoride
ions.