Spectrochemical Series
What
is Spectrochemical Series?
Which
Factors influence the order of ligands in Spectrochemical series?
Spectrochemical
series:
In coordination
chemistry, ligands do not influence the metal ions equally. Some ligands
produce small crystal field stabilizing energy (∆) are called weak field
ligands. Ligands that produce large crystal field stabilizing energy (∆) are
called strong field ligands. A series has been developed called spectrochemical
series.
This series is
fundamental because it connects ligand strength, crystal field splitting and
spin state making it one of the most powerful tools for predicting the
properties of coordination compounds.
Definition:
Spectrochemical series is
arrangement of ligands in order of their ability to increase crystal field
stabilization. It is based on spectroscopic data, experimental observation and
magnetic properties of complexes.
Ligands on left side
produce small splitting while on the right side produce large splitting,
affecting the electron pairing and spin state.
Factors
affecting the sequence of ligands in the spectrochemical series:
Spectrochemical series is
not absolute. The observed order in the series varies somewhat from one complex
to another. Let’s discuss some factors which determine electrochemical series.
1.Sigma
Donation ability:
If six donor ligands are
in coordination compound are split into two, one having appropriate symmetry
for interacting with metal dz2 and
other with appropriate symmetry for interacting with dx2-y2
orbitals. The stronger the bonding interaction ,the higher the crystal field splitting(∆).
A strong σ bonding interaction
requires a good energy match between the metal and the ligand.
 |
| Effect of σ Donation and ∏ Interaction on Ligand strength |
Size of a
ligand is significant, because closer the ligand can approach the metal ,the
better the orbital overlap and consequently larger the crystal field splitting(∆).
Smaller ligands tend to
have larger crystal field splitting(∆) than larger ligands e.g. in halides.
I‑ < Br- < Cl -<
F-
3.∏
Bonding Interactions:
π
donor ligands have interaction between filled p orbital on ligand and metal d
orbital π donor ligands have multiple lone pairs on donor atom
(one pair can be a σ donor, another a π donor).
Halides are typical
examples. Called ‘weak field’ ligands.
Exception: O-based
ligands are not good π donors, O is too electronegative. H2O is considered σ
only, HO- is a very slightly better π donor (three lone pairs!), so is ‘weaker’
field than H2O.
Note: NH3, with only one
lone pair, is only a σ donor, is stronger field than H2O.
π
acceptor ligands have interaction between empty π* orbital on ligand and metal
d orbital
π acceptor ligands are
multiply bonded species that have empty π* orbitals to accept electron density
from the metal center Called ‘strong field’ ligands.
Affect
of π interactions on d orbital splitting
Both π donor and acceptor ligands have the right symmetry to interact with the t2g orbitals on the metal, so π interactions will change this from a nonbonding orbital set to an orbital set with more bonding or antibonding character π donor ligands interact with t2g non bonding orbitals. π donor orbitals are lower in energy than d orbitals (remember, these are nonbonding p orbitals on the ligand) This causes t2g orbitals that we associate with Δo to become antibonding in nature, raising their energy,and making Δo smaller in magnitude.
π acceptor ligands interact with t2g non bonding orbitals. π acceptor orbitals are higher in energy than d orbitals (remember, these are π* orbitals on the ligand) This causes t2g orbitals that we associate with Δo to become bonding in nature, lowering their energy, and making Δo larger in magnitude.
Importance
of Spectrochemical Series:
Spin
state of the complexes
Weak ligands→ high spin
Strong ligands → low spin
Colour
of complexes
Larger ∆→ absorption of
high energy light
Smaller ∆→ absorption of
lower energy light
Magnetic
properties
Weak ligands→
paramagnetic
Strong ligands→
diamagnetic
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