Spectral properties of d-block elements and coordination complexes

Spectral properties
• Sana 524
YouTube Video
• Spectrochemical Series
• https://www.youtube.com/watch?v=CedCBlRK0Bs
• Using the Spectrochemical Series to draw a metal complex ion's
crystal field splitting
• https://www.youtube.com/watch?v=h47CZz-t9iE

Course Outlines
Chemistry of d-Block Elements and Coordination Complexes
• Back ground of coordination chemistry → Done
• General chemical and physical properties of transition elements →
Done
• Comparison of the elements of first transition series (3d) with
those of second (4d) and third (5d) series → Done
• Nomenclature and Structure of coordination complexes with
coordination number 2-6 → Done
• Chelates and chelate effect → Done
• Theories of coordination complexes
• Werner's Theory → Done
• Valence Bond Theory (VBT)
• Crystal Field Theory (CFT)
• Molecular Orbital Theory (MOT)
• Sidgwick’s electronic interpretation
of coordination → Done
• Effective atomic number (EAN)
• Jahn-Teller theorem
• Magnetic properties → Done
• Spectral properties →
Continue
• Isomerism
• Stereochemistry
• Stability constants of
coordination complexes

Spectrochemical
series
A spectrochemical series is a list of ligands ordered on ligand strength
and a list of metal ions based on oxidation number, group and its
identity.
The spectrochemical series is a list of ligands based on the strength of
their interaction with metal ions
The spectrochemical series ranks ligands according the energy difference
Δ (Δo) between the t2g and eg orbitals in their (octahedral) complexes.

Spectrochemical series
• A spectrochemical series is a list of ligands ordered on
ligand strength and a list of metal ions based on oxidation
number, group and its identity.
• In crystal field theory, ligands modify the difference in
energy between the d orbitals (Δ) called the ligand-field
splitting parameter for ligands or the crystal-field splitting
parameter, which is mainly reflected in differences in
color of similar metal-ligand complexes.
Spectrochemical series of ligands
• The magnitude of Δo varies from strong(er) to weak(er) ligands.
• Strong(er) ligands are those which exert a strong(er) field on the
central metal ion and hence have higher splitting power while the
weak(er) ligands are those which have a weak(er) field on the
central metal cation and consequently relatively lower splitting
power.

• Thus strong(er) ligands (e.g. CN-) give larger value of Δo and
weak(er) ligands (e.g. F-) yield a smaller value of Δo (see following
figure).
Figure: Splitting of five d-orbitals in presence of strong(er) and weak(er)
ligands in an octahecral complex. (a) Five d-orbitals in the free metal ion (b)
Splitting of d-orbitals in presence of strong(er) ligands (c) Splitting of d-
orbitals in presence of weak(er) ligands.

• Above figure shows that not only Δo, which represents the
energy difference between the t2g and eg-sets of orbitals, is
smaller in the weak(er) field complex than in the
strong(er) field, but also that both the t2g and eg - levels of
the weak(er) field are correspondingly closer to the level
of the degenerate five d-orbitals of the free isolated
metallic ion than are those, respectively, of the strong(er)
field.
• The common ligands can be arranged in the order of their
increasing splitting power to cause d-orbitals splitting.
• This series is called spectrochemical series.
• For octahedral complexes, ∆oct increases along the following
spectrochemical series of ligands; the [NCS]- ion may coordinate
through the N- or S-donor (SCN means the ligand is bound via
sulfur and NCS via nitrogen) and accordingly, it has two positions
in the series:

• The spectrochemical series is reasonably general.
• Ligands with the same donor atoms are close together in the series.
• This series shows that the value of Δo in the series also increases from left
to right.
• The increase in the value of Δo on proceeding from left to right in the
spectrochemical series is quite evident from the values of Δo for some
octahedral complexes given in following table which clearly shows that
since on proceeding from 6Br- → 3en, the field strength of the ligands
increases, the value of Δo also correspondingly increases.
Small Δ Large Δ
Weak-field ligands Strong-field ligands
π-Donor ligands π-Acceptor ligands
High-spin complexes Low-spin complexes

Characteristics of octahedral complexes
• If we consider octahedral complexes of d-block metal ions, a
number of points arise which can be illustrated by the following
examples:
Table: Δo values (i.e., energy difference between t2g and eg levels) in
cm-1 for some octahedral complexes.

– The complexes of Cr(III) listed in following table illustrate the
effects of different ligand field strengths for a given Mn+ ion;
– The complexes of Fe(II) and Fe(III) in following table illustrate
that for a given ligand and a given metal, ∆oct increases with
increasing oxidation state;
Table:Values of ∆oct for some d-block metal complexes.

– Where analogous complexes exist for a series of Mn+
metals ions (constant n) in a triad, ∆oct increases
significantly down the triad (e.g. following figure):
– For a given ligand and a given oxidation state, ∆oct
varies irregularly across the first row of the d-block, e.g.
over the range 8000 to 14000 cm-1 for the [M(H2O)6]2+
ions.
Figure. The trend in values
of ∆oct for the complexes
[M(NH3)6]3+ where M = Co,
Rh, Ir.
[Co(III) < Rh(III) < Ir(III)]

Spectrochemical series of metals
• Trends in values of ∆oct lead to the conclusion that metal
ions can be placed in a spectrochemical series which is
independent of the ligands:
• Spectrochemical series are empirical generalizations and simple
crystal field theory cannot account for the magnitudes of ∆oct
values.
• In general, it is not possible to say whether a given ligand will exert
a strong field or a weak field on a given metal ion.
• However, when we consider the metal ion, the following two useful
trends are observed:
– ∆ increases with increasing oxidation number
– ∆ increases down a group

Example
• Which complex of the following pairs has the larger
value of Δ0:
– [CoIII(CN)6]3- and [CoIII(NH3)6]3+
– [CoIII(NH3)6]3+ and [CoIIIF6]3-
– [CoIII(H2O)6]3+ and [RhIII(H2O)6]3+
– [Co(H2O)6]2+ and [CoIII(H2O)6]3+
Solution
[CoIII(CN)6]3- and [CoIII(NH3)6]3+
• Since CN- is stronger than NH3 (i.e. CN- ion has greater
field strength than NH3), Δ0 for [CoIII(CN)6]3- is greater than
Δ0 for [CoIII(NH3)6]3+.
[CoIII(NH3)6]3+ and [CoIIIF6]3-
• [CoIII(NH3)6]3+ ion has grater value of Δ0 than that for
[CoIIIF6]3-.

[CoIII(H2O)6]3+ and [RhIII(H2O)6]3+
• Rh is a member of 4d-series while Co is a member of 3d-
series (3d → 4d).
• Thus principal quantum number, n, for Rh is greater than
that of Co and consequently Δ0 for [RhIII(H2O)6]3+ is
greater.
[Co(III) < Rh(III) < Ir(III)]
[Co(H2O)6]2+ and [CoIII(H2O)6]3+
• The charge on Co in [CoIII(H2O)6]3+ is higher than that on
Co [Co(H2O)6]2+ .
• Thus Δ0 for [CoIII(H2O)6]3+ is higher.

Spectral properties of d-block elements and coordination complexes

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