2. METAL TO LIGAND CHARGE TRANSFER
• When the electron is transferred from an orbital located predominantly on a metal center (say, a d-
like orbital) to one localized on a ligand (say, a pi star orbital on a conjugated ligand), that
constitutes a metal-to-ligand charge transfer.
• An example of these type of complexes include [cr(nh3)6]3+, [cr(cl)(nh3)5]2+.
• Other examples of complexes showing these type of transitions are [fe(co)3(bipy)], [ru(bipy)3] 2+ , [w(co)4(phen)] etc.
• In all these cases the π* empty orbital present on the ligand becomes the receptor of electrons with the introduction of
light and the absorption process.
2
3. 3
• In the process of charge relocation the metal is oxidized and the ligand is reduced, therefore for
this type of charge transfer phenomenon, it is important that the metal oxidation as well as ligand
reduction is quite feasible. Easily reducible ligands are those which have a low lying, vacant π*
orbital, such as pyridine, which then forms sturdy colored complexes with the metal ions that are
easily oxidized such as fe2+ and cu+.
• Depending on the number of electrons in the d orbital of the metal ion, two different transitions
are possible; the t2g to π* and eg to π* which both may be Observed depending on the conditions.
5. INTRODUCTION
The nephelauxetic effect is a term used in the inorganic chemistry of
transition metals
It refers to a decrease in the racah interelectronic repulsion parameter
Given the symbol B, that occurs when a transition-metal free ion forms a
complex with ligands
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6. CONTD…
The name "nephelauxetic" comes from the greek for cloud-expanding and
was proposed by the danish inorganic chemist C. K. Jorgensen.
The presence of this effect brings out the disadvantages of crystal field
theory, as this accounts for somewhat covalent character in the metal-ligand
interaction
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7. RACAH REPULSION
PARAMETERS B AND C
Electrons in the partly filled d-orbitals of a metal ion repel one another and give rise to a
number of energy levels depending upon the arrangement of these electrons in the d-
orbitals
The energy of each of these levels can be expressed in terms of some inter electronic
repulsion parameters called racah repulsion parameters B and C
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8. CONTD…
It is observed experimentally that the magnitude of these inter electronic
repulsion parameters always decreases on the complexation of the metal ion.
This is possible only if inter electronic repulsion between the d electrons of the
metal ion decreases on complexation.
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9. CONTINUED
The magnitude of inter electronic repulsion is inversely proportional to the
distance ‘r’ between the regions of maximum charge density of the d orbitals which are
occupied by the electrons
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10. CONTINUED
The extension of the lobes of the d orbitals, that is, the expansion of d electron charge
cloud during complexation by the transition metal ion.
Expansion of d-electron clouds takes place during complexation of trasition metal ion
which is termed nephelauxetic effect
Normally the electron repulsion is found to be weaker in complexes than in free ion and
this means the value of B for the complex is less than for a free ion.
This is due to the delocalization of the electron over the ligands away from the metal
which separates them and hence reduces repulsion.
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11. CONDT…
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The reduction of B from its free ion is normally
reported in terms of nephelauxetic effet β
Where β is the nephelauxetic parameter and refer to the
electron cloud expansion
12. REASONS FOR E- CLOUD EXPANSION
This electron cloud expansion effect may occur for one (or both) of two reasons.
– One is that the effective positive charge on the metal has decreased. Because the
positive charge of the metal is reduced by any negative charge on the ligands, the d-
reduced by any negative charge on the ligands, the d- orbitals can expand slightly.
– The second is the act of overlapping with ligand orbitals and forming covalent
bonds increases orbital size, because the resulting molecular orbital is formed from
two atomic orbitals
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13. NEPHELAUXETIC SERIES
13
The values of β depend on the ligand and vary along the
nephelauxetic series. The series tell us that a small value of
β means that there is large measure of d- electron
delocalization and a greater covalent character in the
complex
14. CONTD…
The nephelauxetic effect does not only depend upon the ligand type, but also upon
the central metal ion. These too can be arranged in order of increasing nephelauxetic
effect as follows:
Mn(ii) < ni(ii) ≈ co(ii) < mo(ii) < re(iv) < fe(iii) < ir(iii) < co(iii) < mn(iv)
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15. Characteristics of Metal
The characteristics of Metal are.
Metal should have high energy of Orbitals
Metal should have filed Orbitals
Metal should have low oxidation state
۔For example
(-1٫ 0)
Metal should be easily oxidizeable (oxidized)
16. Characteristics of ligand
The characteristics of Metal are.
Ligand should have low energy Orbitals.
Ligand should have vacant Orbitals.
Ligand should have high oxidation state.
It should be easily reducible.
17. Comparison between Metal to ligand and
Ligand to metal Charge transfer bands
When the metal or Ligand is changed٫the energy of both the Charge transfer bands
changes.
Keeping the metal constant, if the ligand is more readily polarisable then the
energies of, Ligand to metal Charge transfer bands decrease.
Keeping the metal constant ٫if the ligand is more readily reduceable then the
energies of ٫ metal to ligand charge transfer bands decrease.
With the decrease in oxidation state of the metal for a given ligand the energy of,
Metal to ligand charge transfer bands decrease.
18. Comparison between d-d transitions and charge
transfer transitions
d-d transitions
d-d transitions the metal ion of the complex must contain d-electrons.
d-d transitions are Laporte For bidden.
Charge transfer transitions
metal ion may or may not contain d-electrons.CT transitions may occur in complexes
having Metal with (d-zero) and (d-ten) Configuration. These transitions may also occur in
non-transition compounds.
C T transitions are Laporte allowed.
19. Cont۔۔۔۔
d-d transitions
These transitions do not involve redox process.
d-d transitions occur in visible region.
These are not useful for spectrophotometric estimation of metal ions.
charge transfer transitions
These transitions involve redox process.
These transitions are usually high energy transitions and hence often occur in the near UV region.
These are usually For spectrophotometric estimation of metal ions.
20. Energy decrease between metal and
Ligand
Energy decrease between metal and Ligand
1. Energy different decrease
2. Required energy decrease
3. Wave No. Decrease
4. Frequency decrease
5. Wave length increase
![METAL TO LIGAND CHARGE TRANSFER
• When the electron is transferred from an orbital located predominantly on a metal center (say, a d-
like orbital) to one localized on a ligand (say, a pi star orbital on a conjugated ligand), that
constitutes a metal-to-ligand charge transfer.
• An example of these type of complexes include [cr(nh3)6]3+, [cr(cl)(nh3)5]2+.
• Other examples of complexes showing these type of transitions are [fe(co)3(bipy)], [ru(bipy)3] 2+ , [w(co)4(phen)] etc.
• In all these cases the π* empty orbital present on the ligand becomes the receptor of electrons with the introduction of
light and the absorption process.
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