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Electronic Spectra of Complexes

Color is due to electronic transition b/w levels whose SPACING corresponds tothe wavelength of light available in visible region (complimentary color isobserved).

SPACING depends on:- geometry of complex- nature of ligands- oxidation state of metal

[Co(H2O)6]2+

Oh, Pink[CoCl4]

2-

T d, Blue

[Ni(H2O)6]2+

Oh, Green [Ni(NH3)6]

2+

Oh, Blue

[Cr(H2O)6]2+

Oh, Bright Blue[Cr(H2O)6]

3+

Oh, Violet

Many TM complexes show color, e.g.

       3 NH 

    e

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Electronic Spectra: Examples 

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Electronic Spectra 

MO diagram of octahedralcomplex discussed so faris based on one-electronmodel

Configuration dn → add effect of e–e repulsion → add effect of Ligand Field 

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Effect of e-e repulsion: Term Symbols

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Splitting of Terms in Chemical Environment

(Transformation matrix) (General Transformation matrix)

(else 2l  + 1)

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Construction of Correlation Diagram

Configuration dn → add effect of e–e repulsion → add effect of Ligand Field 

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Correlation Diagram for a d2 system in Oh

(a) Free ion terms

(b) Weak field terms(c) Strong field ground

and excited configuration

(d) Strong field terms

(e) Intermediate field

Total degeneracy

= (2l+1)*(2s+1)

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Splitting of d n  and d 10-n  Terms in O h and T  d Field 

d n  vs d 10-n  

O h vs T  d

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Correlation Diagram for a d2 and d8 system 

Octahedral d8 and

tetrahedral d2  on Left Octahedral d2 and

tetrahedral d8  on Right 

Ot 

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Correlation Diagram for a d4 system 

Octahedral d10-n and tetrahedral dn  on Left 

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Correlation Diagram for a dn system 

(a) d2 and d8 (b) d3 and d7  (c) d4 and d6  (d) d5 

Octahedral d10-n and tetrahedral dn  on Left 

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Interpretation of Electronic Spectrum 

Selection Rules:Laporte allowed transition are those with a change in parity

Spin allowed transition have same spin multiplicity

Interpretation of Electronic Spectrum:1. No. (and position) of spectral lines2. Intensity of spectral lines

3. Width of spectral band

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Molar absorptivities for various types of electronic transition

observed in complexes 

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Ground & excited terms having the same spin for weak field

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Orgel Diagram 

d3 system Co2+ ion

Td (right), Oh (left)

T b S Di

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Tanabe – Sugano Diagrams 

T b S Di

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Tanabe – Sugano Diagrams 

I f F T b S Di

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Info. From Tanabe – Sugano Diagrams d6 system 

e.g. Low spin [Co(en)3]+

I f F T b S Di

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Info. From Tanabe – Sugano Diagrams 

Ratio = 1.32;

corresponds to D/B = 32.8

Gives B = 657/cm and D = 21550/cm

I f f T b S Di

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Info. from Tanabe – Sugano Diagrams 

d3