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What is spectrochemical series? Explain the difference between a weak field ligand and a strong field ligand.

What is spectrochemical series? Explain the difference between a weak field ligand and a strong field ligand.

Grade:9

2 Answers

Sumit Majumdar IIT Delhi
askIITians Faculty 137 Points
6 years ago
Dear student,

Ligands are ranked according to the extent that they split the d electrons. Strong field ligands cause the d orbitals to be split by a large amount. Weak field ligands, on the other hand, cause the d orbitals to be split by only a small amount. The order for some common ligands is as follows:

I-< Br-< S2-<SCN-< Cl-< NO3-< Cl-< F-< OH-< H2O < < O2-< NH3<NO2-< CN-< CO
<-- weaker field ligands stronger field ligands -->
<-- Larger wavelength absorption smaller wavelength absorption -->

This is known as the spectrochemical series. A ligand like CO is called a strong field ligand, whereas as ligand like Cl-is considered a weak field ligand. Consider what this looks like for two octahedral complexes formed by Co3+: [CoCl6]3-( Co3+w/ six Cl-ligands) and [Co(CO)6]3+( Co3+w/ six neutral CO ligands):

68-2331_figdis12-1.gif
Because the spacing between the d orbitals changes, the amount of energy required to promote an electron from the lower d orbitals to the upper ones also changes. Thus, transitions in [Co(CO)6]3+require more energy (a shorter wavelength of light) than those in [CoCl6]3-. Such differences dramatically influence the color of transition metal complexes.
Thanks & Regards
Sumit Majumdar,
askIITians Faculty
Ph.D,IIT Delhi
prabhat kumar pal
11 Points
3 years ago
Really strong field ligand can split d orbitals in two set of orbitals containing 3 and 2 orbitals respectively.The orbitals are known as t2g and eg set of orbitals.

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