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

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):

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.