Question

What is the correlation between d-orbital ligand field strength and the wavelength of maximum absorption (λmax)?

Answer 1

According to the Crystal Field Theory, ligands that have high spin are considered "weak field" and ligands that have low spin are considered "strong field." This relates to the colors seen in a coordination complex. High spin ligands induce the absorption of longer wavelength (lower frequency=lower energy) light than complexes with low spin ligands since their respective Δovalues are smaller than the electron pairing energy.

The energy difference, Δo, determines the color of the coordination complex. According to the spectrochemical series, the high spin ligands are considered "weak field," and absorb longer wavelengths of light (weak Δo), while complexes with low spin ligands absorb light of greater frequency (high Δo). The color seen is the complementary color of that of the wavelength absorbed. To predict which possible colors and their corresponding wavelengths are absorbed, the spectrochemical series can be used:

(Strong field/large Δ0/low spin)    (weak field/ small Δ0/high spin)

CO-, NO-, CN->NO2->en>py≈NH3>EDTA4->SCN->H2O>ONO->ox2->OH->F->SCN->Cl->Br->I