Complex ions and larger Crystals?

1) Which complex ion should have the largest crystal field splitting, delta? Explain Why?

[Cr(H2O)6]^3 or [CrCl3]^3-

2) Which complex ion should ABSORB light of shorter wavelength? Explain Why?

[Cr(H2O)6]^3 or [CrCl3]^3-

1 Answer

  • 1) The aqua complex will have the higher crystal field splitting energy. This is because water is a stronger field ligand than chloride. As for why, I am not really sure, but I think the general trend is that stronger Lewis bases are stronger field ligands – thus, poor bases like Cl- are weak, while stronger bases such as NH3 are strong field. There are other factors that influence these rankings as well – backbonding by ligands with multiple bonds (CO and CN- being examples) makes these ligands very strong field. These rankings are called the spectrochemical series.

    2) Because [Cr(H2O)6]3+ has a larger gap between its two energy levels (this is the crystal field splitting energy), transitions between the t2g and eg levels for this complex will require more energy, and thus shorter-wavelength photons, than for [CrCl6]3-.

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