[licamine]

How can one explain the fact that [MnCl₆]^4- is colorless, whereas [MnCl₄]^2- is weakly colored and [FeCl₄]^2- is strongly colored using Crystal Field Theory? I know that Mn(II) has a d3 configuration and Fe(II) has a d4 configuration, and I can draw the diagrams of d orbitals in tetrahedral and octahedral fields, but I don't get how their specific configurations could explain the fact that there are more or less electronic transitions, thus explaining the different colors.

Thank you very much!

[mjc123]

Well, for a start, Mn(II) is d5 and Fe(II) is d6. The 4s electrons ionise first.
What is special about the d5 configuration? How might this affect possible electronic transitions?
What is a fundamental difference between octahedral and tetrahedral symmetry? (Think of how we label the d orbitals.) How might this affect possible transitions?