Question

why feCN6³- is colourless whereas COF6³- is coloured​

Answer 1

Answer:

And there is 5 unpaired electrons hence it's spin multiplicity = (2S+1) = 6 , and multiplicity of a excited is cannot be six thus The transitions in Fe +3 ion are spin forbidden and are extremely weak so as to make [FeF6]3- almost colorless. Only one band is expected namely the t2g→eg transition (d1).

Answer 2

Answer:

And there is 5 unpaired electrons hence it's spin multiplicity = (2S+1) = 6 , and multiplicity of a excited is cannot be six thus The transitions in Fe +3 ion are spin forbidden and are extremely weak so as to make [FeF6]3- almost colorless. Only one band is expected namely the t2g→eg transition (d1).