Question

Why cobalt hexaflouride is coloured while ferroua hexaflouride is colourless?

Answer 1

Almost all Co3+(d6)CoX3+(d6) complexes are low spin, including [Co(H2O)6]3+,[Co(ox)3]3−,[Co(NH3)6]3+[Co(HX2O)X6]X3+,[Co(ox)X3]X3−,[Co(NHX3)X6]X3+and [Co(CN)6]3−[Co(CN)X6]X3−. The only common high-spin cobalt(III) complex is [CoF6]3−[CoFX6]X3X−. 

Answer 2

Explanation:

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.