discussion

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Why are Ferrocene and Acetyl Ferrocene Colored?

Note: If you are going to use this explanation in the discussion section, reference it, or paraphrase this explanation.

The ferrocene complexes are colored due to the d-d transition occurring within the Iron atom(s). Iron, which is a transition metal, has electrons in the d orbitals, and these d electrons can bond. Transition metals like iron, undergo d orbital splitting, which has an energy gap in the visible region of the electromagnetic spectrum. (It may be helpful to review crystal field theory if you would like more information on this subject. Crystal Field Theory can be found in an inorganic chemistry text book such as Shriver, Atkins, and Langford's Inorganic Chemistry, and Cotton and Wilkinson's Advanced Inorganic Chemistry.) The d orbital splitting in the ferrocene case is more complicated than for the octahedral complexes, in that they (energy levels of the d orbitals) split into 3 levels, and not 2, but the principle is the same. The lowest d-d transition yields a peak into the near UV for ferrocene. It tails into the blue, so blue is absorbed, leaving a yellow color. As you add acetyl groups, the splitting is reduced, pushing the peak into the blue, with a tail absorbing green light, so you get an orange color, and as the peak shifts further, red.

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