[fwlksajzxlc]

Is it correct to say that the green color of chlorophyll comes from pi to pi* excitation of electrons in the conjugated system (alternating single and double bonds) around magnesium?

Chlorophyll structure:


And not from d orbital business actually involving Mg or Mg2+ itself, and possibly the nitrogens?

I recently had a general chemistry final that asked me to choose one or the other, and I said the former.  The diagram provided was the same as above but with Mg2+ instead.  My justification was that Mg2+ does not have d orbitals.  I wasn't sure about this.  Do only transition metals form colored complexes?  In my mind that would explain why iron is why blood is red, and why many websites say chlorphyll is green because of Mg, by analogy.  But I don't quite trust them since they're rather cursory and relatively few in number.

[Rabn]

Hi, The green color of clorophyll is caused by the conjugated double bonds absorbing the other wavelengths. Ions do have orbitals, some that were formerly filled are just vacated. As far as trnsition metals being the cause of color....most certainly that is not the case. A lot of pure organic compunds are yellow. the key to color is the conjugated double. In a lot of cases a transition metal ion is used as kind of a switching board...cholorophyll being a very good example.