[adianadiadi]

We know that there is Mg²⁺ ion in chlorophyll. What will happen if it is replaced by Zn²⁺ ion or Ca²⁺ ion. Any thing special about Mg...? Nature has chosen this ion. Is it just size or also the stability...?

[Babcock_Hall]

It is a forum rule that you must provide your thoughts before we can help you.

[Arkcon]

That chlorophyll center is called a tetrapyrrole ring.  You might want to look up other molecules that use that ring, and see some other metals in the center.  Then, you can try to infer what the metal ion center means.  Its a good way to learn.

[adianadiadi]

It is a forum rule that you must provide your thoughts before we can help you.

I already stated that - may be size or stability are the factors. My understanding/knowledge is not going more than this. If you have anything in your mind, I would like to know..

[Babcock_Hall]

Metal ions differ in their ability to participate in redox chemistry, in the number of ligands that they prefer to coordinate, and in their coordination geometry (there are additional properties that one might consider).  One of these three you can ignore for the metal ions that you mentioned.  Which one?

[Corribus]

They also differ in the spectroscopic/optical properties when bound to a porphyrin (or related) center, which as you might imagine would be important for a photoactive redox center.