[tainted1899]

Hi, I have a question regarding ligand-metal charge transfer.

For the complex [CO (NH3)5 X]2+ where X is Cl, Br, or I, which halogen ligand would cause the MLCT band to have the greatest energy and why?

Does it have something to do with 'I' being the best electron donor of the 3 and forming the strongest bond with the metal centre. But how does this relate to the energy of the band?

Really appreciate any help I can get! Thanks in advance! 

[Dan]

Hi, I have a question regarding ligand-metal charge transfer.

For the complex [CO (NH3)5 X]2+ where X is Cl, Br, or I, which halogen ligand would cause the MLCT band to have the greatest energy and why?

Can you confirm whether you are asking about MLCT or LMCT? It matters.

[tainted1899]

oops...i meant LMCT

[Dan]

OK, I always used to argue this using electronegativity with a good success rate.

So, start with a simplified MO diagram of the levels involved:

Metal pi*                            
                           ^
                           |
                           |
                           |
Ligand pi                            

That thing between the levels is meant to be a vertical arrow indicating the LMCT.
Now, as the electronegativity of the ligand drops, what happens to the energy of the ligand pi orbitals?
So, does the energy of the transition increase or decrease with electronegativity of the ligand?