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Halogen effects--Shielding/Deshielding a proton in NMR

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sunshinegirl314:
In a compound 1-bromo-3-chloro-5-iodobenzene, which hydrogen is the most deshielded, intermediate, and least deshielded?

I thought that the hydrogen between the bromo and the chloro should be most deshielded because these are more electronegative than iodine, but then I was also told to consider the electron-donating effects of the halogens.  And somebody suggested that the hydrogen between the bromine and iodine was most deshielded.

This confuses me because I thought halogens are electron-withdrawing groups, so shouldn't deshielding be based solely on electronegativity? How is there electron-donating character.

Also, what is the relation between coupling constant and deshielding of protons?

Help would be greatly appreciated...

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