Electronegativity won't provide a satisfactory answer here. Iodine shows anomalous behaviour, often referred to as the "heavy atom effect". I do not know/remember the explanation of this effect, and don't have time to look it up right now, but Mack: "heavy atom effect" should give you something to look for in textbooks/google.
This paper looks interesting. I have not had time to read it all, and to be honest I am not at all confident that I will understand the detail, but this line stuck out:
"The number of different, sometimes mutually contradictory, explanations suggested for the observed trends is remarkable."
From
CEJ 1998, 4(1), 118-126