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Iodine is a transition metal I will die on this hill.
Care to defend your position? Iodine is certainly not in the d-block...
The intended joke is that hypervalent iodine compounds like Dess-Martin periodinane flip between different oxidation states like you often see for transition metals. As an example, the mechanism usually drawn for oxidations by DMP is similar to those drawn for PCC/Jones reagent, where the electrons removed from the substrate are "banked" at the metal center. Obviously, redox chemistry is not at all limited to transition metals, but I am often surprised at iodine's propensity to engage in it. A lot of research over the past decade or two has also developed redox catalysis with these reagents, reactivity which is commonly (though again not always) the purview of transition metals.