TPRH How to tell difference between Lewis base and reductant?

[Padfoot]

On pg 302 of tprh, problem 1 asks "When oxygen is binding to the iron atom in hemerythrin, it is acting as a"
A. Lewis acid
B. Lewis base
C. oxidant
D. reductant

The answer is B, but isn't a reductant similar to a lewis base? Since they are both giving electrons to something else?

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[Cawolf]

This is my take on it - may or may not be right.

A Lewis base will donate electrons to form a bond (as stated in the problem), while a reducing agent will donate the electrons completely.

In this case a covalent bond is formed, so the oxygen is acting as a Lewis base.

 

[amy_k]

*A Lewis base can also be a reductant if there is a redox reaction occuring too.

To be classified as a reductant in this case, oxygen must supply the electrons for reduction, but the Iron core of Hemerythrin is oxidized as a result of the binding process, so oxygen ends up being reduced and is therefore the oxidant. Based on their answer I would read this question to mean that while oxygen is in the initial process of binding to the Hemerythrin molecule to form the transition state it is acting as a Lewis base donating electrons. But the end result is oxidation of the iron core once the peroxy-iron complex is formed giving oxy-hemerythrin so technically Lewis base and Oxidant could be correct. It may be one of those cases where you need to stick to just the info they want you to know for the MCAT - that Oxygen binds as a Lewis Base?