What is the molecular structure of the coating on cast iron cookware known as seasoning?
Some hints:


if you mean that "what exactly" in the original sense of the words,
then the answer is: nobody knows and will ever know. 
If you are content with some more vague answer, then 
look at the pyrolysis chemistry occuring from 150 °C 
up with fats, proteins and starch/sugars, and all that 
at once. This is a vast field of chemistry, and all 
is gives to a trained chemist is a idea of the structures 
of that coating. 
Bonds to iron will not occur, first because on cast iron and 
on steel pans there is always a layer of iron oxide, 
on cast iron  you find a glaze formed by fusion of the
molds finish layer (often containing graphite and clay)