One of the somewhat obvious, but still noteworthy, facts about the transferric elements is that they are rare. This is especially true in the role of biological systems.
Of the elements from Hydrogen to Iron, 15 to 17 of them are necessary or for life. The next four elements after iron, cobalt, nickel, copper, and zinc, are all present in living things, in quite small quantities. From the 31st element to the 92nd, the heaviest occurring natural element (Uranium), only three (Selenium, Molybdenum and Iodine) are necessary for living creatures, or at least for eukaryotes. While it is not surprising that living things should use what is common rather than what is rare, the degrees of magnitude more rare that transferric elements are, and the near absence of them in biological processes, is something to keep in mind.