[peterjones]

Okay, here is the example. Its a iron atom in the as of hemoglobin bounded to five nitrogens. then an oxygen comes along and binds as to the iron. It makes it iron(III), making the hybridization of iron Sp3d2( I believe this is my guess). So, obviously, its going to be in the d orbital. But my question is how does the iron know which d-orbital it need to get the right bonding geometry?

I assume its has something to do the electronegative of iron, but the process really doesn't make sense to me.

Or does it not matter and it can bind to whichever d-orbital, because they are all d orbitals

 

[Corribus]

I now have a vision of an iron atom scratching its head because it can't remember which d-orbital to put an electron into.

You've got it backwards.  The iron doesn't choose anything.  The electron goes into the lowest energy available orbital, simple as that.  In an isolated atom, all the d-orbitals are degenerate (equivalent energy).  However when the atom is surrounded by other atoms, as it is in a heme, some of the d-orbitals become lower in energy than others.  This is the basis of ligand field theory.  If an oxidation or reduction occurs at this point, it is this ordering of energies that determines from which/into which an electron is removed/added by an oxidation/reduction reaction.