Carbon monoxide binds to Fe in hemoglobin some 200 times more strongly than oxygen. This is the reason why CO is a toxic substance. The metal-to-ligand sigma bond is formed by donating a lone pair from the donor atom to an empty sp3d2 orbital on Fe.
(a) On the basis of electronegativities, would you expect the C or O atom to form the bond to Fe?
(b) Draw a diagram illustrating the overlap of the orbitals involved in the bonding.