The element iron is a transition metal. Transition metals are defined as elements which have incomplete d shell with electrons. It has the 4s2-3d6 electron configuration.
In contrast to non-metals which obey the octet rule for their electronic stability such as the noble gases, transition metals obey different rules in their electronic and energetic stability.
d5 and d10 electronic configuration of transition metals in general are of particular stability. Therefore, d5 electronic configuration of iron or the +3 oxidation state is of particular stability.
In the free ion this configuration confers magnetic properties to iron because it is high spin configuration.
In complexes of iron as well as with other transition metals, the five d orbitals are split to two energetically separate sets. These two sets are divided to t2g set which contain three energetically equivalent d orbitals and to eg set which contains two energetically equivalent d orbitals.
t2g set lies energetically lower than the eg set of d orbitals. In iron with +3 oxidation state with high spin configuration the five d electrons occupy three of them the t2g set of orbitals while the other 2 d electrons occupy the eg set of orbitals.
This gives the iron ion magnetic properties as all the electrons are aligned in one direction thus making a magnetic atom.
If the difference in energy between the t2g level and the eg level is too high then all the five electrons in iron will occupy the t2g level. Thus making the properties of the metal diamagnetic.
This phenomenon of high or low spin of the metal depends on the ligand attached to the metal. Water complexes are usually high spin, while carbon monoxide or cyanide complexes are low spin.
This is so due to the high splitting of the d orbitals by the CO and CN- ligands which occur as a result to back donation that these two ligands do. Sigma donors which do not have back bonding are usually high spin in their complexes with transition metals in general and with iron in particular.
Water complexes are in general high spin with iron and with other transition metals. In the case of water there is no back bonding so that the splitting of the t2g and eg levels is not big.
d10 electronic configuration of iron and in other transition metals which is manifested as F-2 is of particular stability. This configuration is always diamagnetic regardless of the type of ligands that are attached to the metal.
Iron metal is produced industrially by a reduction process of iron oxides such as hematite or magnetite. Many options occur for a reducing agent in this process. The choice depends on the availability and price of the reducing agent.
On an industrial scale coke or elemental carbon is used as the reducing agent. In this process, coke is first oxidized to carbon monoxide or CO. This process has a disadvantage that the amount of oxygen used to oxidize coke must be limited so that carbon dioxide or CO2 does not generate during the oxidation process.
Carbon monoxide or CO is treated either with hematite or with magnetite to produce iron metal and CO2. Other impurities in the product such as SiO2 is removed by the administration of CaCO3 which upon heating releases CaO which reacts with SiO2 to form CaSiO3 which is removed from iron. Thus purifying iron further.
Iron ions in the +2coxidation state have biological role. They are the carrier of oxygen in the blood to the cells of the body. Iron is bound to the heme portion of hemoglobin molecule where it binds oxygen and releases CO2 in the lungs.
Oxygen is transported by iron to the cells of the body to participate in the oxidative phosphorylation process to generate energy rich molecules in the form of ATP or adenosine triphosphate. Iron is also important biologically as it occurs in the cytochromes of the respiratory chain in the oxidative phosphorylation process. It participates there in the oxidation-reduction reactions that takes place as part of the respiratory process. Iron occurs there as Fe-S clusters.
Anemia due to iron deficiency is treated with iron sulphate tablets or FeSO4. Deficiency of iron in the blood leads to anemia, while excess iron in the blood leads to a medical condition that is called hemochromatosis. This condition can be treated with chelating agents for iron ions in the blood.
