To be able to use Crystal Field Theory (CFT) successfully, it is essential that you can determine the electronic configuration of the central metal ion in any complex. This requires being able to recognize all the entities making up the complex and knowing whether the ligands are neutral or anionic, so that you can determine the oxidation number of the metal ion. The oxidation number is then used to determine the electron configuration and the number of electrons in d-orbitals for a metal ion. Example \(\PageIndex{1}\) What is the electronic configuration of Fe(III)? Solution Start with the electron configuration of the neutral atom. It is sufficient to use the noble gas configuration. Fe: [Ar] 4s 2 3d 6 Then, remove (or add) electrons to reflect the oxidation state of the metal. Remember to remove electrons from the orbital with the highest principal quantum number first! Fe(III) has lost 3 electrons relative to Fe(0). Fe 3 + : [Ar] 3d 5 From the electron configuration, we see that Fe 3 + has 5 electrons in its outermost d-orbitals. Oxidation Numbers and their Relative Stabilities The IUPAC definition of the oxidation number in a coordination compound is: the charge a central atom in a coordination entity would bear if all the ligands were removed along with the electron pairs that were shared with the central atom. It is represented by a Roman numeral. The transition metals show a wide range of oxidation numbers. The Table \(\PageIndex{1}\) summarizes the known oxidation numbers of the first row transition elements. Sc Ti V Cr Mn Fe Co Ni Cu Zn NaN I I I I I I I I NaN NaN II II II II II II II II II III III III III III III III III III NaN NaN IV IV IV IV IV IV IV NaN NaN NaN NaN V V V V V NaN NaN NaN NaN NaN NaN VI VI VI NaN NaN NaN NaN NaN NaN NaN NaN VII NaN NaN NaN NaN NaN * The oxidation number zero usually assigned to the elemental state has been omitted from the Table. The elements Cr to Co form several metal carbonyl compounds where the metals are considered to have an oxidation number of zero. A number of important conclusions can be drawn from Table \(\PageIndex{1}\). There is an increase in the number of oxidation numbers from Sc to Mn. All seven oxidation numbers are exhibited by Mn. The oxidation number of VII represents the formal loss of all seven electrons from 3d and 4s orbitals. In fact all of the elements in the series can utilize all the electrons in their 3d and 4s orbitals. There is a decrease in the number of oxidation states from Mn to Zn. This is because the pairing of d-electrons occurs after Mn (Hund's rule) which in turn decreases the number of available unpaired electrons and hence, the number of oxidation states. The stability of higher oxidation states decreases in moving from Sc to Zn. Mn(VII) and Fe(VI) are powerful oxidizing agents and the higher oxidation states of Co, Ni and Zn are unknown. The relative stability of the +2 state with respect to higher oxidation states, particularly the +3 state increases in moving from left to right. This is justifiable since it will be increasingly difficult to remove the third electron from the d orbitals. There is a tendency of intermediate oxidation states to disproportionate. For example, Mn(VI) → Mn(IV) + Mn(VII) and Cu(I) → Cu(0) + Cu(II). The lower oxidation numbers are usually found in ionic compounds and higher oxidation numbers tend to be involved in covalent compounds. The relative stability of oxidation numbers is an extremely important topic in transition metal chemistry and is usually discussed in terms of the standard reduction potential (E°) values. Thermodynamically E° values are equated to ΔG° values by the relationship: ΔG° = -nFE° where n = number of electrons involved and F = Faraday of electricity. Hence, the E° values indicate the possibility of spontaneous change from one oxidation state to the other. This value however, does not give any information about the reaction rate. Predictions regarding the stability of a particular oxidation state of an element can be made from the Tables of Redox values found in any standard text book or online.