Oxidation States of Transition Metals

The Transition Metals are characterized by atoms whose outermost electrons (valence electrons) are distributed between two sublevels of different energy levels. During the oxidation of these metal atoms, the transfer of electrons may involve only one or both these sublevels. If only one sublevel is involved, the metal always loses 2 electrons, forming an oxidation state of (+2). If two sublevels are involved, a varying number of electrons (from 2 to 7) may be lost, resulting in "multiple oxidation" states. Hence, it is not possible to predict which oxidation state will be exhibited by the metal. Quantitative analysis of the compound formed, however, will reveal the simplest whole number ratio (empirical formula) of the metal and nonmetal atoms. Once this information is obtained, one can quickly determine the oxidation state of the metal. See the following examples.

Example 1

The experimentally determined formula for a compound produced by the reaction of iron and chlorine is FeCl₃. What is the oxidation state of the iron ion in the compound?

One must rely on the guidelines concerning the charge of the nonmetal ions in order to determine the oxidation state of a Transition metal. The guidelines for the oxidation states of nonmetals, state that chlorine, a Group VII member, always forms a charge of ( -1) when it reacts with metals. As there are three chloride ions, the total negative charge is (-3).

The compound, FeCl₃, has no net charge, and one must assume that the positive charge of the iron ion in the compound must be equal to the total negative charge of (-3).

Since there is only one iron ion for three chlorine ions, the oxidation state of iron must be therefore be "+3".

Example 2

The formula for a compound formed by the reaction of manganese and oxygen is MnO2, what is the oxidation state of manganese in the compound?

Referring to the guidelines for nonmetals, it is known that oxygen always forms an ion with a charge of ( -2 ) when it reacts with metals. charged-ion. Since there are two oxygen ions in the compound, the total charge is (-4). As MnO₂ has not net charge, it must mean the single manganese ion has a positive charge equal to this total negative charge. Hence, the charge of the manganese ion must be (+4).

Example 3

An oxide of chromium has the formula, Cr₂O₃. What is the oxidation state of each chromium ion?

As each oxygen ion is assigned a charge of (-2) when it reacts with a metal, the total negative charge is (-6). As the compound has no net charge, this must mean the the total charge of the two chromium ions equals (+6). Hence, the charge of each ion is (+3)

Example 4

An ionic compound has the formula, K₂CrO₄. What is the oxidation state of chromium in this compound?

Potassium is a Representative Element which belongs to Group I. Hence, each potassium ion carries a "+1" charge for a total charge of "+2". Each oxygen ion, carries a (-2) charge whenever it reacts with a metal. The total negative charge due to the 4 oxygens is (-8).

Adding the total positive charge from the two potassium ions and total negative charge of the two oxygen ions gives a net charge of (-6).

Since the compound has no net charge, and there is only 1 chromium ion, its charge must be (+6).