Transition Metals: Variable Oxidation States

Topic 15: Transition Metals 

1. In order to develop their practical skills, students will be encouraged to carry out a range of practical experiments related to this topic. Possible experiments to be discussed include the stepwise reduction of vanadium(V) to vanadium(II), investigating the reactions of copper(II) ions or chromium(III) ions, using sodium hydroxide and ammonia solution to identify transition metal ions, investigating autocatalysis and preparing a complex transition metal salt.
2. Mathematical skills that will be developed in this topic include investigating the geometry of different transition metal complexes.
3. Within this topic, students will consider the model for the filling of electron orbitals encountered earlier in their course, and see how limitations in that model indicate the need for more sophisticated explanations. They will also appreciate that catalyst research is a frontier area, and one that provides an opportunity to show how the scientific community reports and validates new knowledge.

Edexcel A level Chemistry (2017)

Topic 15A: Principles of transition metal chemistry

15A/3 To be able to understand why transition metals show variable oxidation number

What is oxidation state?

Oxidation state is the oxidation number a particular species carries.

So the sodium ion Na⁺ has oxidation number +1 and sodium is in oxidation state +1.

Or the chloride ion Cl^— has oxidation number —1 and chlorine is in oxidation state —1.

Transition metals show variable oxidation states unlike group 1 or Group 17 elements.

The illustration below shows these oxidation states some of which of more stable (shown in colour) at room temperature than others. 

Variable Oxidation States 

Question is why do these metals exhibit variable oxidation states?

The answer has to do with the fact that the 4s energy level and the 3d energy level are very close in value so that the metals can bond to other elements using electrons from both the 4s and the 3d levels. 

Here are the 3d and 4s electron arrangements for the stable oxidation states of these metals

So take the very stable purple manganate(VII) ion (MnO₄^—) the manganese ion (oxidation state 7+) engages in 4 covalent bonds with the oxide ions (oxidation state —2) giving rise to the tetrahedral structured ion overall charge —1 (see below).

This ion also illustrates another feature of the higher oxidation states of the transition metals which is that they are stabilized as oxo anions i.e the metal ion is combined with oxide ions.

Here are a few common examples you will have come across in your college or A level chemistry studies:

Orange Dichromate (VI)           Cr₂O₇^2—

Yellow Chromate (VI)               CrO₄^2—

Black Manganese (IV)oxide       MnO₂

Yellow Metavanadate (V)          VO₃^—

Purple Manganate(VII)             MnO₄^—

Green Manganate(VI)               MnO₄^2—