Have you seen all the different beautiful colours of makeup? Did you know they are made from complexes of transition metals? What if you put those complexes in redox reactions? how would they react?

In this article, we will go over the basics of redox chemistry and transition metal ions to talk about the way transition metal ion complexes behave in electrochemical settings. You probably haven't thought about those two concepts together, but you will definitely see way more links than you thought existed. Did you know how important transition metals are to redox chemistry? and redox chemistry is the basis for electrochemistry.

• Definition of oxidation states and their variability.
• Explanation of transition metal special chemical properties, along with the reason for variable oxidation states.
• What transition metal ion complexes are, and what they rely on.
• How oxidation states and ligands are able to alter the reduction potentials of transition metals.
• Examples of trends that arise when discussing the redox potential based on oxidation states and ligands.
• Applications of the electrochemistry involved.

Influences on the Redox Potential Of Transition Metals

In this article, we will be discussing how the redox potential of different transition metals is able to change based on different factors. It is important to understand those factors as they will determine the redox potential which comes from the chemical state of the transition metal ions.

To get down to the basics, there are two main influences which you need to understand that can possibly change the redox potential of transition metal ions.

First we will discuss the two factors that influence the redox potential of transition metals, and then discuss how they influence redox potential as opposed to the chemical environment of the transition metal ions alone. The two aspects of transition metal chemistry we will be discussing will be variable oxidation states and complexation.

Variable Oxidation States

So what are oxidation states and how can they be variable if they are so crucial towards the chemical nature of ions?

So what does this really mean?

Variable oxidation states concern the way transition metals are able to exist in different oxidation states when in ionic form. Transition metals, which means elements from the d-block of the periodic table, are able to have multiple oxidation states.

This is an intrinsic chemical property of transition metals. If you take other elements, say for example the alkali metals, they are only able to exist in one oxidation state (+1). This is because of their electronic configuration. Transition metals are different from other elements as they are able to form oxidation states, which allows them to form different compounds.

For example, transition metal ions can have different oxides based on their oxidation number. Different oxidation numbers will yield different molecular (empirical) formulas for the same anion.

Transition metal complexes

Here we will discuss the other factor that affects the redux potential of transition metals, which is complexation.

So what are transition metal ion complexes?

Transition metal ion complexes occur due to the ions being bonded datively by ligands, which means they are surrounded by molecules which contain a lone pair of electrons. A dative (coordinate) bond arises by the donation of this lone pair to create a covalent bond.

Complexation results in the d-block of transition metal ions being split, which gives them different chemical and physical properties. This is why transition metal ions behave differently when in a complex and when not. Read more about this in our article Transition Metal Ions in Aqueous Solution.

Below you can see how a transition metal ion complex looks like.

So how does complexation and variable oxidation states impact the redux potential of transition metals?

First of all, what is a redux potential?

If we extrapolate this definition to transition metal complexes, we can say that the redox potential of transition metals will be dependent on their complexation as well as the oxidation state.

Firstly, because different oxidation states will form different transition metal ion complexes, it is crucial to distinguish between the variable oxidation states when performing redox experiments.

Secondly, as ligands are very crucial to complexation, different ligands split the d-orbital to different extents, which will ultimately be reflected in the redox potential of transition metals.

Examples of Redox Potential Of Transition Metals Trends

So when we perform experiments with transition metals to see their redox potentials, what trends do we observe?

Looking at a periodic table where reduction potentials are displayed to see if we can observe any distinctive trends, we can see that there is no single obvious trend. Rather across a period from left to right, there is no definitive trend to distinguish between transition metal elements of ascending atomic number. Take a look at the diagram below which has examples of transition metals in some of their oxidation states along with their oxidation number.

Fig. 2: Reduction potentials of transition metal ions. Source: Chegg.com

Can you notice any specific cohesive trends or abnormalities? This might be due to the variability of transition metals, from different ionic forms and complexation states when in those ionic forms. Therefore we need to look at other factors and trends.

We can discuss this again through the level of complexation, or in other terms, the effect of the ligands.

Ligands on Redox Potential of Transition Metals

Here we will discuss how ligands, or in other terms, the complexation, are able to impact the redox potential of transition metals.

Take a look at the following two oxidation potentials for two different electrolytes. The only difference is the solvent, which is to say that the transition metal ion complex has different ligands.

$$[Ni(H_{2}O)_{6}]^{2+} + 2e^{-} \rightleftharpoons Ni + 6H_{2}O E^{o} = -0.26 V$$

$$[Ni(NH_{3})_{6}]2^{+} + 2e^{-} \rightleftharpoons Ni + 6NH_{3} E^{o} = -0.49 V$$

Can you tell see how the reduction potential significantly decreases with a different ligand? That is because different ligands are able to interact with the transition metal ion differentially. That is to say, they can bind to the ion stronger or weaker and split the d-orbital to different extents.

Applications of Transition Metal Redox Potentials

Here we will discuss the applications that can arise them the electrochemistry of transition metal ions.

The applications of transition metal ion complexes comes from the way their redox potentials are able to be used in many different disciplines of science. Most commonly, which particularly looks at their physical chemistry importance, would be for electrochemical experiments and studies. The way in which redox chemistry is so tightly linked with oxidation states can be quantified and analysed through a different light. If considering variable oxidation states as being different reduction (or oxidation) potentials, then we can start thinking of the way the internal electronic structure as well as the environment of the transition metal ion is able to produce the specific redox chemistry observed.

Likewise, we can use the information from redox potentials of transition metal ions to understand transition metal ion complexes in solutions from a different viewpoint. Reduction potentials give critical information on the strength of the ligand as well as how the ligand is able to interact with the transition metal ion. By examining this data it is apparent that we can generate hypotheses on the nature of the ligand, and compare the ligands in way that have not been done before to determine their strenghts.

Having learned all this, you should be able to tackle different aspects of not only redox chemistry, but also metal chemistry. In this article, we went over the basics of redox potentials and transition metal complexes, most important how they act when transition metal d-orbitals are split under different contexts.