Question

Write half-equations and an overall equation to represent the oxidation of tetraammineplatinum(II) ion to trans-tetraamminedichloroplatinum(IV) ion by ${\mathrm{Cl}}_2$ Then make sketches of the two complex ions.

Short answer

The half-reactions are:$[\mathrm{Pt}({\mathrm{NH}}_3{)}_4{]}^{2+}\to [\mathrm{Pt}({\mathrm{NH}}_3{)}_4{]}^{4+}+2e^{-}$ and ${\mathrm{Cl}}_2+2e^{-}\to 2{\mathrm{Cl}}^{-}$.The overall redox reaction after balancing is: $[\mathrm{Pt}({\mathrm{NH}}_3{)}_4{]}^{2+}+{\mathrm{Cl}}_2\to [\mathrm{Pt}({\mathrm{NH}}_3{)}_4{\mathrm{Cl}}_2{]}^{2+}$.The complex ions' sketches show Pt as the center atom surrounded by the respective ligands.

Step-by-step solution

Determine and write down the half-reactions

The two half reactions would look like this:1. $[\mathrm{Pt}({\mathrm{NH}}_3{)}_4{]}^{2+}\to [\mathrm{Pt}({\mathrm{NH}}_3{)}_4{]}^{4+}+2e^{-}$ -- This is the oxidation half-reaction where Platinum ion is losing electrons.2. ${\mathrm{Cl}}_2+2e^{-}\to 2{\mathrm{Cl}}^{-}$ -- This is the reduction half-reaction where Chlorine is gaining electrons.

Write down the overall redox reaction

Combine the two half reactions to form the overall redox reaction. Don’t forget to balance the electrons.The overall reaction after balancing would look like:$[\mathrm{Pt}({\mathrm{NH}}_3{)}_4{]}^{2+}+{\mathrm{Cl}}_2\to [\mathrm{Pt}({\mathrm{NH}}_3{)}_4{\mathrm{Cl}}_2{]}^{2+}$

Sketch the two complex ions

First, sketch the tetraammineplatinum(II) ion which is a square planar complex. The Pt(II) ion is at the center and is surrounded by 4 NH3 groups at corners. In the sketch, show Pt in the center with all 4 NH3 groups attached. Then, sketch the trans-tetraamminedichloroplatinum(IV) ion which is also a square planar complex. The Pt(IV) ion is in the center surrounded by 4 ligands - 2 NH3 groups and 2 Cl- ions. Cl- ions are trans, that is, opposite to each other.

Key concepts

Oxidation Half-Reaction

In redox reactions, oxidation refers to the loss of electrons. Each atom or molecule can gain or lose electrons, which impacts the chemical state and charge of the resulting species. In the given exercise, the oxidation half-reaction involves the transformation of the tetraammineplatinum(II) ion $[\mathrm{Pt}({\mathrm{NH}}_3{)}_4{]}^{2+}$.
This platinum complex starts with a +2 charge and, through the loss of electrons, gets oxidized to $[\mathrm{Pt}({\mathrm{NH}}_3{)}_4{]}^{4+}$.
Here’s how the reaction looks: - $[\mathrm{Pt}({\mathrm{NH}}_3{)}_4{]}^{2+}\to [\mathrm{Pt}({\mathrm{NH}}_3{)}_4{]}^{4+}+2e^{-}$
The key here is understanding that the magnesium ion is going from a lower charge to a higher charge, indicating a loss of two electrons.
This kind of half-reaction is always accompanied by a corresponding reduction half-reaction, which you'll find out about next.

Reduction Half-Reaction

Reduction, in the context of redox reactions, is the process of gaining electrons. For every oxidation half-reaction, there's a corresponding reduction half-reaction. In this exercise, chlorine gas ${\mathrm{Cl}}_2$ acts as the electron acceptor.
Imagine chlorine as being very eager to gain electrons.

• The reduction half-reaction is: ${\mathrm{Cl}}_2+2e^{-}\to 2{\mathrm{Cl}}^{-}$

This means that the two electrons lost by the platinum complex are gained by the chlorine molecules.
This changes each chlorine atom into a chloride ion ${\mathrm{Cl}}^{-}$, where both now exist as negatively charged ions.
It's essential to recognize how these electrons are transferred between these species, influencing the overall reaction.

Complex Ions

When discussing coordination chemistry, terms like complex ions frequently appear. A complex ion refers to a central metal atom or ion, such as platinum in this exercise, bound to surrounding molecules or ions called ligands.
These ligands like ${\mathrm{NH}}_3$ and ${\mathrm{Cl}}^{-}$ are usually electron-pair donors.

• The tetraammineplatinum(II) ion $[\mathrm{Pt}({\mathrm{NH}}_3{)}_4{]}^{2+}$ represents a classic example with ammonia as its ligand.
• Upon oxidation, this evolves into $[\mathrm{Pt}({\mathrm{NH}}_3{)}_4{\mathrm{Cl}}_2{]}^{2+}$, where chlorine ions replace two ammonia molecules as ligands.

Recognizing these changes and how ligands interact with the central metal ion helps in understanding how the structure and function of complex ions are manipulated in chemical reactions.

Square Planar Complexes

Square planar complexes are coordination compounds with a distinctive geometry. Unlike more common shapes like tetrahedral or octahedral, square planar configurations are typically seen in complexes with a central metal atom bound to four ligands arranged around it in a flat, square configuration.
In the exercise you've worked through:

• Both $[\mathrm{Pt}({\mathrm{NH}}_3{)}_4{]}^{2+}$ and $[\mathrm{Pt}({\mathrm{NH}}_3{)}_4{\mathrm{Cl}}_2{]}^{2+}$ are square planar complexes.
• In the platinum(II) complex, four ammonia molecules serve as ligands.
• Post-reaction, the platinum(IV) complex has a slightly altered structure, with two ammonia and two trans chlorine ligands, maintaining the planar layout.

Understanding these geometric arrangements provides insights into the ways these molecules react and how molecular structures are maintained or modified through chemical processes.