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Chapter 21: Problem 46

Draw all geometrical isomers of \(\mathrm{Pt}(\mathrm{CN})_{2} \mathrm{Br}_{2}\left(\mathrm{H}_{2} \mathrm{O}\right)_{2}\). Which of these isomers has an optical isomer? Draw the various optical isomers.

Short Answer

Expert verified
There are three geometrical isomers for the \(\mathrm{Pt}(\mathrm{CN})_{2}\mathrm{Br}_{2}\left(\mathrm{H}_{2} \mathrm{O}\right)_{2}\) complex, namely the trans, cis-a, and cis-b isomers. However, none of these isomers possesses optical isomers, as they all have a plane of symmetry.

Step by step solution

01

Identify the ligand positions

To start, we need to recognize that the coordination number of platinum is 6 in the given complex, meaning that there are six ligand positions available around the central atom. Therefore we will have the six ligands, two cyano ligands (\(\mathrm{CN}\)), two bromo ligands (\(\mathrm{Br}\)), and two aqua ligands (\(\mathrm{H_2O}\)) occupying these positions.
02

Determine possible arrangements of ligands

Since the platinum center has a coordination number of 6, it forms an octahedral arrangement with the ligands. There are two main geometrical isomer classes for octahedral arrangements, namely fac and mer. In the fac isomer, the three identical ligands are in the facial positions (coplanar), and in the mer isomer, the three identical ligands are in the meridional positions (along one equatorial line). However, in our given complex, we do not have three identical ligands. Instead, we have three pairs of identical ligands, so we need to consider all the possible arrangements of these three pairs.
03

Draw the geometrical isomers

There are three possible geometrical isomers for this complex: 1. The trans isomer: the two cyano ligands are opposite to each other, so are the bromo and aqua ligands. Pt || CN - Br | | Br - H2O || H2O 2. The cis-a isomer: the two cyano ligands are next to each other, so are the bromo ligands, while the two aqua ligands are in opposite positions. Pt || CN - H2O | | Br - CN || Br 3. The cis-b isomer: the two cyano ligands are next to each other, so are the aqua ligands, while the two bromo ligands are in opposite positions. Pt || CN - Br | | H2O - CN || H2O
04

Identify isomers with optical isomers

In order to have an optical isomer, a molecule must be non-superimposable on its mirror image, meaning it must lack a plane of symmetry. In our case, all three geometrical isomers (trans, cis-a, and cis-b) have a plane of symmetry. Hence, none of them possesses optical isomers. In conclusion, there are three different geometrical isomers for the given complex, but none of these isomers has an optical isomer.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Coordination Chemistry
Coordination chemistry is a branch of inorganic chemistry that deals with compounds featuring a central metal atom or ion (usually called a metal center) surrounded by molecules or ions known as ligands. These ligands can be anything from simple ions like water ()
Octahedral Complexes
Octahedral complexes are a type of coordination compound where the central metal ion is surrounded by six ligands positioned at the corners of an octahedron. This arrangement results in an octahedral geometry, a term used to describe the shape of the complex where ligands are symmetrically arranged around the metal ion in three-dimensional space. For example, in the )
Optical Isomerism
Optical isomerism is a form of stereoisomerism related to the chiral nature of molecules. Chirality occurs when a molecule cannot be superimposed on its mirror image, much like how your left hand does not perfectly overlap with your right hand when mirrored. The result is a pair of isomers, called enantiomers, which are non-superimposable mirror images of each other. These molecules exhibit optical activity, which means they can rotate the plane of polarized light.)

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Most popular questions from this chapter

The complex ion \(\mathrm{Fe}(\mathrm{CN})_{6}{ }^{3-}\) is paramagnetic with one unpaired electron. The complex ion \(\mathrm{Fe}(\mathrm{SCN})_{6}{ }^{3-}\) has five unpaired electrons. Where does \(\mathrm{SCN}^{-}\) lie in the spectrochemical series relative to \(\mathrm{CN}^{-}\) ?

Draw all the geometrical isomers of \(\left[\mathrm{Cr}(\mathrm{en})\left(\mathrm{NH}_{3}\right)_{2} \mathrm{BrCl}\right]^{+} .\) Which of these isomers also have an optical isomer? Draw the various isomers.

Qualitatively draw the crystal field splitting for a trigonal bipyramidal complex ion. (Let the \(z\) axis be perpendicular to the trigonal plane.)

When an aqueous solution of \(\mathrm{KCN}\) is added to a solution containing \(\mathrm{Ni}^{2+}\) ions, a precipitate forms, which redissolves on addition of more KCN solution. Write reactions describing what happens in this solution. [Hint: \(\mathrm{CN}^{-}\) is a Brónsted-Lowry base \(\left(K_{\mathrm{b}}=10^{-5}\right)\) and a Lewis base. \(]\)

Consider the following data: $$\begin{aligned} \mathrm{Co}^{3+}+\mathrm{e}^{-} \longrightarrow \mathrm{Co}^{2+} & & \mathscr{E}^{\circ}=1.82 \mathrm{~V} \\ \mathrm{Co}^{2+}+3 \mathrm{en} \longrightarrow \mathrm{Co}(\mathrm{en})_{3}^{2+} & K &=1.5 \times 10^{12} \\ \mathrm{Co}^{3+}+3 \mathrm{en} \longrightarrow \mathrm{Co}(\mathrm{en}){ }^{3+} & K &=2.0 \times 10^{47} \end{aligned}$$ where en \(=\) ethylenediamine. a. Calculate \(\mathscr{E}^{\circ}\) for the half-reaction $$\mathrm{Co}(\mathrm{en})_{3}^{3+}+\mathrm{e}^{-} \longrightarrow \mathrm{Co}(\mathrm{en})_{3}^{2+}$$ b. Based on your answer to part a, which is the stronger oxidizing agent, \(\mathrm{Co}^{3+}\) or \(\mathrm{Co}(\mathrm{en})_{3}{ }^{3+}\) ? c. Use the crystal field model to rationalize the result in part b.
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