Chapter 13, COORDINATION CHEMISTRY: REACTION MECHANISMS AND METHODS OF SYNTHESIS; SUBSTITUTION REACTIONS Video Solutions, Inorganic Chemistry

Problem 1

Outline the preparation of the cis and trans isomers of $\left[\mathrm{P}\left(\mathrm{NH}_3\right)\left(\mathrm{NO}_2\right) \mathrm{Cl}_2\right]^{-}$, given that the substituent trans effects are in the order $\mathrm{NO}_2^{-}>\mathrm{Cl}^{-}>\mathrm{NH}_3$.

Vishal Sharma

Numerade Educator

02:46

Problem 2

a. On page 707 (reaction 13-14) we discussed the effects of leaving groups on substitutions at square planar centers. The compound needed in these studies was trans$\left[\mathrm{Pt}\right.$ (amine) (DMSO) $\mathrm{Cl}_2$ ]. Given that the trans effects of the ligands are in the order DMSO (S-bonded) $>\mathrm{Cl}^{-}>$amine, describe a preparation of the cis and trans isomers from $\left[\mathrm{PtCl}_4\right]^{2-}$.
b. If cis-[Pt(amine $\left.)_2(\mathrm{DMSO}) \mathrm{Cl}\right]^{+}$is heated with one equivalent of $\mathrm{HCl}$, one ligand is substituted with chloride. What is the composition of the final product and what is its structure?

Adriano Chikande

Numerade Educator

01:26

Problem 3

The basicities (as measured by $p K_a$ values) of trimethyl-, triethyl-, and tri-n-propylphosphine are nearly identical. However, in reaction 13-5, if tri-n-propylphosphine is replaced by $\mathrm{Et}_3 \mathrm{P}$ the rate is enhanced; even greater enhancement is gained by replacement with $\mathrm{Me}_3 \mathrm{P}$. Why should this be the case?

Aadit Sharma

Numerade Educator

Problem 4

Of what mechanistic significance are the $\triangle S^{ \pm}$values in Table 13-2?

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02:10

Problem 5

Why should the rate constants in reaction 13-6 be so much greater than those in 13-7?

David Collins

Numerade Educator

01:39

Problem 6

Why do phosphines have a high trans effect? (Hint: Consider the way in which they are bonded to a transition metal ion.)

Aadit Sharma

Numerade Educator

01:39

Problem 7

Explain more fully why a ligand such as an olefin or $\mathrm{CN}^{-}$can have a small trans influence but a large trans effect.

Aadit Sharma

Numerade Educator

01:49

Problem 8

Table 13-6 presents the predicted stereochemical results for the substitution of the group $\mathrm{X}$ in the complex cis- $\left[\mathrm{Co}(\mathrm{en})_2 \mathrm{AX}\right]^{n+}$, Verify these results.

Nicole Smina

Numerade Educator

02:27

Problem 9

On page 713 it was noted that the rate laws given in Figure 13-6 can all reduce to the simple form Rate $=k_{\text {obss }}[$ Complex $][\mathrm{X}]$ under certain circumstances. Show how this may be possible. Consult Chapter 7, Table 7-1.

David Collins

Numerade Educator

03:28

Problem 10

In the text you learned of the rate laws for the substitution of a monodentate ligand on an octahedral complex by another monodentate ligand. However, another very important process is the substitution of two monodentate ligands by a chelating ligand. Wilkins [Acc. Chem. Res, 3, 408 (1970)] shows that the steps currently considered important when the substitution ligand is bidentate are
$$
\begin{aligned}
& \left.\left(\mathrm{H}_2 \mathrm{O}\right)_5 \mathrm{M}(\mathrm{L}-\mathrm{L}) \quad \stackrel{k_3}{k_{\rightarrow}}\left(\mathrm{H}_2 \mathrm{O}\right)_4 \mathrm{M}-\mathrm{L}\right)+\mathrm{H}_2 \mathrm{O} \\
&
\end{aligned}
$$

That is, after formation of an outer sphere complex, one end of the bidentate ligand substitutes for one water molecule, followed by substitution of the other end for yet another water. Assuming that the first reaction is faster than the other two (and that $\left.k_x / k_{-x}=K_{\text {dit }}\right)$, and that a steady state exists for the intermediate $\left(\mathrm{H}_2 \mathrm{O}\right)_5 \mathrm{M}(\mathrm{L}-\mathrm{L})$, derive the rate constant expression. Show that when $k_{\mathrm{b}}>k_{\mathrm{w}}$, the rate constant expression reduces to that commonly observed for the substitution of a monodentate ligand by another monodentate ligand.

David Collins

Numerade Educator

03:28

Problem 11

Perhaps the chief conclusion to be reached from the discussion of reaction mechanisms in this chapter is that four-coordinate planar complexes react predominantly by an associative mechanism, whereas octahedral complexes react predominantly by dissociative pathways. The question that is now of interest is the mechanism of substitution in five-coordinate complexes. Clearly, both associative and dissociative mechanisms are possible, since a five-coordinate complex can potentially either add or lose a ligand. Some research has recently been published on such reactions, and this question concerns one of those publications.

The replacement of a phosphine ligand on an iron- or cobalt-dithiolene complex proceeds according to the stoichiometry:
figure cant copy
a. For a dissociative mechanism, the reactions involved would be
$$
\begin{aligned}
& \mathrm{M}\left(\mathrm{S}_2 \mathrm{C}_2 \mathrm{Ph}_2\right)_2 \mathrm{X} \quad \stackrel{k_1}{\underset{k_2}{=}} \mathrm{M}\left(\mathrm{S}_2 \mathrm{C}_2 \mathrm{Ph}_2\right)_2+\mathrm{X} \\
& \mathrm{M}\left(\mathrm{S}_2 \mathrm{C}_2 \mathrm{Ph}_2\right)_2+\mathrm{L} \stackrel{k_3}{\longrightarrow} \mathrm{M}\left(\mathrm{S}_2 \mathrm{C}_2 \mathrm{Ph}_2\right)_2 \mathrm{~L}
\end{aligned}
$$

Write the rate law for this mechanism, assuming that the four-coordinate intermediate is in a steady state. What is the expression for $k_{\text {obs }}$ when excess $L$ is used (that is, you operate under pseudo-first order conditions)? To what does this expression reduce when $k_3 k_2$ ?
b. For an associative mechanism, the reactions involved would be
$$
\mathrm{M}\left(\mathrm{S}_2 \mathrm{C}_2 \mathrm{Ph}_2\right)_2 \mathrm{X}+\mathrm{L} \stackrel{k_2}{\stackrel{k_1}{\rightleftharpoons}} \mathrm{M}\left(\mathrm{S}_2 \mathrm{C}_2 \mathrm{Ph}_2\right)_2 \mathrm{XL} \xrightarrow{k_1} \mathrm{M}\left(\mathrm{S}_2 \mathrm{C}_2 \mathrm{Ph}_2\right)_2 \mathrm{~L}+\mathrm{X}
$$

Assuming steady state conditions for the six-coordinate intermediate, write a rate law for the associative mechanism. Again assume pseudo-first order conditions in $L$ and write the expression for $k_{\text {obs }}$
c. Given below is a plot of $k_{\text {obs }}$ vs. [L] for the reaction of $\mathrm{Co}\left(\mathrm{S}_2 \mathrm{C}_2 \mathrm{Ph}_2\right)_2 \mathrm{PPh}_3+\mathrm{L}$. When $\mathrm{L}=\mathrm{P}(\mathrm{OEt})_3, k_{\text {obs }}=0.05+42.5[\mathrm{~L}]$
figure cant copy
From this plot, and the fact that added $\mathrm{X}$ (the leaving group) has a small effect on the reactions of the cobalt complex, what can you conclude about the mechanism of the reaction? That is, is the reaction strictly dissociative or strictly associative, or are both pathways used? If both pathways are used, which predominates? [If you wish further information on reactions of five-coordinate complexes, consult the following, more recent references: D. A. Sweigart and P. Heidtmann, J. C. S. Chem. Commun., 556 (1973); J. C. S. Dalton, 1686 (1975).]

David Collins

Numerade Educator

01:00

Problem 12

$\mathrm{Cd}^{2+}$ and $\mathrm{Hg}^{2+}$ exchange coordinated water very rapidly and are in Class $\mathbf{I}$. Why is $\mathrm{Zn}^{2+}$ in Class II and not Class I?

Aadit Sharma

Numerade Educator

06:46

Problem 13

Why does $\mathrm{Ga}^{3+}$ exhibit a greater rate of water exchange than $\mathrm{Al}^{3+}$ ? What would the rate of exchange be for $\mathrm{Tl}^{3+}$ with respect to $\mathrm{Al}^{3+}$ and $\mathrm{Ga}^{3+}$ ? What about $\mathrm{Tl}^{+}$?

Ali Daher

Numerade Educator

03:09

Problem 14

The values of $\Delta S^1$ in Table 13-5 are generally positive or are only slightly negative. Why should this be the case? In contrast, the values of $\Delta S^t$ in Table 13-2 are quite negative. Why?

Dennis Howard

Numerade Educator

02:52

Problem 15

The values of $\Delta H^*$ in Table 13-5 are, in general, greater than the corresponding values in Table 13-2. Why?

Hast Aggarwal

Numerade Educator

04:52

Problem 16

Given below are thermodynamic data for the reactions of halides and pseudo-halides with $\mathrm{Fe}$ (III) and $\mathrm{Hg}$ (II) in water.
$$
\begin{array}{clccc}
\hline \text { Metal Ion } & \text { Halide } & \Delta H^{\bullet}(\mathbf{k c a l} / \mathrm{mol}) & \Delta G^{\circ}(\mathbf{k c a l} / \mathrm{mol}) & \Delta S^{\circ}(\mathbf{e} . \mathrm{u} .) \\
\hline \mathrm{Hg}^{2+} & \mathrm{F}^{-} & 0.85 & -1.4 & 8 \\
& \mathrm{Cl}^{-} & -5.9 & -9.19 & 11.1 \\
& \mathrm{Br}^{-} & -9.6 & -12.8 & 10.9 \\
& \mathrm{I}^{-} & -18.0 & -17.5 & -2 \\
\mathrm{Fe}^{3+} & \mathrm{CN}^{-} & -23.0 & -23.2 & 0.7 \\
& \mathrm{~F}^{-} & 7.5 & -6.68 & 48 \\
& \mathrm{Cl}^{-} & 8.5 & -2.02 & 35 \\
& \mathrm{Br}^{-} & 6.1 & -0.82 & 23 \\
& \mathrm{SCN}^{-} & -1.5 & -4.1 & 8.7 \\
& \mathrm{~N}_3^{-} & -1.6 & -6.8 & 17.3 \\
\hline
\end{array}
$$
Plot $\Delta G, \Delta H$, and $T \Delta S$ against the halide in the order given, and notice the great difference in the plots for the two different metal ions. Comment on these differences. For example, which factor- $\Delta H$ or $\Delta S$-controls the reactions of $\mathrm{Fe}$ (III) and $\mathrm{Hg}(\mathrm{II})$ ? Does complex stability reflect the factors that went into the classification of metal ions into $a$, $b$, and borderline groups? (See Tables 13-7 and 13-8).

Rashmi Sinha

Numerade Educator

04:28

Problem 17

Figure 13-15 shows the trend in $\Delta H$ for diamine, amino acid, and dicarboxylate ligands. Supportive data are presented in Table 13-9. Speculate on the reason that $\Delta H$ for dicarboxylate ligands is greater than zero, whereas that for diamine ligands is less than zero.

Cameron Oden

Numerade Educator

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Problem 18

One of the most important ligands in inorganic chemistry is the acetylacetonate ion, $\left[\mathrm{H}_3 \mathrm{C}-\mathrm{CO}-\mathrm{CH}-\mathrm{CO}-\mathrm{CH}_3\right]^{-}$, a bidentate ligand that forms complexes with virtually all metal ions. The thermodynamic data for the formation of $1: 1$ complexes (reaction below) are given in tabular form below. Assuming room temperature, calculate $\Delta G^{\circ}$ or $\log K$. Plot $\Delta H^{\circ}$ and $\Delta G^{\circ}$ or $\log K$ us. metal ion as illustrated in Figures 13-14 and 13-15,
figure cant copy
and compare the resulting plots with those for diamine and dicarboxylate ligands in Figures 13-14 and 13-15. Also compare the data below with those in Table 13-9. Comment on similarities and differences.
$$
\begin{array}{lccccccl}
& \mathbf{M n}^{2+} & \mathrm{Fe}^{2+} & \mathbf{C o}^{2+} & \mathbf{N i}^{2+} & \mathbf{C u}^{2+} & \mathbf{Z n}^{2+} & \\
\hline \Delta H^{\circ} & -2.5 & - & -1.2 & -6.7 & -4.7 & -1.9 & \text { (kcal/mol) } \\
\Delta S^{\circ} & 11 & - & 21 & 12 & 22 & 17 & \text { (e.u.) } \\
\hline
\end{array}
$$

Susan Hallstrom

Numerade Educator

02:46

Problem 19

In the course of experiments on the biological functioning of platinum compounds, cisand trans- $\left[\mathrm{Pt}\left(\mathrm{NH}_3\right)_2 \mathrm{Cl}_2\right]$ were allowed to react with methionine, with the results outlined below. What can these reactions tell about the ability of $\mathrm{Cl}^{-}$to act as a leaving group and about the trans effect of the thioether sulfur?
figure cant copy

Adriano Chikande

Numerade Educator

02:04

Problem 20

What spectral transition(s) gives rise to the blue color of cobalt glass? Why are octahedral $\mathrm{Co}$ (III) complexes usually not blue, in contrast with tetrahedral $\mathrm{Co}$ (II) complexes?

Aadit Sharma

Numerade Educator

00:48

Problem 21

What is the chirality of compound 44 ?

Nikhil Choudhary

Numerade Educator

02:17

Problem 22

Outline the synthesis of trans-Pt(py) $\left(\mathrm{PEt}_3\right) \mathrm{Cl}_2$ starting with $\left[\mathrm{PtCl}_4\right]^{2-}$.

Ahmed Ali

Numerade Educator

01:37

Problem 23

The compound below has been found to be effective against certain types of cancer. Synthesize the compound starting with $\left[\mathrm{PtCl}_4\right]^{2-}$.
figure cant copy

Arpit Gupta

Numerade Educator