Chapter 24

Write the formula and name of (a) a complex ion having \(\mathrm{Cr}^{3+}\) as the central ion and two \(\mathrm{NH}_{3}\) molecules and four \(\mathrm{Cl}^{-}\) ions as ligands (b) a complex ion of iron(III) having a coordination number of 6 and \(\mathrm{CN}^{-}\) as ligands (c) a coordination compound comprising two types of complex ions: one a complex of \(\mathrm{Cr}(\mathrm{III})\) with ethylenediamine (en), having a coordination number of 6 the other, a complex of \(\mathrm{Ni}(\mathrm{II})\) with \(\mathrm{CN}^{-}\), having a coordination number of 4

What are the coordination number and the oxidation state of the central metal ion in each of the following complexes? Name each complex. (a) \(\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{6}\right]^{2+}\) (b) \(\left[\mathrm{AlF}_{6}\right]^{3-}\) (c) \(\left[\mathrm{Cu}(\mathrm{CN})_{4}\right]^{2-}\) (d) \(\left[\operatorname{CrBr}_{2}\left(\mathrm{NH}_{3}\right)_{4}\right]^{+}\) (e) \(\left[\operatorname{Co}(\text { ox })_{3}\right]^{4-}\) (f) \(\left[\mathrm{Ag}\left(\mathrm{S}_{2} \mathrm{O}_{3}\right)_{2}\right]^{3-}\)

Supply acceptable names for the following. (a) \(\left[\mathrm{Co}(\mathrm{OH})\left(\mathrm{H}_{2} \mathrm{O}\right)_{4}\left(\mathrm{NH}_{3}\right)\right]^{2+}\) (b) \(\left[\mathrm{Co}(\mathrm{ONO})_{3}\left(\mathrm{NH}_{3}\right)_{3}\right]\) (c) \(\left[\operatorname{Pt}\left(\mathrm{H}_{2} \mathrm{O}\right)_{4}\right]\left[\mathrm{PtCl}_{6}\right]\) (d) \(\left[\mathrm{Fe}(\mathrm{ox})_{2}\left(\mathrm{H}_{2} \mathrm{O}\right)_{2}\right]^{-}\) (e) \(\mathrm{Ag}_{2}\left[\mathrm{HgI}_{4}\right]\)

Write appropriate formulas for the following. (a) potassium hexacyanoferrate(III) (b) bis(ethylenediamine)copper(II) ion (c) pentaaquahydroxoaluminum(III) chloride (d) amminechlorobis(ethylenediamine) chromium(III) sulfate (e) tris(ethylenediamine)iron(III) hexacyanoferrate(II)

Draw Lewis structures for the following ligands: (a) \(\mathrm{H}_{2} \mathrm{O} ;\) (b) \(\mathrm{CH}_{3} \mathrm{NH}_{2} ;\) (c) \(\mathrm{ONO}^{-} ;\) (d) SCN \(^{-}\).

Draw Lewis structures for the following ligands: (a) hydroxo; (b) sulfato; (c) oxalato; (d) thiocyanato- \(N\) -.

Draw a plausible structure to represent: (a) \(\left[\mathrm{PtCl}_{4}\right]^{2-}\) (b) \(\operatorname{fac}-\left[\operatorname{Co}\left(\mathrm{H}_{2} \mathrm{O}\right)_{3}\left(\mathrm{NH}_{3}\right)_{3}\right]^{2+}\) (c) \(\left[\mathrm{CrCl}\left(\mathrm{H}_{2} \mathrm{O}\right)_{5}\right]^{2+}\)

Draw plausible structures of the following chelate complexes. (a) \(\left[\operatorname{Pt}(\text { ox })_{2}\right]^{2-}\) (b) \(\left[\mathrm{Cr}(\mathrm{ox})_{3}\right]^{3-}\) (c) \([\mathrm{Fe}(\text { EDTA })]^{2-}\)

Draw plausible structures corresponding to each of the following names. (a) pentamminesulfatochromium(III) ion (b) trioxalatocobaltate(III) ion (c) triamminedichloronitrito-O-cobalt(III)

Draw plausible structures corresponding to each of the following names. (a) pentamminenitrito- \(N\) -cobalt(III) ion (b) ethylenediaminedithiocyanato-S-copper(II) (c) hexaaquanickel(II) ion

Which of these general structures for a complex ion would you expect to exhibit cis and trans isomerism? Explain. (a) tetrahedral (b) square-planar (c) linear

Which of these octahedral complexes would you expect to exhibit geometric isomerism? Explain. (a) \(\left[\mathrm{CrOH}\left(\mathrm{NH}_{3}\right)_{5}\right]^{2+}\) (b) \(\left[\mathrm{CrCl}_{2}\left(\mathrm{H}_{2} \mathrm{O}\right)\left(\mathrm{NH}_{3}\right)_{3}\right]^{+}\) (c) \(\left[\mathrm{CrCl}_{2}(\mathrm{en})_{2}\right]^{+}\) (d) \(\left[\mathrm{CrCl}_{4}(\mathrm{en})\right]^{-}\) (e) \(\left[\mathrm{Cr}(\mathrm{en})_{3}\right]^{3+}\)

If \(\mathrm{A}, \mathrm{B}, \mathrm{C},\) and \(\mathrm{D}\) are four different ligands, (a) how many geometric isomers will be found for square-planar \([\mathrm{PtABCD}]^{2+} ?\) (b) Will tetrahedral \([\mathrm{ZnABCD}]^{2+}\) display optical isomerism?

Write the names and formulas of three coordination isomers of \(\left[\mathrm{Co}(\mathrm{en})_{3}\right]\left[\mathrm{Cr}(\mathrm{ox})_{3}\right]\).

Draw a structure for cis-dichlorobis(ethylenediamine)cobalt(III) ion. Is this ion chiral? Is the trans isomer chiral? Explain.

Describe how the crystal field theory explains the fact that so many transition metal compounds are colored.

Cyano complexes of transition metal ions (such as \(\mathrm{Fe}^{2+}\) and \(\mathrm{Cu}^{2+}\) ) are often yellow, whereas aqua complexes are often green or blue. Explain the basis for this difference in color.

If the ion \(\mathrm{Co}^{2+}\) is linked with strong-field ligands to produce an octahedral complex, the complex has one unpaired electron. If \(\mathrm{Co}^{2+}\) is linked with weak-field ligands, the complex has three unpaired electrons. How do you account for this difference?

Predict: (a) which of the complex ions, \(\left[\mathrm{MoCl}_{6}\right]^{3-}\) and \(\left[\mathrm{Co}(\mathrm{en})_{3}\right]^{3+},\) is diamagnetic and which is paramagnetic; (b) the number of unpaired electrons expected for the tetrahedral complex ion \(\left[\mathrm{CoCl}_{4}\right]^{2-}\).

Predict: (a) whether the square-planar complex ion \(\left[\mathrm{Cu}(\mathrm{py})_{4}\right]^{2+}\) is diamagnetic or paramagnetic (b) whether octahedral \(\left[\mathrm{Mn}(\mathrm{CN})_{6}\right]^{3-}\) or tetrahedral \(\left[\mathrm{FeCl}_{4}\right]^{-}\) has the greater number of unpaired electrons.

In Example \(24-5,\) we chose between a tetrahedral and a square-planar structure for \(\left[\mathrm{Ni}(\mathrm{CN})_{4}\right]^{2-}\) based on magnetic properties. Could we similarly use magnetic properties to establish whether the ammine complex of \(\mathrm{Ni}(\mathrm{II})\) is octahedral \(\left[\mathrm{Ni}\left(\mathrm{NH}_{3}\right)_{6}\right]^{2+}\) or tetrahedral \(\left[\mathrm{Ni}\left(\mathrm{NH}_{3}\right)_{4}\right]^{2+} ?\) Explain.

In both \(\left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{2+}\) and \(\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]^{4-}\) ions, the iron is present as \(\mathrm{Fe}(\mathrm{II}) ;\) however, \(\left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{2+}\) is paramagnetic, whereas \(\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]^{4-}\) is diamagnetic. Explain this difference.

Write equations to represent the following observations. (a) A mixture of \(\hat{M g}(O H)_{2}(s)\) and \(Z n(O H)_{2}(s)\) is treated with \(\mathrm{NH}_{3}(\mathrm{aq}) .\) The \(\mathrm{Zn}(\mathrm{OH})_{2}\) dissolves, but the \(\mathrm{Mg}(\mathrm{OH})_{2}(\mathrm{s})\) is left behind. (b) When \(\mathrm{NaOH}(\mathrm{aq})\) is added to \(\mathrm{CuSO}_{4}(\mathrm{aq}),\) a pale blue precipitate forms. If \(\mathrm{NH}_{3}(\) aq) is added, the precipitate redissolves, producing a solution with an intense deep blue color. If this deep blue solution is made acidic with \(\mathrm{HNO}_{3}(\mathrm{aq}),\) the color is converted back to pale blue.

Write equations to represent the following observations. (a) A quantity of \(\mathrm{CuCl}_{2}(\mathrm{s})\) is dissolved in concentrated HCl(aq) and produces a yellow solution. The solution is diluted to twice its volume with water and assumes a green color. On dilution to ten times its original volume, the solution becomes pale blue. (b) When chromium metal is dissolved in \(\mathrm{HCl}(\mathrm{aq}), \mathrm{a}\) blue solution is produced that quickly turns green. Later the green solution becomes blue-green and then violet.

Write a series of equations to show the stepwise displacement of \(\mathrm{H}_{2} \mathrm{O}\) ligands in \(\left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{3+}\) by ethylenediamine, for which \(\log K_{1}=4.34, \log K_{2}=3.31\), and \(\log K_{3}=2.05 .\) What is the overall formation constant, \(\beta_{3}=K_{f},\) for \(\left[\mathrm{Fe}(\mathrm{en})_{3}\right]^{3+} ?\)

A tabulation of formation constant data lists the following log \(K\) values for the formation of \(\left[\mathrm{CuCl}_{4}\right]^{2-}\): \(\log K_{1}=2.80, \quad \log K_{2}=1.60, \quad \log K_{3}=0.49, \quad\) and \(\log K_{4}=0.73 .\) What is the overall formation constant \(\beta_{4}=K_{\mathrm{f}}\) for \(\left[\mathrm{CuCl}_{4}\right]^{2-} ?\)

Explain the following observations in terms of complex-ion formation. (a) \(\mathrm{Al}(\mathrm{OH})_{3}(\mathrm{s})\) is soluble in \(\mathrm{NaOH}(\mathrm{aq})\) but insoluble in \(\mathrm{NH}_{3}(\mathrm{aq})\) (b) \(\mathrm{ZnCO}_{3}(\mathrm{s})\) is soluble in \(\mathrm{NH}_{3}(\mathrm{aq}),\) but \(\mathrm{ZnS}(\mathrm{s})\) is not. (c) The molar solubility of AgCl in pure water is about \(1 \times 10^{-5} \mathrm{M} ;\) in \(0.04 \mathrm{M} \mathrm{NaCl}(\mathrm{aq}),\) it is about \(2 \times 10^{-6}; \mathrm{M}\) but in \(1 \mathrm{M} \mathrm{NaCl}(\mathrm{aq}),\) it is about \(8 \times 10^{-5} \mathrm{M}\).

Explain the following observations in terms of complex-ion formation. (a) \(\mathrm{CoCl}_{3}\) is unstable in aqueous solution, being reduced to \(\mathrm{CoCl}_{2}\) and liberating \(\mathrm{O}_{2}(\mathrm{g}) .\) Yet, \(\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{6}\right] \mathrm{Cl}_{3}\) can be easily maintained in aqueous solution. (b) AgI is insoluble in water and in dilute \(\mathrm{NH}_{3}(\mathrm{aq})\) but AgI will dissolve in an aqueous solution of sodium thiosulfate.

Which of the following would you expect to react as a Bronsted-Lowry acid: \(\left[\mathrm{Cu}\left(\mathrm{NH}_{3}\right)_{4}\right]^{2+},\left[\mathrm{FeCl}_{4}\right]^{-},\) \(\left[\mathrm{Al}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{3+}, \operatorname{or}\left[\mathrm{Zn}(\mathrm{OH})_{4}\right]^{2-} ? \mathrm{Why} ?\)

Write simple chemical equations to show how the complex ion \(\left[\mathrm{CrOH}\left(\mathrm{H}_{2} \mathrm{O}\right)_{5}\right]^{2+}\) acts as \((\mathrm{a})\) an acid; (b) a base.

Show that the oxidation of \(\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{6}\right]^{2+}\) to \(\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{6}\right]^{3+}\) referred to on page 1098 should occur spontaneously in alkaline solution with \(\mathrm{H}_{2} \mathrm{O}_{2}\) as an oxidizing agent.

Draw dashed and solid wedge diagrams of transplatin, trans-Pt( \(\mathrm{NH}_{3}\) ) \(_{2} \mathrm{Cl}_{2}\), and cisplatin, \(\operatorname{cis}-\mathrm{Pt}\left(\mathrm{NH}_{3}\right)_{2} \mathrm{Cl}_{2}\) Then, explain how transplatin can be more reactive yet less effective at killing cancer cells than is cisplatin.

From each of the following names, you should be able to deduce the formula of the complex ion or coordination compound intended. Yet, these are not the best systematic names that can be written. Replace each name with one that is more acceptable: (a) cupric tetraammine ion; (b) dichlorotetraammine cobaltic chloride; (c) platinic(IV) hexachloride ion; (d) disodium copper tetrachloride; (e) dipotassium antimony(III) pentachloride.

How many isomers are there of the complex ion \(\left[\mathrm{CoCl}_{2}(\mathrm{en})\left(\mathrm{NH}_{3}\right)_{2}\right]^{+} ?\) Sketch their structures.

The cis and trans isomers of \(\left[\mathrm{CoCl}_{2}(\mathrm{en})_{2}\right]^{+}\) can be distinguished via a displacement reaction with oxalate ion. What difference in reactivity toward oxalate ion would you expect between the cis and trans isomers? Explain.

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.

Without performing detailed calculations, show why you would expect the concentrations of the various ammine-aqua complex ions to be negligible compared with that of \(\left[\mathrm{Cu}\left(\mathrm{NH}_{3}\right)_{4}\right]^{2+}\) in a solution having a total \(\mathrm{Cu}(\mathrm{II})\) concentration of \(0.10 \mathrm{M}\) and a total concentration of \(\mathrm{NH}_{3}\) of \(1.0 \mathrm{M}\). Under what conditions would the concentrations of these ammine-aqua complex ions (such as \(\left.\left[\mathrm{Cu}\left(\mathrm{H}_{2} \mathrm{O}\right)_{3} \mathrm{NH}_{3}\right]^{2+}\right)\) become more significant relative to the concentration of \(\left[\mathrm{Cu}\left(\mathrm{NH}_{3}\right)_{4}\right]^{2+} ?\) Explain.

Verify the statement on page 1101 that neither \(\mathrm{Ca}^{2+}\) nor \(\mathrm{Mg}^{2+}\) found in natural waters is likely to precipitate from the water on the addition of other reagents if the ions are complexed with EDTA. Assume reasonable values for the total metal ion concentration and that of free EDTA, such as \(0.10 \mathrm{M}\) each.

Estimate the total \(\left[\mathrm{Cl}^{-}\right]\) required in a solution that is initially \(0.10 \mathrm{M} \mathrm{CuSO}_{4}\) to produce a visible yellow color. \(\left[\mathrm{Cu}\left(\mathrm{H}_{2} \mathrm{O}\right)_{4}\right]^{2+}+4 \mathrm{Cl}^{-} \rightleftharpoons\left[\mathrm{CuCl}_{4}\right]^{2-}+4 \mathrm{H}_{2} \mathrm{O}\) \(K_{f}=4.2 \times 10^{5}\) Assume that \(99 \%\) conversion of \(\left[\mathrm{Cu}\left(\mathrm{H}_{2} \mathrm{O}\right)_{4}\right]^{2+}\) to \(\left[\mathrm{CuCl}_{4}\right]^{2-}\) is sufficient for this to happen, and ignore the presence of any mixed aqua- chloro complex ions.

A Cu electrode is immersed in a solution that is \(1.00 \mathrm{M} \mathrm{NH}_{3}\) and \(1.00 \mathrm{M}\) in \(\left[\mathrm{Cu}\left(\mathrm{NH}_{3}\right)_{4}\right]^{2+} .\) If a standard hydrogen electrode is the cathode, \(E_{\text {cell }}\) is \(+0.08 \mathrm{V} .\) What is the value obtained by this method for the formation constant, \(K_{f},\) of \(\left[\mathrm{Cu}\left(\mathrm{NH}_{3}\right)_{4}\right]^{2+} ?\)

The following concentration cell is constructed. \(\mathrm{Ag} | \mathrm{Ag}^{+}\left(0.10 \mathrm{M}\left[\mathrm{Ag}(\mathrm{CN})_{2}\right]^{-}, 0.10 \mathrm{M} \mathrm{CN}^{-}\right)\) $$\| \mathrm{Ag}^{+}(0.10 \mathrm{M}) | \mathrm{Ag}$$ If \(K_{f}\) for \(\left[\mathrm{Ag}(\mathrm{CN})_{2}\right]^{-}\) is \(5.6 \times 10^{18},\) what value would you expect for \(E_{\text {cell }}\) ? [Hint: Recall that the anode is on the left.]

The compound \(\mathrm{CoCl}_{2} \cdot 2 \mathrm{H}_{2} \mathrm{O} \cdot 4 \mathrm{NH}_{3}\) may be one of the hydrate isomers \(\left[\mathrm{CoCl}\left(\mathrm{H}_{2} \mathrm{O}\right)\left(\mathrm{NH}_{3}\right)_{4}\right] \mathrm{Cl} \cdot \mathrm{H}_{2} \mathrm{O}\) or \(\left[\mathrm{Co}\left(\mathrm{H}_{2} \mathrm{O}\right)_{2}\left(\mathrm{NH}_{3}\right)_{4}\right] \mathrm{Cl}_{2} .\) A \(0.10 \mathrm{M}\) aqueous solution of the compound is found to have a freezing point of \(-0.56^{\circ} \mathrm{C} .\) Determine the correct formula of the compound. The freezing-point depression constant for water is \(1.86 \mathrm{mol}\) \(\mathrm{kg}^{-1}\) \(^{\circ} \mathrm{C}\), and for aqueous solutions, molarity and molality can be taken as approximately equal.

Explain why aqueous solutions of \(\left[\operatorname{Sc}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right] \mathrm{Cl}_{3}\) and \(\left.\mathrm{Zn}\left(\mathrm{H}_{2} \mathrm{O}\right)_{4}\right] \mathrm{Cl}_{2}\) are colorless, but an aqueous solution of \(\left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right] \mathrm{Cl}_{3}\) is not.

Provide a valence bond description of the bonding in the \(\mathrm{Cr}\left(\mathrm{NH}_{3}\right)_{6}^{3+}\) ion. According to the valence bond description, how many unpaired electrons are there in the \(\mathrm{Cr}\left(\mathrm{NH}_{3}\right)_{6}^{3+}\) complex? How does this prediction compare with that of crystal field theory?

Acetyl acetone undergoes an isomerization to form a type of alcohol called an enol. The enol, abbreviated acacH, can act as a bidentate ligand as the anion acac^-. Which of the following compounds are optically active: \(\operatorname{Co}(\mathrm{acac})_{3} ;\) trans\(\left[\mathrm{Co}(\mathrm{acac})_{2}\left(\mathrm{H}_{2} \mathrm{O}\right)_{2}\right] \mathrm{Cl}_{2} ; \operatorname{cis}-\left[\mathrm{Co}(\mathrm{acac})_{2}\left(\mathrm{H}_{2} \mathrm{O}\right)_{2}\right] \mathrm{Cl}_{2} ?\)

We have seen that complex formation can stabilize oxidation states. An important illustration of this fact is the oxidation of water in acidic solutions by \(\mathrm{Co}^{3+}(\) aq) but not by \(\left[\mathrm{Co}(\mathrm{en})_{3}\right]^{3+} .\) Use the following data. \(\left[\mathrm{Co}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{3+}+\mathrm{e}^{-} \longrightarrow\left[\mathrm{Co}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{2+}\) $$ E^{\circ}=1.82 \mathrm{V} $$ \(\left[\operatorname{Co}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{2+}+3 \mathrm{en} \longrightarrow\left[\mathrm{Co}(\mathrm{en})_{3}\right]^{2+}+6 \mathrm{H}_{2} \mathrm{O}(1)\) $$ \log \beta_{3}=12.18 $$ \(\left[\mathrm{Co}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{3+}+3 \mathrm{en} \longrightarrow\left[\mathrm{Co}(\mathrm{en})_{3}\right]^{3+}+6 \mathrm{H}_{2} \mathrm{O}(1)\) $$ \log \beta_{3}=47.30 $$ Calculate \(E^{\circ}\) for the reaction $$ \left[\mathrm{Co}(\mathrm{en})_{3}\right]^{3+}+\mathrm{e}^{-} \longrightarrow\left[\mathrm{Co}(\mathrm{en})_{3}\right]^{2+} $$ Show that \(\left[\mathrm{Co}(\mathrm{en})_{3}\right]^{3+}\) is stable in water but \(\mathrm{Co}^{3+}(\mathrm{aq})\) is not.

The amino acid glycine \(\left(\mathrm{NH}_{2} \mathrm{CH}_{2} \mathrm{CO}_{2} \mathrm{H}\right.,\) denoted Hgly) binds as an anion and is a bidentate ligand. Draw and name all possible isomers of \(\left[\mathrm{Co}(\mathrm{gly})_{3}\right]\) How many isomers are possible for the compound $$\left[\mathrm{Co}(\mathrm{gly})_{2} \mathrm{Cl}\left(\mathrm{NH}_{3}\right)\right]\left[\text {Hint:} \mathrm{NH}_{2} \mathrm{CH}_{2} \mathrm{CO}_{2}^{-}\text {is }\right.$$ the glycinate anion.]

A structure that Werner examined as a possible alternative to the octahedron is the trigonal prism. (a) Does this structure predict the correct number of isomers for the complex ion \(\left[\mathrm{CoCl}_{2}\left(\mathrm{NH}_{3}\right)_{4}\right]^{+} ?\) If not, why not? (b) Does this structure account for optical isomerism in \(\left[\mathrm{Co}(\mathrm{en})_{3}\right]^{3+} ?\) Explain.

In your own words, describe the following terms or symbols: (a) coordination number; (b) \(\Delta_{\mathrm{o}} ;\) (c) ammine complex; (d) enantiomer.

Briefly describe each of the following ideas, phenomena, or methods: (a) spectrochemical series; (b) crystal field theory; (c) optical isomer; (d) structural isomerism.