Chapter 22

Give a systematic name or the formula for the following: (a) \(\left(\mathrm{NH}_{4}\right)_{2}\left[\mathrm{CuCl}_{4}\right]\) (b) \(\mathrm{Mo}(\mathrm{CO})_{6}\) (c) tetraaquadichlorochromium(III) chloride (d) aquabis(ethylenediamine)thiocyanatocobalt(III) nitrate

A The complex ion \(\left[\mathrm{Co}\left(\mathrm{CO}_{3}\right)_{3}\right]^{3-},\) an octahedral complex with bidentate carbonate ions as ligands, has one absorption in the visible region of the spectrum at \(640 \mathrm{nm} .\) From this information, (a) Predict the color of this complex, and explain your reasoning. (b) Is the carbonate ion a weak- or strong-field ligand? (c) Predict whether \(\left[\mathrm{Co}\left(\mathrm{CO}_{3}\right)_{3}\right]^{3-}\) will be paramagnetic or diamagnetic.

A You have isolated a solid organometallic compound containing manganese, some number of CO ligands, and one or more \(\mathrm{CH}_{3}\) ligands. To find the molecular formula of the compound, you burn \(0.225 \mathrm{g}\) of the solid in oxygen and isolate \(0.283 \mathrm{g}\) of \(\mathrm{CO}_{2}\) and \(0.0290 \mathrm{g}\) of \(\mathrm{H}_{2} \mathrm{O} .\) The molar mass of the compound is \(210 \mathrm{g} / \mathrm{mol}\). Suggest a plausible formula and structure for the molecule. (Make sure it satisfies the EAN rule. The \(\mathrm{CH}_{3}\) group can be thought of as a \(\mathrm{CH}_{3}^{-}\) ion, a two-electron donor ligand.)

In this question, we explore the differences between metal coordination by monodentate and bidentate ligands. Formation constants, \(K_{t}\), for \(\left[\mathrm{Ni}\left(\mathrm{NH}_{3}\right)_{6}\right]^{2+}(\mathrm{aq})\) and \(\left[\mathrm{Ni}(\mathrm{en})_{3}\right]^{2+}(\mathrm{aq})\) are as follows: $$\begin{aligned} &\mathrm{Ni}^{2+}(\mathrm{aq})+6 \mathrm{NH}_{3}(\mathrm{aq}) \longrightarrow\left[\mathrm{Ni}\left(\mathrm{NH}_{3}\right)_{6}\right]^{2+}(\mathrm{aq})\\\ &K_{f}=10^{8} \end{aligned}$$ $$\mathrm{Ni}^{2+}(\mathrm{aq})+3 \mathrm{en}(\mathrm{aq}) \longrightarrow\left[\mathrm{Ni}(\mathrm{en})_{3}\right]^{2+}(\mathrm{aq}) \quad \quad K_{\mathrm{f}}=10^{18}$$ The difference in \(K_{f}\) between these complexes indicates a higher thermodynamic stability for the chelated complex, caused by the chelate effect. Recall that \(K\) is related to the standard free energy of the reaction by \(\Delta_{r} G^{\circ}=-R T \ln K\) and \(\Delta_{r} G^{\circ}=\Delta_{r} H^{\circ}-T \Delta_{r} S^{\circ} .\) We know from experiment that \(\Delta_{r} H^{\circ}\) for the NH seaction is \(-109 \mathrm{kJ} / \mathrm{mol}-\mathrm{rxn},\) and \(\Delta_{r} H^{\circ}\) for the ethylenediamine reaction is \(-117 \mathrm{kJ} / \mathrm{mol}\) -rxn. Is the difference in \(\Delta_{r} H^{\circ}\) sufficient to account for the \(10^{10}\) difference in \(K_{f} ?\) Comment on the role of entropy in the second reaction.

As mentioned on page \(1047,\) transition metal organometallic compounds have found use as catalysts. One example is Wilkinson's catalyst, a rhodium compound \(\left[\mathrm{RhCl}\left(\mathrm{PR}_{3}\right)_{3}\right]\) used in the hydrogenation of alkenes. The steps involved in the catalytic process are outlined below. Indicate whether the rhodium compounds in each step have 18 - or 16 -valence electrons. (See Study Question \(34 .)\) Step \(1 .\) Addition of \(\mathrm{H}_{2}\) to the rhodium center of Wilkinson's catalyst. (For electron-counting purposes \(\mathrm{H}\) is considered a hydride ion, \(\left.\mathrm{H}^{-}, \text {a two-electron donor. }\right)\) Step \(2 .\) Loss of a PR \(_{3}\) ligand (a two-electron donor) to open a coordination site. (PR \(_{3}\) is a phosphine such as \(\mathrm{P}\left(\mathrm{C}_{6} \mathrm{H}_{5}\right)_{3},\) triphenylphosphine.) Step \(3 .\) Addition of the alkene to the open site. Step 4. Rearrangement to add H to the double bond. (Here the \(-\mathrm{CH}_{2} \mathrm{CH}_{3}\) group is a two-electron donor and can be thought of as a \(\left[\mathrm{CH}_{2} \mathrm{CH}_{3}\right]^{-}\) anion for electron counting purposes.) Step \(5 .\) Loss of the alkane. Step \(6 .\) Regeneration of the catalyst. $$\text { Net reaction: } \mathrm{CH}_{2}=\mathrm{CH}_{2}+\mathrm{H}_{2} \longrightarrow \mathrm{CH}_{3} \mathrm{CH}_{3}$$