Question

The following statements discuss some coordination compounds. For each coordination compound, give the complex ion and the counterions, the electron configuration of the transition metal, and the geometry of the complex ion. a. \(\mathrm{CoCl}_{2} \cdot 6 \mathrm{H}_{2} \mathrm{O}\) is a compound used in novelty devices that predict rain. b. During the developing process of black-and-white film, silver bromide is removed from photographic film by the fixer. The major component of the fixer is sodium thiosulfate. The equation for the reaction is: \(\begin{aligned} \operatorname{AgBr}(s)+2 \mathrm{Na}_{2} \mathrm{S}_{2} \mathrm{O}_{3}(a q) & \longrightarrow \\\ \mathrm{Na}_{3}\left[\mathrm{Ag}\left(\mathrm{S}_{2} \mathrm{O}_{3}\right)_{2}\right](a q)+& \mathrm{NaBr}(a q) \end{aligned}\) c. In the production of printed circuit boards for the electronics industry, a thin layer of copper is laminated onto an insulating plastic board. Next, a circuit pattern made of a chemically resistant polymer is printed on the board. The unwanted copper is removed by chemical etching, and the protective polymer is finally removed by solvents. One etching reaction is: \(\mathrm{Cu}\left(\mathrm{NH}_{3}\right)_{4} \mathrm{Cl}_{2}(a q)+4 \mathrm{NH}_{3}(a q)+\mathrm{Cu}(s) \longrightarrow\) \(2 \mathrm{Cu}\left(\mathrm{NH}_{3}\right)_{4} \mathrm{Cl}(a q)\)

Short answer

The complex ions and counterions, electron configurations, and geometries for each compound are as follows: a. \(\mathrm{CoCl}_{2} \cdot 6 \mathrm{H}_{2} \mathrm{O}\): - Complex ion: \(\mathrm{Co(H_2O)_6}^{2+}\) - Counterions: 2 chloride ions - Electron configuration: \([Ar] 3d^5\) - Geometry: Octahedral b. Sodium thiosulfate fixer in the reaction: - Complex ion: \(\mathrm{Ag(S_2O_3)_2}^{3-}\) - Counterions: Sodium ions - Electron configuration: \([Kr] 4d^{10}\) - Geometry: Linear c. \(\mathrm{Cu(NH_3)_4Cl_2}\): - Complex ion: \(\mathrm{Cu(NH_3)_4}^{2+}\) - Counterions: 2 chloride ions - Electron configuration: \([Ar] 3d^7\) - Geometry: Square planar

Step-by-step solution

Complex Ion and Counterions

The given compound is \(\mathrm{CoCl}_{2} \cdot 6 \mathrm{H}_{2} \mathrm{O}\). The complex ion is formed by the cobalt metal ion and the six water molecules: \(\mathrm{Co(H_2O)_6}^{2+}\). The counterions are the two chloride ions, from \(\mathrm{CoCl}_2\).

Electron Configuration

Cobalt is in the 2+ oxidation state in this compound. The atomic number of cobalt is \(27\), so its electron configuration is \([Ar] 4s^2 3d^7\). When it forms a \(2+\) ion, it loses two electrons, so the configuration becomes \([Ar] 3d^5\).

Geometry

The complex has 6 ligands (water molecules) surrounding the cobalt ion. This corresponds to an octahedral geometry. ##b. Silver Bromide Fixer##

Complex Ion and Counterions

In the reaction, the complex ion that forms is \(\mathrm{Ag(S_2O_3)_2}^{3-}\). The counterions are the sodium ions, which come from sodium thiosulfate.

Electron Configuration

Silver is in the 1+ oxidation state in this compound. The atomic number of silver is \(47\), so its electron configuration is \([Kr] 5s^2 4d^9\). When it forms a \(1+\) ion, it loses one electron, so the configuration becomes \([Kr] 4d^{10}\).

Geometry

The complex has 2 ligands (thiosulfate ions) surrounding the silver ion. This corresponds to a linear geometry. ##c. Copper Ammonia Complex##

Complex Ion and Counterions

The given compound is \(\mathrm{Cu(NH_3)_4Cl_2}\). The complex ion is formed by the copper metal ion and the four ammonia molecules: \(\mathrm{Cu(NH_3)_4}^{2+}\). The counterions are the two chloride ions.

Electron Configuration

Copper is in the 2+ oxidation state in this compound. The atomic number of copper is \(29\), so its electron configuration is \([Ar] 4s^2 3d^9\). When it forms a \(2+\) ion, it loses two electrons, so the configuration becomes \([Ar] 3d^7\).

Geometry

The complex has 4 ligands (ammonia molecules) surrounding the copper ion. This corresponds to a square planar geometry.

Key concepts

Complex Ion

When it comes to coordination compounds, a complex ion is at the heart of the matter. A complex ion is a metal ion bonded to one or more ligands by coordinate bonds. Ligands are ions or molecules that can donate a pair of electrons to the metal ion, creating a more stable electron configuration.

Ligands can be a variety of substances, ranging from water molecules, as seen in the compound \[\[\begin{align*}\mathrm{CoCl_{2} \r\cdot 6 H_{2}O}\end{align*}\]\], to the thiosulfate ions in sodium thiosulfate used in the photographic fixer. In \[\[\begin{align*}\mathrm{Ag(S_2O_3)_{2}^{3-}}\end{align*}\]\], silver forms a complex ion with two thiosulfate ions. The number and type of ligands determine the name and charge of the complex ion.

The counterions are usually the negatively charged ions that balance out the positive charge of the metal ion in the compound. In coordination chemistry, understanding the composition of complex ions is crucial since their formation is essential in many chemical processes, such as precipitation, electroplating, and the biological function of metalloproteins.

Electron Configuration

The electron configuration of a metal within a coordination compound is vital for understanding its chemistry. It tells us how the electrons are distributed among an atom's shells and subshells. The electrons in the outermost orbitals are significant because they participate in chemical bonding.

For example, cobalt (\[\[\begin{align*}\mathrm{Co}\end{align*}\]\]) in \[\[\begin{align*}\mathrm{CoCl_{2} \r\cdot 6 H_{2}O}\end{align*}\]\] has an electron configuration of \[\[\begin{align*}\text{[Ar]} 4s^2 3d^7\end{align*}\]\] after losing two electrons, it becomes \[\[\begin{align*}\text{[Ar]}3d^5\end{align*}\]\], with the electrons now more localized around the cobalt nucleus. Likewise, in copper (\[\[\begin{align*}\mathrm{Cu}\end{align*}\]\]) from the printed circuit boards' etching reaction, it changes from \[\[\begin{align*}\text{[Ar]} 4s^2 3d^9\end{align*}\]\] to \[\[\begin{align*}\text{[Ar]} 3d^7\end{align*}\]\] when in the \[\[\begin{align*}\mathrm{Cu}^{2+}\end{align*}\]\] form. Understanding these changes is essential as they influence the compound's reactivity, color, and magnetic properties.

Geometrical Structure

The geometrical structure, also known as molecular geometry, describes the three-dimensional arrangement of the atoms within a molecule. In the context of coordination compounds, this pertains to the layout of the ligands attached to the central metal ion. Different geometries arise due to the preference of the central atom, the size and bonding preferences of the ligands, and the electron-pair repulsion theory, which explains that electron pairs around a central atom tend to be oriented as far apart as possible.

For instance, the octahedral geometry in \[\[\begin{align*}\mathrm{CoCl_{2} \r\cdot 6 H_{2}O}\end{align*}\]\] results from six ligands symmetrically surrounding the cobalt ion, whereas the silver thiosulfate complex \[\[\begin{align*}\mathrm{Ag(S_2O_3)_{2}^{3-}}\end{align*}\]\] exhibits a linear geometry with two ligands. The square planar geometry of the copper ammonia complex, \[\[\begin{align*}\mathrm{Cu(NH_3)_4}^{2+}\end{align*}\]\], displays the four ammonia ligands arranged at the corners of a square, with the copper ion at the center. Knowing the geometrical structure helps us predict the physical and chemical properties of the compound, such as how it will interact with light or with other molecules.