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 thiosul-fate. The equation for the reaction is: $$\operatorname{AgBr}(s)+2 \mathrm{Na}_{2} \mathrm{S}_{2} \mathrm{O}_{3}(a q) \longrightarrow \\ \quad\quad\quad\quad\quad\quad\quad\quad\quad\quad \mathrm{Na}_{3}\left[\mathrm{Ag}\left(\mathrm{S}_{2} \mathrm{O}_{3}\right)_{2}\right](a q)+\mathrm{NaBr}(a q)$$ 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: $$\mathrm{Cu}\left(\mathrm{NH}_{3}\right)_{4} \mathrm{Cl}_{2}(a q)+4 \mathrm{NH}_{3}(a q)+\mathrm{Cu}(s) \longrightarrow \\\ \quad\quad\quad\quad\quad\quad\quad\quad\quad\quad\quad\quad\quad\quad\quad\quad 2 \mathrm{Cu}\left(\mathrm{NH}_{3}\right)_{4} \mathrm{Cl}(a q)$$ Assume these copper complex ions have tetrahedral geometry.

Short answer

a. In the compound \(\mathrm{CoCl}_{2} \cdot 6 \mathrm{H}_{2} \mathrm{O}\), the complex ion is \(\left[\mathrm{Co}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{2+}\) with counterions 2 \(\mathrm{Cl}^-\). The electron configuration of Co(II) is \(\mathrm{[Ar]}3d^7\), and the complex ion has octahedral geometry. b. In the compound \(\mathrm{Na}_{3}\left[\mathrm{Ag}\left(\mathrm{S}_{2}\mathrm{O}_{3}\right)_{2}\right]\), the complex ion is \(\left[\mathrm{Ag}\left(\mathrm{S}_{2} \mathrm{O}_{3}\right)_{2}\right]^{3-}\) with counterions 3 \(\mathrm{Na}^+\). The electron configuration of Ag(I) is \(\mathrm{[Kr]}4d^{10}\), and the complex ion has linear geometry. c. In the compound \(\mathrm{Cu}\left(\mathrm{NH}_{3}\right)_{4} \mathrm{Cl}_{2}\), the complex ion is \(\left[\mathrm{Cu}\left(\mathrm{NH}_{3}\right)_{4}\right]^{2+}\) with counterions 2 \(\mathrm{Cl}^-\). The electron configuration of Cu(II) is \(\mathrm{[Ar]}3d^9\), and the complex ion has tetrahedral geometry.

Step-by-step solution

Complex Ion and Counterions

The complex ion is \(\left[\mathrm{Co}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{2+}\), and the counterions are 2 \(\mathrm{Cl}^-\).

Electron Configuration

Cobalt has a ground state electron configuration of \(\mathrm{[Ar]}3d^{7}4s^2\). When it forms a coordination complex in oxidation state +2, it loses 2 electrons, giving it the electron configuration \(\mathrm{[Ar]}3d^7\).

Geometry of the Complex Ion

The complex ion has 6 water molecules attached to the cobalt in octahedral geometry. b. Coordination compound: \(\mathrm{Na}_{3}\left[\mathrm{Ag}\left(\mathrm{S}_{2}\mathrm{O}_{3}\right)_{2}\right]\)

Complex Ion and Counterions

The complex ion is \(\left[\mathrm{Ag}\left(\mathrm{S}_{2} \mathrm{O}_{3}\right)_{2}\right]^{3-}\), and the counterions are 3 \(\mathrm{Na}^+\).

Electron Configuration

Silver has a ground state electron configuration of \(\mathrm{[Kr]}4d^{10}5s^1\). When it forms a coordination complex in oxidation state +1, it loses 1 electron, giving it the electron configuration \(\mathrm{[Kr]}4d^{10}\).

Geometry of the Complex Ion

The complex ion has 2 thiosulfate groups attached to the silver in a linear geometry. c. Coordination compound: \(\mathrm{Cu}\left(\mathrm{NH}_{3}\right)_{4} \mathrm{Cl}_{2}\)

Complex Ion and Counterions

The complex ion is \(\left[\mathrm{Cu}\left(\mathrm{NH}_{3}\right)_{4}\right]^{2+}\), and the counterions are 2 \(\mathrm{Cl}^-\).

Electron Configuration

Copper has a ground state electron configuration of \(\mathrm{[Ar]}3d^{10}4s^1\). When it forms a coordination complex in oxidation state +2, it loses 2 electrons, giving it the electron configuration \(\mathrm{[Ar]}3d^9\).

Geometry of the Complex Ion

The complex ion has 4 ammonia molecules attached to the copper in tetrahedral geometry.

Key concepts

Complex Ion

A complex ion is formed when a central metal atom or ion is surrounded by a number of ligands. Ligands are ions or molecules that bind to the metal to form the complex. The binding can involve ionic interactions, covalent bonds, or a combination of both.
For example, in the coordination compound \([\mathrm{Co}(\mathrm{H}_2 \mathrm{O})_6]^{2+}\), the central cobalt ion is surrounded by six water molecules, which act as ligands. This setup forms the complex ion, which carries a specific charge often due to the metal's oxidation state.
The charge of the complex ion is crucial as it determines the number and type of counterions needed to balance the charge of the entire coordination compound.

Electron Configuration

Electron configuration represents the arrangement of electrons in an atom's orbitals. This configuration changes when forming a coordination complex, especially for transition metals which often lose electrons.
The cobalt ion's ground state is \[ \mathrm{[Ar]}3d^{7}4s^2 \]. When cobalt forms a complex ion in the +2 oxidation state, it loses two electrons: the two 4s electrons. Its electron configuration then becomes \[ \mathrm{[Ar]}3d^7\].
Silver, in its ground state, has the electron configuration \[ \mathrm{[Kr]}4d^{10}5s^1 \]. In a coordination complex with +1 oxidation state, it loses the 5s electron, resulting in \[ \mathrm{[Kr]}4d^{10} \].
Understanding these configurations gives insight into the chemical and physical properties of the complexes, such as color and magnetic properties.

Geometry of Coordination Complex

The geometry of a coordination complex is determined by the number and spatial arrangement of the ligands around the central metal.
- **Octahedral Geometry:** In complexes like \[\mathrm{Co}(\mathrm{H}_2 \mathrm{O})_6]^{2+}\], six ligands are spaced evenly around the central metal, forming an octahedral shape. This geometric arrangement is quite common in coordination chemistry.
- **Linear Geometry:** A different structural design is seen when a metal forms two strong linear bonds, like in \([\mathrm{Ag}(\mathrm{S}_2 \mathrm{O}_3)_2]^{3-}\), maintaining a linear arrangement. - **Tetrahedral Geometry:** Complexes such as \([\mathrm{Cu}(\mathrm{NH}_3)_4]^{2+}\) often have a tetrahedral geometry, where four ligands are positioned symmetrically around the central metal. This spatial orientation allows for interactions distinct in terms of coordination stability and reactivity.

Counterions

Counterions balance the charge of the complex ion in a coordination compound. They are essential for maintaining electrical neutrality. Without counterions, the overall charge of a compound could make it highly reactive or unstable.
For instance, in the compound \[ \mathrm{CoCl}_2 \cdot 6 \mathrm{H}_2 \mathrm{O} \], the complex ion \[ [\mathrm{Co}(\mathrm{H}_2 \mathrm{O})_6]^{2+} \] is balanced by two \[ \mathrm{Cl}^- \] ions. Without these Cl- ions, the compound couldn't maintain a stable ionic formation.
Similarly, \[ [\mathrm{Ag}(\mathrm{S}_2 \mathrm{O}_3)_2]^{3-} \] requires three \[ \mathrm{Na}^+ \] counterions to achieve neutrality. Counterions can also influence the solubility and reactivity of the coordination compound in various solutions.