Problem 36 - Chemistry: The Central Science

Question

Write the formula for each of the following compounds, being sure to use brackets to indicate the coordination sphere: (a) tetraaquadibromemanganese(III) perchlorate (b) bis(bipyridyl)cadmium(II) chloride (c) potassium tetrabromo(ortho-phenanthroline)-cobaltate (III) (d) cesium diamminetetracyanochromate(III) (e) tris(ethylenediamine)rhodium(III) tris(oxalato)cobaltate(III)

Step-by-Step Solution

Answer

The formulae for the given coordination complexes are as follows: (a) \[ \text{[Mn(H}_2\text{O)}_4\text{(Br)}_2\text{](ClO}_4\text{)}_3 \] (b) \[ \text{[Cd(bipy)}_2\text{](Cl)}_2 \] (c) \[ \text{K[Co(phen)(Br)}_4\text{]} \] (d) \[ \text{Cs[Cr(NH}_3\text{)}_2\text{(CN)}_4\text{]} \] (e) \[ \text{[Rh(en)}_3\text{][Co(ox)}_3\text{]} \]

Step 1: (a) tetraaquadibromemanganese(III) perchlorate

In this compound: - Central metal ion: manganese with oxidation state +3 (Mn(III)) - Ligands: Four aqua (H2O) and two bromide ligands (Br) - Counterion: perchlorate (ClO4) So the formula for the compound becomes: \[ \text{[Mn(H}_2\text{O)}_4\text{(Br)}_2\text{](ClO}_4\text{)}_3 \]

Step 2: (b) bis(bipyridyl)cadmium(II) chloride

In this compound: - Central metal ion: cadmium with oxidation state +2 (Cd(II)) - Ligands: Two bipyridyl ligands (bipy), bipyridyl is a common ligand with the formula C10H8N2 - Counterion: chloride (Cl) So the formula for the compound becomes: \[ \text{[Cd(bipy)}_2\text{](Cl)}_2 \]

Step 3: (c) potassium tetrabromo(ortho-phenanthroline)-cobaltate(III)

In this compound: - Central metal ion: cobalt with oxidation state +3 (Co(III)) - Ligands: Four bromide ligands (Br) and one ortho-phenanthroline ligand (phen), ortho-phenanthroline is a ligand with the formula C12H8N2 - Counterion: potassium (K) So the formula for the compound becomes: \[ \text{K[Co(phen)(Br)}_4\text{]} \]

Step 4: (d) cesium diamminetetracyanochromate(III)

In this compound: - Central metal ion: chromium with oxidation state +3 (Cr(III)) - Ligands: Two ammonia (NH3) and four cyanide ligands (CN) - Counterion: cesium (Cs) So the formula for the compound becomes: \[ \text{Cs[Cr(NH}_3\text{)}_2\text{(CN)}_4\text{]} \]

Step 5: (e) tris(ethylenediamine)rhodium(III) tris(oxalato)cobaltate(III)

In this compound, there are two coordination spheres, Rhodium and Cobalt. For the rhodium coordination sphere: - Central metal ion: rhodium with oxidation state +3 (Rh(III)) - Ligands: Three ethylenediamine ligands (en), ethylenediamine is a ligand with the formula NH2CH2CH2NH2 For the cobalt coordination sphere: - Central metal ion: cobalt with oxidation state +3 (Co(III)) - Ligands: Three oxalate ligands (ox), oxalate is a ligand with the formula C2O4 Both coordination spheres have the same oxidation state, and it's a 1:1 ratio, so no additional counterions are needed. So the formula for the compound becomes: \[ \text{[Rh(en)}_3\text{][Co(ox)}_3\text{]} \]

Key Concepts

Complex Ions

In the realm of coordination chemistry, complex ions are intriguing entities composed of a central metal ion which is surrounded by molecules or ions known as ligands. These ligands are not just idle bystanders; they donate electron pairs to the metal, forming a delicate dance of coordination bonds. Imagine a metal ion at the center of a grand ballroom, with ligands gracefully extending their electron 'hands' to form a circle around it.

Consider the example from the textbook exercises. In compound (a), a manganese ion (Mn) forms a complex with water and bromide ions, resulting in a coordination sphere that is securely encased within brackets in the chemical formula. This notation is critical, as it distinctly delineates the metal-ligand sphere from the rest of the compound, which in this case includes three perchlorate counterions.

Oxidation States

Peering into the electron configuration of atoms within coordination compounds, we encounter the notion of oxidation states, a theoretical construct that sheds light on the electronic relationships within the compound. Oxidation states help to indicate the electric charge of the central metal ion if all ligands were removed along with the electron pairs they shared. This conventional method aids our understanding of the compound's reactivity and bonding.

Complex ions often bear a net charge due to the central metal's oxidation state, and this charge is balanced by counterions outside the coordination sphere. For instance, in compound (d), the chromium ion (Cr) has an oxidation state of +3. Counterions like the cesium ion (Cs) exist to compensate, ensuring that the compound remains electrically neutral overall. Recognizing these oxidation states is a key to predicting and rationalizing the stabilities and reactivity patterns of coordination compounds.

Ligands

Diving into the sea of coordination compounds, we encounter a colorful variety of molecules and ions, christened ligands. These are the atoms or groups that muster electron pairs for the bonding extravaganza with metal ions. Ligands come in many forms, from simple species like water (H2O) and ammonia (NH3) to more complex organic molecules like bipyridyl and ethylenediamine.

Ligands are not only the 'guests' at the coordination party, but they influence the properties and geometries of the central metal atoms. As seen in compound (e), ethylenediamine acts as a bidentate ligand, which means it forms two bonds with the rhodium ion. In contrast, oxalate, another bidentate ligand, binds to the cobalt ion. Each ligand imparts specific characteristics to the complex, affecting aspects such as its color, magnetic properties, and its ability to participate in chemical reactions.