Problem 24 - Chemistry Central Science

Question

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

Step-by-Step Solution

Answer

The chemical formulas for the coordination compounds are: (a) \([Mn(H_2O)_4Br_2](ClO_4)_3\) (b) \([Cd(bpy)_2]Cl_2\) (c) \(K[CoBr_4(o-phen)]\) (d) \(Cs[Cr(NH_3)_2(CN)_4]\) (e) \([Rh(en)_3]_3[Co(ox)_3]_2\)

Step 1: (a) Tetraaquadibromomanganese(III) perchlorate

In this compound, the central metal atom is manganese (Mn) with an oxidation state of +3. The ligands are four water molecules (tetraaqua) and two bromide ions (dibromo). The counterion is perchlorate (ClO4^-). So, the formula for this compound is \([Mn(H_2O)_4Br_2](ClO_4)_3\).

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

In this compound, the central metal atom is cadmium (Cd) with an oxidation state of +2. The ligands are two bipyridyl molecules (bpy; C_10H_8N_2). The counterion is chloride (Cl^-). So, the formula for this compound is \([Cd(bpy)_2]Cl_2\).

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

In this compound, the central metal atom is cobalt (Co) with an oxidation state of +3. The ligands are four bromide ions (tetrabromo) and one ortho-phenanthroline molecule (o-phen; C_12H_8N_2). The counterion is potassium (K^+). So, the formula for this compound is \(K[CoBr_4(o-phen)]\).

Step 4: (d) Cesium diamminetetracyanochromate(III)

In this compound, the central metal atom is chromium (Cr) with an oxidation state of +3. The ligands are two ammonia molecules (diammine) and four cyanide ions (tetracyano). The counterion is cesium(Cs^+). So, the formula for this compound is \(Cs[Cr(NH_3)_2(CN)_4]\).

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

This compound contains two complex ions: one with a rhodium (Rh) central atom and the other with a cobalt (Co) central atom, both with an oxidation state of +3. For the rhodium complex, the ligands are three ethylenediamine molecules (en; C_2H_8N_2). For the cobalt complex, the ligands are three oxalate ions (ox; C_2O_4^{2-}). The formula for this compound is \([Rh(en)_3]_3[Co(ox)_3]_2\).

Key Concepts

Inorganic Chemistry

Inorganic chemistry is a branch of chemistry focused on compounds that are not based on carbon-hydrogen bonds. Unlike organic compounds, inorganic compounds often contain metal atoms and might include minerals, metals, and synthetic compounds. Many catalysts, materials, and pigments fall under inorganic chemistry. Coordination compounds, a key topic in inorganic chemistry, are of particular interest due to their complex structures and wide range of applications in various fields, including medicine, materials science, and catalysis.

In coordination compounds, central metal atoms or ions are surrounded by a set of molecules or ions known as ligands, which can be neutral or negatively charged entities. These ligands donate electron pairs to the metal to form coordinate covalent bonds, creating a coordination complex enclosed in square brackets as in the provided exercise examples. The type and arrangement of ligands around the central atom can drastically affect the properties of the compound, making the study of these complexes an essential part of inorganic chemistry.

Oxidation States

Understanding Oxidation States in Coordination Compounds

Identifying the oxidation state of the metal in a coordination compound is foundational for writing correct formulas. The oxidation state indicates the degree of oxidation of the metal atom, represented by a numeral in parentheses following the metal’s name. It is a hypothetical charge that the atom would have if all its bonds to atoms of different elements were 100% ionic.

Oxidation states allow us to understand the electron transfer processes occurring within these complexes and are key to predicting chemical reactivity and stability. The step-by-step solution illustrates the application of oxidation states, with the numerals (II), (III) providing crucial information for deducing the correct formulas of the compounds. A consistent approach to determining and using oxidation states is imperative to mastery in coordination chemistry.

Ligands in Coordination Chemistry

Variety and Role of Ligands

Ligands in coordination chemistry are ions or molecules that can donate electron pairs to form coordinate covalent bonds with a metal ion, creating a complex. They can be water molecules, halide ions, ammonia, or organic molecules like ethylenediamine and ortho-phenanthroline, as showcased in the exercise. The term 'bis', 'tris', etc., informs us of the number of identical ligands in a complex, indicating that there are two or three of these ligands, respectively.

Ligands are not just simple spectators; they define the geometry and reactivity of the central metal, affect colors, magnetic properties, and biological activity of the compounds they form. Appreciating the interaction between ligands and the metal center is pivotal to predicting the behavior of coordination compounds. Each ligand’s role is critical in applications ranging from industrial catalysis to medicinal chemistry.

Counterions in Coordination Compounds

Stabilizing Charge with Counterions

Counterions are ions outside of the coordination sphere that balance the charge of the complex ions in coordination compounds. Their presence is required to maintain electrical neutrality. For example, in tetraaquadibromomanganese(III) perchlorate, perchlorate acts as the counterion to balance the +3 charge of the manganese complex ion.

Counterions can be simple ions such as Na+, K+, or Cl-, or more complex ions like perchlorate (ClO4-). Their sizes, charges, and their own chemistry can influence the overall properties of the coordination compound, including its solubility, crystallinity, and even its biological activity. Understanding the necessity for counterions and the choice of appropriate counterions is crucial for the design and application of coordination compounds in real-world contexts.