Problem 35 - Chemistry The Central Science In Si Units

Question

Write the formula for each of the following compounds, being sure to use brackets to indicate the coordination sphere: (a) triamminetriaquachromium(III) nitrate (b) dichlorobis(ethylenediamine)platinum(II) (c) pentacarbonyliron(0) (d) ammonium diaquabis(oxalato)Co(II) (e) tris(bipyridyl)cobalt(III) sulfate

Step-by-Step Solution

Answer

(a) [Cr(NH3)3(H2O)3](NO3)3; (b) [Pt(en)2Cl2]; (c) [Fe(CO)5]; (d) (NH4)2[Co(C2O4)2(H2O)2]; (e) [Co(bipy)3]2(SO4)3.

Step 1: Understanding the Compound Name

Each compound name consists of a central metal atom, its oxidation state, and surrounding ligands. Coordination compounds are written with the metal and its ligands in square brackets, indicating the coordination sphere.

Step 1: Triamminetriaquachromium(III) nitrate

This compound consists of chromium(III) coordinating with three ammonia (NH3) and three water (H2O) ligands. The oxidation state of chromium is +3 and nitrate (NO3-) is the counter ion. - Coordination sphere: [Cr(NH3)3(H2O)3] - Add the counter ion outside the brackets: [Cr(NH3)3(H2O)3](NO3)3.

Step 2: Dichlorobis(ethylenediamine)platinum(II)

This compound consists of platinum(II) coordinating with two ethylenediamine (en) ligands and two chloride ions. - Coordination sphere: [Pt(en)2Cl2] - No counter ions are needed here since this formula is neutral.

Step 3: Pentacarbonyliron(0)

This is a zero-charged iron complex with five carbon monoxide (CO) ligands. - Coordination sphere: [Fe(CO)5] - No counter ions are needed here as the complex carries no net charge.

Step 4: Ammonium diaquabis(oxalato)Co(II)

This compound consists of cobalt(II) coordinating with two oxalate ligands and two water molecules. Ammonium (NH4)+ is the counter ion. - Coordination sphere: [Co(C2O4)2(H2O)2]^{2-} - Add the counter ion outside the brackets to neutralize the charge: (NH4)2[Co(C2O4)2(H2O)2].

Step 5: Tris(bipyridyl)cobalt(III) sulfate

This compound consists of cobalt(III) coordinating with three bipyridyl (bipy) ligands. Sulfate (SO4)^{2-} is the counter ion. - Coordination sphere: [Co(bipy)3]^{3+} - Add the counter ion outside the brackets to neutralize the charge: [Co(bipy)3]2(SO4)3.

Key Concepts

Coordination Sphere

In the world of coordination compounds, the coordination sphere is a critical concept. It refers to the central part of a coordination compound consisting of a metal ion surrounded by its ligands. The entire assembly is typically enclosed within square brackets. The term 'coordination sphere' helps us differentiate between the ligands directly attached to the metal ion and other species outside the brackets, known as counter ions.
The coordination sphere not only indicates the immediate environment of the metal ion but also plays a big role in defining the compound's properties and reactivity. For example, in the compound \([Cr(NH_3)_3(H_2O)_3](NO_3)_3\), everything inside the brackets—three ammonia and three water molecules and the chromium ion itself—constitutes the coordination sphere. Understanding the arrangement and the number of ligands around the metal ion is crucial for predicting the chemical behavior of the compound.

Ligands

Ligands are molecules or ions directly bonded to the central metal ion in a coordination compound. They function as Lewis bases by donating electron pairs to the metal ion, which acts as a Lewis acid. Ligands can be simple ions like Cl⁻ or NH₃, or larger molecules like ethylenediamine (en) and bipyridyl (bipy).
Ligands are categorized based on several factors such as their charge, size, and the number of donor atoms, known as denticity. Denticity refers to the number of coordinate bonds a ligand forms with the central metal. For instance, in \([Pt(en)_2Cl_2]\), ethylenediamine acts as a bidentate ligand, meaning it forms two bonds with the platinum ion. This concept helps in constructing the geometric configuration and stability of coordination compounds.

Oxidation State

The oxidation state of the central metal ion within the coordination sphere determines not only its chemical properties but also the naming of the compound. The oxidation state is indicated in parentheses right after the metal's name in the chemical formula. It describes the degree of oxidation or the hypothetical charge the metal would have if all ligands and electrons were removed.
For example, in the compound dichlorobis(ethylenediamine)platinum(II), the oxidation state of platinum is +2. This helps in understanding the overall charge of the coordination sphere and, consequently, the expected interactions with counter ions. Calculating the oxidation state involves considering the charges of the ligands and ensuring the neutrality or overall charge alignment in the entire complex.

Counter Ions

Counter ions are ions located outside the coordination sphere and serve the purpose of balancing the overall charge of the coordination compound. They are not bonded directly to the central metal ion but complete the salt formation by stabilizing the ionic charges.
In some coordination compounds, counter ions are necessary for charge neutrality. For instance, in \(([NH_4]_2[Co(C_2O_4)_2(H_2O)_2]\), the ammonium ion acts as the counter ion. Meanwhile, some compounds, such as \([Fe(CO)_5]\), are already neutral, and thus do not need any counter ions. Recognizing when and which counter ions are present is essential for constructing the chemical formula correctly and understanding the full structure of the compound.