Question

Write the formula for each of the following compounds, being sure to use brackets to indicate the coordination sphere: (a) hexaamminechromium(III) nitrate (b) tetramminecarbonatocobalt(III) sulfate (c) dichlorobis(ethylenediamine)platinum(IV) bromide

Short answer

The formulas for the given coordination compounds are: (a) hexaamminechromium(III) nitrate: \([Cr(NH3)6](NO3)3\) (b) tetramminecarbonatocobalt(III) sulfate: \([Co(NH3)4(CO3)]2SO4\) (c) dichlorobis(ethylenediamine)platinum(IV) bromide: \([Pt(en)2Cl2]Br2\)

Step-by-step solution

(a) hexaamminechromium(III) nitrate

To write the formula for hexaamminechromium(III) nitrate: 1. Identify the central metal: chromium(III), with the symbol Cr and oxidation state +3. 2. Identify the ligand: ammine, with the symbol NH3. 3. Write down the formula for the coordination sphere: [Cr(NH3)6]^(3+). This indicates a chromium atom surrounded by six ammine ligands. 4. Identify the counter anion: nitrate, with the formula NO3^(-). 5. Determine the number of nitrates required to balance the charge: The coordination sphere has a 3+ charge, so we need three nitrates to balance it. 6. Write the final formula for the compound: [Cr(NH3)6](NO3)3.

(b) tetramminecarbonatocobalt(III) sulfate

To write the formula for tetramminecarbonatocobalt(III) sulfate: 1. Identify the central metal: cobalt(III), with the symbol Co and oxidation state +3. 2. Identify the ligands: ammine (NH3) and carbonate (CO3^(2-)). 3. Write down the formula for the coordination sphere: [Co(NH3)4(CO3)]^(1+). This indicates a cobalt atom surrounded by four ammine ligands and one carbonate ligand. With a 3+ oxidation state for the cobalt and a 2- charge for the carbonate, the overall charge of the coordination sphere is +1. 4. Identify the counter anion: sulfate, with the formula SO4^(2-). 5. Determine the number of sulfates required to balance the charge: The coordination sphere has a 1+ charge, so we need two coordination spheres to balance one sulfate ion's charge. 6. Write the final formula for the compound: [Co(NH3)4(CO3)]2SO4.

(c) dichlorobis(ethylenediamine)platinum(IV) bromide

To write the formula for dichlorobis(ethylenediamine)platinum(IV) bromide: 1. Identify the central metal: platinum(IV), with symbol Pt and oxidation state +4. 2. Identify the ligands: chloro (Cl^(-)) and ethylenediamine (en, or H2NCH2CH2NH2). 3. Write down the formula for the coordination sphere: [Pt(en)2Cl2]^(2+). This indicates a platinum atom surrounded by two ethylenediamine ligands and two chloro ligands. The platinum atom has an oxidation state of +4, and the two chloro ligands each have a charge of -1, giving the coordination sphere an overall charge of +2. 4. Identify the counter anion: bromide, with the formula Br^(-). 5. Determine the number of bromides required to balance the charge: The coordination sphere has a 2+ charge, so we need two bromide ions to balance it. 6. Write the final formula for the compound: [Pt(en)2Cl2]Br2.

Key concepts

Coordination Chemistry

Coordination chemistry is a fascinating branch of chemistry that focuses on the study of coordination compounds. These compounds typically consist of a central metal atom or ion bonded to surrounding molecules or ions, termed ligands. Understanding coordination chemistry is crucial for grasping how various elements interact at the molecular level.

In coordination chemistry, a coordination compound is made up of two main parts:

• The coordination sphere: which includes the central metal and the surrounding ligands.
• The counter ions: these maintain electric neutrality but are not directly bonded to the metal center.

This type of chemistry plays an essential role in biological functions, industrial processes such as catalysis, and even in the formulation of synthetic materials.

Ligands

Ligands are the entities that attach to the central metal atom in a coordination compound. Their main role is to donate electron pairs to the metal, effectively forming a coordinate covalent bond. By doing so, they help stabilize metal ions, which often exist in high oxidation states.

Ligands vary widely in their nature, including:

• Neutral molecules like ammonia (NH₃), often called ammine when in coordination compounds.
• Anions like chloride (Cl⁻), which contribute to the charge balance in the coordination sphere.
• Polyatomic ions like carbonate (CO₃²⁻), which may coordinate in various modes depending on their structure and flexibility.

The diversity of ligands enables the formation of a vast array of coordination compounds, each with unique properties and reactivities.

Oxidation States

In the context of coordination chemistry, the oxidation state of a metal is a key factor that determines its coordination behavior. It refers to the hypothetical charge on the metal atom if all ligands and electron pairs were removed. The oxidation state influences the types and numbers of ligands a metal can bind, greatly affecting the overall charge and stability of the coordination compound.

For example, in our coordination compounds:

• Chromium has an oxidation state of +3, allowing it to bind six ammine ligands in hexaamminechromium(III) nitrate.
• Cobalt in tetramminecarbonatocobalt(III) sulfate also has an oxidation state of +3, coordinating with four ammine ligands and one carbonate.
• Platinum holds a +4 oxidation state in dichlorobis(ethylenediamine)platinum(IV) bromide, coordinating with two ethylenediamine molecules and two chloride ions.

Knowing the oxidation state is essential for writing formulas accurately and understanding the reactivity and bonding in coordination complexes.

Coordination Spheres

The coordination sphere is the inner core of a coordination compound, comprised of the central metal ion bonded directly to its ligands. It is typically represented in formula notation with square brackets [ ], separating it from other components of the compound like counter ions. This helps highlight which atoms are directly bonded to the metal.

The number of ligands attached to the metal in the coordination sphere is known as the coordination number. This number can vary, but is typically between 2 and 9:

• A coordination number of 6, as seen in [Cr(NH₃)₆](NO₃)₃, suggests an octahedral geometry.
• A coordination number of 4, like in [Co(NH₃)₄(CO₃)]₂SO₄, often suggests a tetrahedral or square planar geometry.
• Additional ligands like in [Pt(en)₂Cl₂]Br₂, allow different ligand arrangements and geometries.

Understanding coordination spheres provides insight into the compound's geometry and reactivity, crucial for applications in synthesis and catalysis.