Problem 23 - 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) hexaamminechromium(III) nitrate (b) tetraamminecarbonatocobalt(III) sulfate (c) dichlorobis(ethylenediamine)platinum(IV) bromide (d) potassium diaquatetrabromovanadate(III) (e) bis(ethylenediamine) zinc(II) tetraiodomercurate(II)

Step-by-Step Solution

Answer

The short versions of the answers are: (a) \([Cr(NH3)_6](NO3)_3\) (b) \([Co(NH3)_4CO3](SO4)_2\) (c) \([PtCl_2(H_2NCH_2CH_2NH_2)_2](Br)_4\) (d) \(K_3[V(H_2O)_2Br_4]\) (e) \([Zn(H_2NCH_2CH_2NH_2)_2](HgI_4)\)

Step 1: Identify the central ion and its oxidation state

The central ion is chromium (Cr) with an oxidation state of +3 (chromium(III)).

Step 2: Write the symbols and number of ligands

We have hexaammine, which means 6 ammine ligands (NH3).

Step 3: Form the coordination sphere

Combine the central ion and its ligands in brackets: \([Cr(NH3)_6]\)

Step 4: Write the counter-ions and indicate their quantity

The counter-ion is nitrate (NO3-) with a charge of -1. To balance the charge, we need 3 nitrate ions, so the formula is: \([Cr(NH3)_6](NO3)_3\) #For (b): tetraamminecarbonatocobalt(III) sulfate#

Step 1: Identify the central ion and its oxidation state

The central ion is cobalt (Co) with an oxidation state of +3 (cobalt(III)).

Step 2: Write the symbols and number of ligands

We have tetraammine, which means 4 ammine ligands (NH3), and carbonato (CO3^2-).

Step 3: Form the coordination sphere

Combine the central ion and its ligands in brackets: \([Co(NH3)_4CO3]\)

Step 4: Write the counter-ions and indicate their quantity

The counter-ion is sulfate (SO4^2-) with a charge of -2. To balance the charge, we need 2 sulfate ions, so the formula is: \([Co(NH3)_4CO3](SO4)_2\) #For (c): dichlorobis(ethylenediamine)platinum(IV) bromide#

Step 1: Identify the central ion and its oxidation state

The central ion is platinum (Pt) with an oxidation state of +4 (platinum(IV)).

Step 2: Write the symbols and number of ligands

We have dichloro, which means 2 chloro ligands (Cl), and bis(ethylenediamine), which means 2 ethylenediamine ligands (\(H_2NCH_2CH_2NH_2\)).

Step 3: Form the coordination sphere

Combine the central ion and its ligands in brackets: \([PtCl_2(H_2NCH_2CH_2NH_2)_2]\)

Step 4: Write the counter-ions and indicate their quantity

The counter-ion is bromide (Br-) with a charge of -1. To balance the charge, we need 4 bromide ions, so the formula is: \([PtCl_2(H_2NCH_2CH_2NH_2)_2](Br)_4\) #For (d): potassium diaquatetrabromovanadate(III)#

Step 1: Identify the central ion and its oxidation state

The central ion is vanadium (V) with an oxidation state of +3 (vanadate(III)).

Step 2: Write the symbols and number of ligands

We have diaqua, which means 2 aqua ligands (H2O), and tetrabromo, which means 4 bromo ligands (Br).

Step 3: Form the coordination sphere

Combine the central ion and its ligands in brackets: \([V(H_2O)_2Br_4]\)

Step 4: Write the counter-ions and indicate their quantity

The counter-ion is potassium (K+) with a charge of +1. To balance the charge, we need 3 potassium ions, so the formula is: \(K_3[V(H_2O)_2Br_4]\) #For (e): bis(ethylenediamine) zinc(II) tetraiodomercurate(II)#

Step 1: Identify the central ion and its oxidation state

The central ion is zinc (Zn) with an oxidation state of +2 (zinc(II)).

Step 2: Write the symbols and number of ligands

We have bis(ethylenediamine), which means 2 ethylenediamine ligands (\(H_2NCH_2CH_2NH_2\)).

Step 3: Form the coordination sphere

Combine the central ion and its ligands in brackets: \([Zn(H_2NCH_2CH_2NH_2)_2]\)

Step 4: Write the counter-ions and indicate their quantity

The counter-ion is tetraiodomercurate(II) (\(HgI_4^{2-}\)) with a charge of -2. To balance the charge, we need 1 tetraiodomercurate(II) ion, so the formula is: \([Zn(H_2NCH_2CH_2NH_2)_2](HgI_4)\)

Key Concepts

Oxidation State

The oxidation state, also known as oxidation number, reflects the degree of oxidation of an atom within a chemical compound. It's a theoretical charge that helps us understand the electronic makeup of compounds. For transition metal coordination compounds, like those given in the exercises, determining the oxidation state is crucial for predicting the overall compound structure and knowing the relationships between components.

• The oxidation state of the central metal ion can often be found in the chemical name, like in chromium(III) or cobalt(III). The Roman numeral indicates the charge of the metal ion.
• Oxidation states impact the number of electrons that the central metal atom shares with its ligands, influencing the geometry and magnetic properties of the complex.
• Knowing the oxidation state enables chemists to understand the metal's role and behavior in biological systems or industrial applications.

Determining the oxidation state often involves balancing the charges of the metal ion with its ligands and counter-ions to achieve a neutral or specific charged complex.

Coordination Sphere

The coordination sphere is a fundamental concept in coordination chemistry. It includes the central metal atom and all the ligands directly bonded to it. Understanding the coordination sphere is essential for writing correct chemical formulas and describing the structure of the complexes.

• The coordination sphere is usually enclosed in square brackets [ ], indicating that the contents are considered as a single entity in chemical equations.
• Each ligand within the coordination sphere is directly bonded to the central metal ion and contributes to the stability and reactivity of the complex.
• The arrangement of ligands around the central atom can affect the compound's physical and chemical properties, such as color, solubility, and reactivity.

Learning how to identify and construct coordination spheres in compounds is a key skill in inorganic chemistry, aiding in the prediction of compound behavior.

Ligands

Ligands are molecules or ions that bind to the central metal atom in a coordination compound. Their role is to donate electron pairs to the metal atom, forming coordinate covalent bonds. Understanding ligands is crucial for predicting the structure and properties of coordination complexes.

• Ligands can be neutral molecules like water (H₂O) or ammonia (NH₃), or they can be ions like chloride (Cl^-) or hydroxide (OH^-).
• Each ligand can have one or more donor atoms, influencing the coordination number, which is the total number of bonds formed between the ligands and the central ion.
• The identity and arrangement of ligands determine the geometric shape of the coordination compound, which affects its magnetic and optical properties.

By studying ligands, chemists can manipulate complexes for designing specific properties in various fields, from catalysis to drug design.

Counter-Ions

Counter-ions are the ions outside the coordination sphere that balance the charge of the complex ion within the sphere. They are not directly bonded to the central metal atom, but they are essential for achieving electroneutrality in the compound.

• Counter-ions maintain the overall charge balance of the complex. For example, if the coordination sphere is positively charged, the counter-ions will typically be negatively charged.
• These ions can significantly influence the solubility and stability of the coordination compound in different environments.
• Counter-ions can vary widely and include common ions such as nitrate (NO₃^-), sulfate (SO₄^2-), bromide (Br^-), and potassium (K⁺).

Identifying and understanding counter-ions is crucial for writing the proper formulas and predicting reactions involving coordination complexes.