Problem 28 - Chemistry: An Atoms-Focused Approach

Question

What are the names of the following coordination compounds? a. \(\left(\mathrm{NH}_{4}\right)_{3}\left[\mathrm{Co}(\mathrm{CN})_{6}\right]\) b. \(\left[\mathrm{Co}(\mathrm{en})_{2} \mathrm{Cl}\right]\left(\mathrm{NO}_{3}\right)_{2}\) c. \(\left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{4}(\mathrm{OH})_{2}\right] \mathrm{Cl}\)

Step-by-Step Solution

Answer

a. \(\left(\mathrm{NH}_{4}\right)_{3}\left[\mathrm{Co}(\mathrm{CN})_{6}\right]\) b. \(\left[\mathrm{Co}(\mathrm{en})_{2}\mathrm{Cl}\right]\left(\mathrm{NO}_{3}\right)_{2}\) c. \(\left[\mathrm{Fe}\left(\mathrm{H}_{2}\mathrm{O}\right)_{4}(\mathrm{OH})_{2}\right] \mathrm{Cl}\) Answer: a. Hexaamminecobalt(III) hexacyanide. b. Bis(ethylenediamine)cobalt(I) chloro nitrate. c. Tetraaqua dihydroxo iron(I) chloride.

Step 1: Identify the cation and the anion

In this compound, the cation is \(\mathrm{NH}_{4}\) and the anion is \([\mathrm{Co}(\mathrm{CN})_{6}]\).

Step 2: Name the cation and the ligands

The cation is ammonium, and the ligand \(\mathrm{CN}\) (one of the common exceptions) is cyanide. There are 6 cyanide ions, so we use the Greek prefix "hexa".

Step 3: Name the central metal and its oxidation state

The central metal is cobalt. To determine the oxidation state, we sum the charges of all the ligands (6*(-1)) and balance it with the cation ((3*(+1)) which gives us +3. Cobalt is thus named "cobalt(III)".

Step 4: Combine the names

The complete name of the compound is: hexaamminecobalt(III) hexacyanide. b. \(\left[\mathrm{Co}(\mathrm{en})_{2}\mathrm{Cl}\right]\left(\mathrm{NO}_{3}\right)_{2}\)

Step 1: Identify the cation and the anion

In this compound, the cation is \([\mathrm{Co}(\mathrm{en})_{2}\mathrm{Cl}]\) and the anion is \(\mathrm{NO}_{3}\).

Step 2: Name the cation and the ligands

The anion is nitrate. The ligands are \(\mathrm{en}\) and \(\mathrm{Cl}\), which are named ethylenediamine and chloro, respectively. We also use the prefix "bis" to indicate there are 2 ethylenediamine ligands.

Step 3: Name the central metal and its oxidation state

The central metal is cobalt. The sum of the charges of the ligands and the metal is \(2\times(0)+(-1)=0+(-1)=-1\). Since the charge of the cation is +1 due to the 2 nitrate ions, the oxidation state of cobalt is +1. Cobalt is thus named "cobalt(I)".

Step 4: Combine the names

The complete name of the compound is: bis(ethylenediamine)cobalt(I) chloro nitrate. c. \(\left[\mathrm{Fe}\left(\mathrm{H}_{2}\mathrm{O}\right)_{4}(\mathrm{OH})_{2}\right] \mathrm{Cl}\)

Step 1: Identify the cation and the anion

In this compound, the cation is \([\mathrm{Fe}\left(\mathrm{H}_{2}\mathrm{O}\right)_{4}(\mathrm{OH})_{2}]\) and the anion is \(\mathrm{Cl}\).

Step 2: Name the cation and the ligands

The anion is chloride. The ligands are \(\mathrm{H_{2}O}\) and \(\mathrm{OH}\), which are named aqua and hydroxo, respectively. We use the prefixes "tetra" for the 4 aqua ligands and "di" for the 2 hydroxo ligands.

Step 3: Name the central metal and its oxidation state

The central metal is iron. The sum of the charges of the ligands and the metal is \(4\times(0)+2\times(-1)=-2\). Since the compound has an overall charge of -1 due to the chloride ion, the oxidation state of iron is +1. Iron is thus named "iron(I)".

Step 4: Combine the names

The complete name of the compound is: tetraaqua dihydroxo iron(I) chloride.

Key Concepts

Oxidation State

Understanding the oxidation state of the central metal in a coordination compound is key to naming these compounds accurately. The oxidation state reveals how many electrons the metal has gained or lost when forming the compound. Here's how to determine it:

• Add up the charges of all ligands. For instance, cyanide (\(\text{CN}^-\)) has a charge of \(-1\).
• Consider the charge of the central metal, which is balanced by the charges of the ligands and any counter ions.
• Lastly, use this sum to deduce the metal's oxidation state, often represented in Roman numerals in the compound's name.

This process helps to understand the nature of the metal's participation in the structure, providing insights into the compound's properties.

Ligands

Ligands are essential in coordination compounds, acting as the "arms" that bind to the central metal. They can be ions or molecules, and naming them forms a crucial step in naming the compound:

• Common ligands include water (\(\text{H}_2\text{O}\)) known as "aqua," and ammonia (\(\text{NH}_3\)) known as "ammine."
• Ligands are given prefixes based on their quantity: "mono" (1), "di" (2), "tri" (3), etc. Special prefixes like "bis" and "tris" are used when ligands are complex, like ethylenediamine (en).
• Negative ligands often end with an "o," such as chloro from \(\text{Cl}^-\).

The arrangement and type of ligands influence the compound's geometry and reactivity, making ligand identification a crucial aspect of coordination chemistry.

Naming Coordination Compounds

Naming coordination compounds involves a step-by-step approach to convey the structure and components of the compound accurately. Here's a breakdown:

• Begin by identifying the cation and the anion. If the complex is an anion, its name ends with "ate."
• Name the ligands first, in alphabetical order, followed by the central metal with its oxidation state in parentheses.
• Use appropriate prefixes for the number of each type of ligand.

For example, in "hexaamminecobalt(III) chlorate," "hexa" indicates six ligands, "ammine" refers to the ligands attached to cobalt, and "cobalt(III)" specifies the metal and its oxidation state. This systematic method ensures the name accurately reflects the compound's chemical reality.

Cation and Anion Identification

Identifying the cation and anion in coordination compounds is crucial for determining their names. Here's how to do it:

• The cation is typically the positively charged part and named first in the compound name.
• The anion, conversely, is negatively charged and named last.
• Inspection of chemical formulas reveals which part is cationic or anionic. For example, \(\text{[Co(en)_2Cl]}\) is a cation in the compound \(\text{[Co(en)_2Cl]}(\text{NO}_3)_2\).

Knowing which is the cation and which is the anion helps in properly balancing the charges and understanding the compound's composition. This distinction is fundamental in chemical nomenclature and analysis.