Question

Name the following coordination compounds. a. \(\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{6}\right] \mathrm{Cl}_{2}\) d. \(\mathrm{K}_{4}\left[\mathrm{PtCl}_{6}\right]\) b. \(\left[\mathrm{Co}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right] \mathrm{I}_{3}\) e. \(\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{5} \mathrm{Cl}\right] \mathrm{Cl}_{2}\) c. \(\mathrm{K}_{2}\left[\mathrm{PtCl}_{4}\right]\) f. \(\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{3}\left(\mathrm{NO}_{2}\right)_{3}\right]\)

Short answer

The names of the coordination compounds are: a. hexaamminecobalt(III) chloride d. potassium hexachloroplatinate(IV) b. hexaaquacobalt(III) iodide e. pentaamminechloridocobalt(III) chloride c. potassium tetrachloroplatinate(II) f. triamminetrititrocobalt(III)

Step-by-step solution

(a. Identifying elements and naming \(\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{6}\right] \mathrm{Cl}_{2}\))

In this coordination compound, the central metal atom is cobalt (Co) and the ligand is ammonia (NH\(_{3}\)) with a coordination number of 6. The compound has two chlorine (Cl) counterions. The name of this compound is: hexaamminecobalt(III) chloride. Notice that roman numeral III is used to represent the +3 oxidation state of cobalt.

(d. Identifying elements and naming \(\mathrm{K}_{4}\left[\mathrm{PtCl}_{6}\right]\))

In this coordination compound, the central metal atom is platinum (Pt) and the ligand is chloride (Cl) with a coordination number of 6. The compound has four potassium (K) counterions. The name of this compound is: potassium hexachloroplatinate(IV). Notice that roman numeral IV is used to represent the +4 oxidation state of platinum.

(b. Identifying elements and naming \(\left[\mathrm{Co}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right] \mathrm{I}_{3}\))

In this coordination compound, the central metal atom is cobalt (Co) and the ligand is water (H\(_{2}\)O) with a coordination number of 6. The compound has three iodide (I) counterions. The name of this compound is: hexaaquacobalt(III) iodide. Notice that roman numeral III is used to represent the +3 oxidation state of cobalt.

(e. Identifying elements and naming \(\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{5} \mathrm{Cl}\right] \mathrm{Cl}_{2}\))

In this coordination compound, the central metal atom is cobalt (Co). There are two types of ligands: ammonia (NH\(_{3}\)) with a coordination number of 5 and chloride (Cl) with a coordination number of 1. The compound has two chlorine (Cl) counterions. The name of this compound is: pentaamminechloridocobalt(III) chloride. Notice that roman numeral III is used to represent the +3 oxidation state of cobalt.

(c. Identifying elements and naming \(\mathrm{K}_{2}\left[\mathrm{PtCl}_{4}\right]\))

In this coordination compound, the central metal atom is platinum (Pt) and the ligand is chloride (Cl) with a coordination number of 4. The compound has two potassium (K) counterions. The name of this compound is: potassium tetrachloroplatinate(II). Notice that roman numeral II is used to represent the +2 oxidation state of platinum.

(f. Identifying elements and naming \(\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{3}\left(\mathrm{NO}_{2}\right)_{3}\right]\))

In this coordination compound, the central metal atom is cobalt (Co). There are two types of ligands: ammonia (NH\(_{3}\)) and nitrite (NO\(_{2}\)) both with a coordination number of 3. The compound has no counterions. The name of this compound is: triamminetrititrocobalt(III). Notice that roman numeral III is used to represent the +3 oxidation state of cobalt.

Key concepts

Complex Naming

In chemistry, naming coordination compounds can initially appear challenging, but it's actually quite systematic once you understand the rules. These names provide detailed information about the components and structure of the compound. To start, the name begins with the ligands followed by the central metal atom or ion.
Ligands are listed alphabetically, ignoring any prefixes. Use special prefixes like "tetra-, penta-, and hexa-" to indicate the number of identical ligands. For example, "hexaammine" reveals six ammonia molecules bound to cobalt.
When naming the metal, if the whole complex is negatively charged, the metal's name might end with "-ate". Lastly, we specify the oxidation state of the metal, written in Roman numerals in parentheses, like "(III)" for a +3 oxidation state. The counterions, which balance the charge, appear at the end of the compound's name, such as "chloride." This way, the whole name, for instance, might be "hexaamminecobalt(III) chloride."

Oxidation State

Determining the oxidation state of the central metal atom in coordination compounds is crucial for naming purposes. It informs us about the metal ion's charge, which helps predict the compound's properties. Here's how to find it:
1. Identify the total charge of the complex ion.
2. Assign known charges to the ligands and counterions, remembering that common charges are chlorine (-1), ammonia (0), and water (0).
3. Use the formula: Total charge = Metal oxidation state + sum of ligand charges.
Consider \([ ext{Co}( ext{NH}_3)_6]^{3+}\), where cobalt has an oxidation state of +3. The ligands are neutral, and the charge of the whole complex ion is +3. By balancing these, we deduce cobalt's oxidation state. Knowing the oxidation state aids in not just naming but understanding the reactivity and stability of the compound.

Ligands

Ligands play a pivotal role in the structure and function of coordination compounds. They are molecules or ions that donate an electron pair to the central metal atom. Understanding their types and roles helps one grasp the nature of these compounds.
Ligands can be categorized based on the number of donor atoms present:

• Monodentate ligands: single donor atom, e.g., ammonia (NH\(_3\))
• Bidentate ligands: two donor atoms, e.g., ethylenediamine (en)
• Polydentate ligands: multiple donor atoms, like EDTA

They influence the stability and geometry of the complex. For instance, ammonia is a common ligand with a lone pair on nitrogen, binding to metals like cobalt. By naming ligands first in the compound name, we highlight their importance prior to naming the metal or indicating oxidation state.

Coordination Number

The coordination number of a coordination compound refers to the number of ligand atoms bonded directly to the central metal atom. It's essential for understanding the geometry and stability of the compound.
To determine this number, count the total donor atoms from ligands attached to the metal. For instance, in \([ ext{Co}( ext{NH}_3)_6]\), cobalt has a coordination number of 6, as it bonds with six ammonia molecules.
Coordination numbers range typically from 2 to 12 but are most often in the range of 4 to 6. They directly influence the shape of the compound:

• 4 usually forms square planar or tetrahedral shapes.
• 6 tends towards octahedral geometry.

Understanding this concept helps predict and explain the compound's chemical behavior, including its reactivity and interaction with other molecules.