Problem 42 - General Chemistry

Question

Give the coordination number of the transition element in each of the following complexes. a. \(\left[\mathrm{Ni}\left(\mathrm{NH}_{3}\right)_{6}\right]\left(\mathrm{ClO}_{3}\right)_{2}\) b. \(\left[\mathrm{Cu}\left(\mathrm{NH}_{3}\right)_{4}\right] \mathrm{SO}_{4}\) c. \(\left[\mathrm{Cr}(\mathrm{en})_{3}\right] \mathrm{Cl}_{3}\) d. \(\mathrm{K}_{2}\left[\mathrm{Ni}(\mathrm{CN})_{4}\right]\)

Step-by-Step Solution

Answer

a: 6, b: 4, c: 6, d: 4

Step 1: Understand the Coordination Number

The coordination number of a transition metal complex is the number of ligand atoms that are directly bonded to the central metal ion. It essentially reflects how many points of attachment are present around the metal atom.

Step 2: Determine Coordination Number for a

In the complex \(\left[\mathrm{Ni}\left(\mathrm{NH}_{3}\right)_{6}\right]\left(\mathrm{ClO}_{3}\right)_{2}\), Ni is surrounded by 6 NH\(_3\) molecules. Since NH\(_3\) is a monodentate ligand, it attaches at one point each, leading to a coordination number of 6.

Step 3: Determine Coordination Number for b

In \(\left[\mathrm{Cu}\left(\mathrm{NH}_{3}\right)_{4}\right]\mathrm{SO}_{4}\), Cu is surrounded by 4 NH\(_3\) molecules. NH\(_3\) is a monodentate ligand, so the coordination number of Cu is 4.

Step 4: Determine Coordination Number for c

For the complex \(\left[\mathrm{Cr}(\mathrm{en})_{3}\right]\mathrm{Cl}_{3}\), Cr is coordinated by 3 en (ethylenediamine) ligands. En is a bidentate ligand (each en ligand connects at two points), so the coordination number is 6 (3 ligands x 2).

Step 5: Determine Coordination Number for d

In the complex \(\mathrm{K}_{2}\left[\mathrm{Ni}(\mathrm{CN})_{4}\right]\), Ni is surrounded by 4 CN\(^-\) ions. Since each CN\(^-\) is a monodentate ligand, the coordination number is 4.

Key Concepts

Transition Metal Complexes

Transition metal complexes are fascinating compounds that consist of a central metal ion bonded to surrounding atoms or molecules. Just imagine a tiny metal ion at the center, acting almost like a hub, with special groups called ligands attached to it. These ligands are connected in a specific way, forming a composition called a coordination complex. Transition metals, which make up the central part, are elements like iron, nickel, copper, and more.
Why are they called 'transition metals' in the first place? They are termed so because they transition from metallic to non-metallic characteristics in their valence shell. This unique property allows them to easily form bonds and create colorful and complex structures.

• Complexes often appear vividly colorful, due to the d-orbital electron configuration and energy differences.
• Many complexes are highly stable, making them important in industrial and biological settings.

Understanding these complexes is crucial as they play key roles in processes like catalysis, electron transfer, and much more.

Ligands

Ligands are the components attached to the central metal ion in a transition metal complex. Think of ligands as arms reaching out to hold onto the central metal ion, creating stability. They can be ions or molecules and have at least one pair of electrons that they can donate to the central metal.
There is a variety of ligands - some are neutral like ammonia (\( ext{NH}_3 \)), and others are negatively charged like the cyanide ion (\( ext{CN}^- \)).
When ligands attach to the metal, they form what is known as a coordinate or dative bond. This type of bond appears when the ligand shares its lone pair of electrons with the metal. Ligands are crucial for the formation of metal complexes and can greatly influence their properties.

Monodentate and Bidentate Ligands

Not all ligands are created equal. Some ligands attach to the central metal ion at just one point; these are called monodentate ligands. Examples include ammonia (\( ext{NH}_3 \)) and chloride ion (\( ext{Cl}^- \)).

• Monodentate ligands: Connect at one point; simplest form of ligands.

Then there are others that can attach at more than one point - akin to using two arms instead of one to hold onto the central metal. These are referred to as bidentate ligands. Bidentate ligands, like ethylenediamine (\( ext{en} \)), come into play by connecting at two locations, allowing greater stability through chelation (a ring-like structure formed with the metal ion).

• Bidentate ligands: Connect at two points; provide additional strength to the complex.

Understanding the difference between monodentate and bidentate ligands helps in determining the coordination number and stability of metal complexes.

Central Metal Ion

The central metal ion is the heart of a transition metal complex. This metal ion is what holds the ligands together, dictating the complex's overall geometry and properties. Imagine it as the central character in an ensemble cast, with ligands as the supporting actors. Transition metals like nickel (\( ext{Ni} \)), copper (\( ext{Cu} \)), and chromium (\( ext{Cr} \)) often serve as the central ion.
This central metal ion is particularly intriguing because it can adopt different charges, influencing the nature of its bonds with ligands. The ability of these ions to switch oxidation states allows them to function prominently in various chemical reactions.

• Central metal ions affect the physical and chemical properties of complexes.
• More transition metal characteristics include variable oxidation states and coordination numbers.

Understanding the role of the central metal ion is key to mastering the concept of transition metal complexes.