Problem 71 - General Chemistry: Principles and Modern Applications

Question

The coordination number of \(\mathrm{Pt}\) in the complex ion \(\left[\mathrm{PtCl}_{2}(\mathrm{en})_{2}\right]^{2+}\) is \((\mathrm{a}) 2 ;(\mathrm{b}) 3 ;(\mathrm{c}) 4 ;(\mathrm{d}) 5 ;(\mathrm{e}) 6\).

Step-by-Step Solution

Answer

The coordination number of \(\mathrm{Pt}\) in the complex ion \( \left[\mathrm{PtCl}_{2}(\mathrm{en})_{2}\right]^{2+} \) is 6.

Step 1: Identify the central atom and its ligands

The central atom in this complex is platinum, \( \mathrm{Pt} \). The ligands - atoms or ions which are directly bonded to the central atom - are chlorine (\( \mathrm{Cl} \)) and ethylenediamine (\( \mathrm{en} \)). Both of the ligands are directly bonded to \( \mathrm{Pt} \). There are 2 \( \mathrm{Cl} \) and 2 \( \mathrm{en} \) ligands, but it's crucial to remember that \( \mathrm{en} \) is a bidentate ligand, meaning each \( \mathrm{en} \) forms 2 bonds with \( \mathrm{Pt} \).

Step 2: Determine the coordination number

To determine the coordination number of \( \mathrm{Pt} \), count the total number of direct bonds to \( \mathrm{Pt} \). There are 2 bonds from the \( \mathrm{Cl} \) ligands, and 2 \(\times\) 2 bonds from the \( \mathrm{en} \) ligands (since \( \mathrm{en} \) is bidentate). Thus, the total number of direct bonds to \( \mathrm{Pt} \) is 2 \( \mathrm{Cl} \) + 2 \( \times\) 2 \( \mathrm{en} \) = 6. Therefore, the coordination number of \( \mathrm{Pt} \) is 6.

Key Concepts

Complex Ions

Complex ions are fascinating structures formed when a central metal atom is bonded to surrounding ions or molecules, known as ligands. This bonding results in a charged ion, where the molecule as a whole can either be positive or negative. For example, in the complex ion \( [\mathrm{PtCl}_{2}(\mathrm{en})_{2}]^{2+} \), the platinum (\( \mathrm{Pt} \)) serves as the central atom. It forms bonds with chlorine (\( \mathrm{Cl} \)) ions and ethylenediamine (\( \mathrm{en} \)) molecules. This entire assembly carries a \( 2+ \) charge, making it a complex cation.

Understanding complex ions is crucial in chemistry as they play significant roles in reactions and properties of various compounds. The combination of the central metal and its ligands can affect the stability, reactivity, and color of the complex. To fully grasp complex ions, consider these aspects:

• The central atom is usually a transition metal.
• Ligands can be simple ions or more complex molecules.
• The charge of the complex ion can be determined by considering the charges of the central metal and its ligands.

Bidentate Ligands

Ligands are the chemical species that donate electrons to a central metal atom in a coordination complex. Bidentate ligands are unique because they can form two bonds with the central metal, thereby increasing the stability of the complex. Ethylenediamine, abbreviated as \( \mathrm{en} \), is the most common example of a bidentate ligand.

Bidentate ligands create a stronger attachment to the metal compared to monodentate ligands. This is because they "bite" twice onto the central atom, hence the name. In the complex ion \( [\mathrm{PtCl}_{2}(\mathrm{en})_{2}]^{2+} \), each \( \mathrm{en} \) ligand binds twice to platinum, contributing to the complex's six-coordinate bonds.

Here are some key points about bidentate ligands:

• They have two donor atoms.
• Their ability to form two bonds increases the complex's stability.
• They are often used in chelation therapies due to their strong binding nature.

Platinum Chemistry

Platinum is a transition metal well-known for its role in forming complex ions and compounds. Its unique chemical properties make it invaluable in industrial and pharmaceutical applications. In coordination chemistry, platinum often acts as the central metal atom due to its ability to hold multiple ligands.

In the complex \( [\mathrm{PtCl}_{2}(\mathrm{en})_{2}]^{2+} \), platinum's ability to coordinate with six bonds illustrates its versatility. This capability is tied to its electron configuration, which allows it to accept electrons from several ligands without destabilizing the atom. Consequently, platinum complexes can exhibit interesting geometries, such as planar or octahedral shapes, which influence the properties and reactivity of the compound.

Here are several important features of platinum in chemistry:

• It has a high affinity for nitrogen and oxygen donor ligands.
• Platinum complexes are often used in catalysis and cancer treatment, such as in drugs like cisplatin.
• The coordination number of platinum can vary, but six is common, as seen in the given complex.