Problem 99 - Chemistry The Molecular Science

Question

The metal ion in \(\left[\mathrm{Pt}\left(\mathrm{NH}_{3}\right)_{2}\left(\mathrm{C}_{2} \mathrm{O}_{4}\right)\right]\) is surrounded by a square planar array of coordinating atoms. (a) Give the oxidation number of the central metal ion. (b) Draw the structural formula of this coordination compound.

Step-by-Step Solution

Answer

(a) The oxidation number of Pt is +2. (b) The structural formula is a square planar arrangement with Pt at the center, \(\mathrm{NH}_3\) and \(\mathrm{C}_2\mathrm{O}_4^{2-}\) opposite each other.

Step 1: Identify Ligands and Their Charges

In the given coordination compound \(\left[\mathrm{Pt}\left(\mathrm{NH}_{3}\right)_{2}\left(\mathrm{C}_{2}\mathrm{O}_{4}\right)\right]\), identify the ligands and their charges. \(\mathrm{NH}_{3}\) is a neutral ligand with no charge. The \(\mathrm{C}_{2}\mathrm{O}_{4}^{2-}\) ligand, also known as oxalate, has a charge of \(-2\).

Step 2: Determine the Charge on the Complex

Since there is no charge given outside the brackets of the formula, the entire coordination complex is neutral (net charge = 0). This is important for balancing the charges when determining the oxidation state of the metal.

Step 3: Calculate Oxidation State of the Metal

Using the known ligand charges, set up an equation for the oxidation number of the central metal ion (Pt). Let the oxidation number of Pt be \(x\). The equation considering a neutral charge on the complex is: \[ x + 2(0) - 2 = 0 \] Simplifying gives: \[ x - 2 = 0 \] \[ x = +2 \] Thus, the oxidation number of Pt is \(+2\).

Step 4: Draw the Structural Formula

To draw the structural formula of the coordination compound, first, acknowledge that Pt has a square planar geometry with 4 coordinating sites occupied by the ligands. Place \(\mathrm{NH}_{3}\) groups opposite each other, and the \(\mathrm{C}_{2}\mathrm{O}_{4}^{2-}\) ligand opposite itself, completing the square planar arrangement.

Key Concepts

Ligands

In coordination chemistry, ligands play a crucial role as the molecules or ions that bind to a central metal atom to form a complex. Ligands can range from simple ions like chloride to complex organic molecules. They usually provide the electrons that form a bond with the metal center. These ligands define the geometry and properties of the complex.
In the compound \(\left[\mathrm{Pt}\left(\mathrm{NH}_{3}\right)_{2}\left(\mathrm{C}_{2}\mathrm{O}_{4}\right)\right]\), note how each type of ligand contributes differently:

• \(\mathrm{NH}_{3}\) (ammonia): A neutral molecule acting as a ligand by donating its lone pair of electrons. It does not alter the overall charge of the complex.
• \(\mathrm{C}_{2}\mathrm{O}_{4}^{2-}\) (oxalate): An example of a bidentate ligand, meaning it attaches to the metal at two points, helping to create stability in the complex. It carries a \(-2\) charge, influencing the oxidation state calculations of the central metal.

Understanding these ligands and their characteristics helps predict the behavior of the complex in various chemical reactions.

Oxidation Number

The oxidation number is a concept used to determine the charge an atom would have if all bonds were ionic. In coordination compounds, it helps predict reactivity and stability. Calculating the oxidation number involves balancing the charges amongst the constituent parts of the compound.
For the compound \(\left[\mathrm{Pt}\left(\mathrm{NH}_{3}\right)_{2}\left(\mathrm{C}_{2}\mathrm{O}_{4}\right)\right]\), assume the oxidation number of platinum is \(x\). Given that the complex overall is neutral:

• The ammonia ligands \(\mathrm{NH}_{3}\) contribute \(0\) to the charge.
• The oxalate ion \(\mathrm{C}_{2}\mathrm{O}_{4}^{2-}\) contributes \(-2\).

Setting up the equation: \[ x + 2(0) - 2 = 0 \]Solving gives \(x = +2\), indicating the oxidation number of platinum is \(+2\). This calculation is essential because a metal’s oxidation state can affect its chemical properties, including its interaction with other substances.

Square Planar Geometry

Square planar geometry is a common arrangement for coordination compounds, especially those involving transition metals with d8 electron configurations, like platinum in the given compound.
In a square planar arrangement, the metal atom is in the center of a square plane formed by four ligand atoms. This geometry significantly influences the properties of the complex:

• It provides a stable and symmetric structure.
• The arrangement can influence the compound's electronic properties, affecting its color, magnetism, and reactivity.

In \(\left[\mathrm{Pt}\left(\mathrm{NH}_{3}\right)_{2}\left(\mathrm{C}_{2}\mathrm{O}_{4}\right)\right]\), platinum is at the center of the plane. The two \(\mathrm{NH}_{3}\) ligand groups are positioned opposite one another, creating a balance, while the bidentate \(\mathrm{C}_{2}\mathrm{O}_{4}^{2-}\) creates bonds that complete the square. Understanding this geometry is crucial for predicting how the complex will interact with other molecules and how it can be manipulated in chemical synthesis.