Problem 72 - Chemistry An Atoms First Approach

Question

The compound cisplatin, \(\operatorname{Pt}\left(\mathrm{NH}_{3}\right)_{2} \mathrm{Cl}_{2},\) has been studied extensively as an antitumor agent. The reaction for the synthesis of cisplatin is: \(\mathrm{K}_{2} \mathrm{PtCl}_{4}(a q)+2 \mathrm{NH}_{3}(a q) \longrightarrow \mathrm{Pt}\left(\mathrm{NH}_{3}\right)_{2} \mathrm{Cl}_{2}(s)+2 \mathrm{KCl}(a q)\) Write the electron configuration for platinum ion in cisplatin. Most \(d^{8}\) transition metal ions exhibit square planar geometry. With this and the name in mind, draw the structure of cisplatin.

Step-by-Step Solution

Answer

The electron configuration for the platinum ion (\(\operatorname{Pt}^{2+}\)) is: \[1s^2 \ 2s^2 2p^6 \ 3s^2 3p^6 4s^2 3d^{10} 4p^6 5s^2 4d^{10} 5p^6 4f^{14} 5d^9\] The structure of cisplatin (\(\operatorname{Pt}\left(\mathrm{NH}_{3}\right)_{2}\mathrm{Cl}_{2}\)) with square planar geometry is: ``` NH3 | Cl ----- Pt ----- Cl | NH3 ```

Step 1: Identify the oxidation state of platinum in cisplatin

The compound cisplatin contains Pt (platinum), \(\mathrm{NH}_3\) (ammonia) and \(\mathrm{Cl}_2\) (chlorine) molecules. By looking at the synthesis reaction: \(\mathrm{K}_{2} \mathrm{PtCl}_{4}(a q)+2 \mathrm{NH}_{3}(a q) \longrightarrow \operatorname{Pt}\left(\mathrm{NH}_{3}\right)_{2} \mathrm{Cl}_{2}(s)+2 \mathrm{KCl}(a q)\) We can identify that Pt is originally bonded with 4 Cl ions in the complex \(\mathrm{PtCl}_{4}^{2-}\). By this, we know that platinum is in the oxidation state of +2 or \(\operatorname{Pt}^{2+}\).

Step 2: Write the electron configuration for platinum ion in cisplatin

To find the electron configuration for platinum ion (\(\operatorname{Pt}^{2+}\)), first, write the electron configuration for the neutral Pt atom. Neutral Pt has 78 electrons, and its electron configuration is: \[1s^2 \ 2s^2 2p^6 \ 3s^2 3p^6 4s^2 3d^{10} 4p^6 5s^2 4d^{10} 5p^6 6s^2 4f^{14} 5d^9\] Now, to write the electron configuration for \(\operatorname{Pt}^{2+}\), we should remove 2 electrons from the neutral Pt atom. We always remove electrons from the highest principal quantum number (n). In this case, it's the 6s^2 orbitals. So the electron configuration for \(\operatorname{Pt}^{2+}\) becomes: \[1s^2 \ 2s^2 2p^6 \ 3s^2 3p^6 4s^2 3d^{10} 4p^6 5s^2 4d^{10} 5p^6 4f^{14} 5d^9\]

Step 3: Draw the structure of cisplatin considering its square planar geometry

Since \(\operatorname{Pt}^{2+}\) is a \(d^8\) transition metal ion, it has a square planar geometry. In this structure, the platinum ion is at the center, and two NH3 molecules and two Cl ions surround it. The cisplatin molecule can be represented as Pt(NH3)2Cl2, with cis geometry having the two NH3 molecules adjacent to each other and the two Cl ions adjacent to each other as well. So the structure of cisplatin will look like: ``` NH3 | Cl ----- Pt ----- Cl | NH3 ``` The structure above represents cisplatin with a square planar geometry.

Key Concepts

Electron Configuration

In the realm of chemistry, the electron configuration of an atom is essential for understanding its chemical behavior and bonding characteristics. For platinum, (\(\text{Pt}^{2+}\)), the task is to determine its electron arrangement, especially when it is a component of a compound such as cisplatin.
Neutral platinum has 78 electrons and its electron configuration is written as:

• \[ 1s^2 \ 2s^2 \ 2p^6 \ 3s^2 \ 3p^6 \ 4s^2 \ 3d^{10} \ 4p^6 \ 5s^2 \ 4d^{10} \ 5p^6 \ 6s^2 \ 4f^{14} \ 5d^9 \]

For the \(\text{Pt}^{2+}\) ion in cisplatin, two electrons are removed. This happens from the highest energy level first, which is the 6s orbital. Thus, the \(\text{Pt}^{2+}\) configuration becomes:

• \[ 1s^2 \ 2s^2 \ 2p^6 \ 3s^2 \ 3p^6 \ 4s^2 \ 3d^{10} \ 4p^6 \ 5s^2 \ 4d^{10} \ 5p^6 \ 4f^{14} \ 5d^8 \]

Understanding this electron configuration is pivotal because it directly influences both the chemical properties and the potential geometries the compound can form.

Oxidation State

To understand the characteristics of a compound like cisplatin, knowing the oxidation state of its components is crucial. The oxidation state of an element describes the degree of oxidation (loss of electrons) in a chemical compound. In the cisplatin synthesis reaction:

• \(\text{K}_2\text{PtCl}_4(aq) + 2 \text{NH}_3(aq) \rightarrow \text{Pt}(\text{NH}_3)_2\text{Cl}_2(s) + 2\text{KCl}(aq)\)

platinum's role is clearly highlighted. Initially bonded with four chloride ions, forming \(\text{PtCl}_4^{2-}\), platinum in cisplatin is said to be in the +2 oxidation state, expressed as \(\text{Pt}^{2+}\). In essence:

• Each chloride (\(\text{Cl}\)) contributes a \(-1\) charge.
• The \(\text{PtCl}_4^{2-}\) complex implies that the overall charge of the compound before removing chlorides must balance the charges out, indicating that \(\text{Pt}\) must be in the +2 state to achieve electrical neutrality in the complex.

The +2 oxidation state significantly affects the bonding and structure of cisplatin, influencing its biological functions as an antitumor agent.

Square Planar Geometry

Cisplatin, structurally noted as \(\text{Pt}(\text{NH}_3)_2\text{Cl}_2\), is particularly fascinating due to its \(d^8\) electron configuration which favors a square planar geometry. This structure is defined by four ligands situated at the corners of a square plane, with the central metal ion at the center. This arrangement occurs due to the presence of a \(d^8\) configuration that stabilizes the compound effectively in the square planar form rather than tetrahedral forms found in other metals.
Here's how it impacts cisplatin:

• The two ammonia (\(\text{NH}_3\)) ligands and the two chloride (\(\text{Cl}\)) ions align in a square plane.
• "Cis" implies the geometric positioning where similar ligands are adjacent rather than opposite, crucial for its biological efficacy.
• This unique geometry places the platinum ion at the center surrounded symmetrically by its four ligands, contributing to its efficient binding properties and interactions within biological systems.

When visualizing cisplatin, think of it like a flat square grid with platinum centrally positioned, ensuring efficient binding which is crucial for its role in cancer treatment.