Question

There are two compounds of the formula Pt \(\left(\mathrm{NH}_{3}\right)_{2} \mathrm{Cl}_{2} :\) The compound on the right is called cisplatin, and the compound on the left is called transplatin. (a) Which compound has a nonzero dipole moment? (b) One of these compounds is an anticancer drug, and one is inactive. The anticancer drug works by its chloride ions undergoing a substitution reaction with nitrogen atoms in DNA that are close together, forming a \(\mathrm{N}-\mathrm{Pt}-\mathrm{N}\) angle of about \(90^{\circ} .\) Which compound would you predict to be the anticancer drug?

Short answer

(a) Cisplatin has a nonzero dipole moment. (b) Cisplatin is the anticancer drug.

Step-by-step solution

Understand the molecular geometry

The two compounds, cisplatin and transplatin, have the same chemical formula but different geometric structures. They are both square planar coordination compounds of the metal platinum, with two ammonia (NH3) ligands and two chloride (Cl) ligands. In cisplatin, the two ammonia ligands and the two chloride ligands are next to each other (cis position), whereas, in transplatin, they are opposite of each other (trans position).

Calculate the dipole moment

A molecule has a nonzero dipole moment if it has polar bonds and the vector sum of the individual bond dipoles does not cancel out. In both cisplatin and transplatin, there are polar bonds between the platinum and the ligands (NH3 and Cl). In the case of cisplatin, since the two ammonia ligands and the two chloride ligands are on the same side of the molecule, their bond dipoles do not cancel out, resulting in a nonzero dipole moment. Conversely, in transplatin, the bond dipoles of NH3 and Cl ligands cancel each other out as they are positioned opposite of each other, giving a net dipole moment of zero. Hence, cisplatin has a nonzero dipole moment. (a) Answer: Cisplatin has a nonzero dipole moment.

Predict the anticancer drug

As mentioned in the exercise, the anticancer drug works by having its chloride ions react with nitrogen atoms in DNA that are close together, forming an N-Pt-N angle of approximately 90 degrees. In cisplatin, the two chloride ligands are adjacent to each other, allowing the possibility of forming such an angle with nearby nitrogen atoms. However, in transplatin, the chloride ligands are positioned on opposite sides, making it unlikely for them to interact with DNA in such a manner. Thus, based on this information, we predict that cisplatin is the anticancer drug. (b) Answer: Cisplatin is the anticancer drug.

Key concepts

Molecular Geometry

Molecular geometry relates to the three-dimensional arrangement of atoms within a molecule. This concept is fundamental in predicting and understanding the chemical behavior of molecules. For instance, the molecular geometry of coordination compounds such as cisplatin and transplatin is square planar. This involves a central metal atom like platinum being surrounded by other atoms or molecules called ligands, at 90-degree angles. This structure dictates the molecule's properties, including its reactivity and interaction with other substances, which is particularly crucial in the case of cisplatin's interaction with DNA for its anticancer activity.

Understanding molecular geometry is not only about recognizing the shape but also about appreciating how the spatial arrangement affects the molecule's physical and chemical characteristics. Having the knowledge that cisplatin and transplatin share a square planar geometry yet have different biological activities can intrigue a student to delve deeper, bridging a gap between theoretical concepts and practical application.

Dipole Moment

The dipole moment is a measure of the separation of positive and negative charges within a molecule, influenced by the nature of the atoms involved and the molecular geometry. Considering the coordination compounds cisplatin and transplatin, understanding their dipole moments holds the key to predicting which compound will have a nonzero dipole moment.

Intrigued students' curiosity is further piqued when they learn how the arrangement of ligands in these isomers influences their dipole moments. The presence of a nonzero dipole moment in cisplatin — attributable to the cis arrangement of polar ligands — sets it apart from transplatin, which has a zero dipole moment due to the trans arrangement causing a cancellation of dipole vectors. This topic showcases a concrete example of how molecular structures dictate chemical behavior.

Coordination Compounds

Coordination compounds consist of a central metal atom bonded to a set of ligands, which can be molecules or ions. These compounds often exhibit extraordinary colors and unique chemical properties, making them fascinating subjects of study in the realms of inorganic chemistry and pharmacology. Notably, cisplatin and transplatin are coordination compounds with platinum as their metal center.

The square planar arrangement around the platinum atom in both cisplatin and transplatin makes them pertinent examples of coordination compounds. Students get to explore how the central metal and ligands contribute to the formation and function of such complexes. Cisplatin's efficacy as an anticancer agent is a testament to the concept that even slight variations in the structural configuration of coordination compounds can lead to significant differences in biological activity.

Cis and Trans Isomers

Isomers are molecules with the same molecular formula but different structural arrangements. Cis and trans isomers represent an interesting case whereby two compounds can have vastly different properties due to the different placements of groups attached to a double bond or a metal center in a coordination compound. As observed in the exercise, cisplatin has ligands on the same side, while transplatin has them opposite.

The interest in these isomers lies in the profound effect that their arrangement has on their function. The spatial arrangement in cisplatin enables it to form a 90-degree N-Pt-N angle conducive to its interaction with DNA, facilitating its application as an effective anticancer drug, as opposed to its inactive trans isomer. This concept provides students with a clear impression of how minuscule changes at the molecular level can result in significant differences in biological properties.