Question

Comment on each of the following observations. (a) \(\left[\mathrm{IF}_{5}\right]^{2-}\) and \(\left[\mathrm{XeF}_{5}\right]^{-}\) are pentagonal planar. (b) \(\left[\mathrm{PCl}_{4}\right]\left[\mathrm{PCl}_{3} \mathrm{F}_{3}\right]\) contains tetrahedral and octahedral \(P\) centres, and the anion possesses isomers. (c) Members of the series of complexes \(\left[\mathrm{PtCl}_{4}\right]^{2-}\) \(\left[\mathrm{PtCl}_{3}\left(\mathrm{PMe}_{3}\right)\right]^{-},\left[\mathrm{PtCl}_{2}\left(\mathrm{PMe}_{3}\right)_{2}\right]\) \(\left[\mathrm{PtCl}\left(\mathrm{PMe}_{3}\right)_{3}\right]^{+}\) do not possess the same number of isomers.

Short answer

(a) Both ions are pentagonal planar. (b) Tetrahedral \([\mathrm{PCl}_4]\) and octahedral \([\mathrm{PCl}_3\mathrm{F}_3]\) exhibit isomerism. (c) Each Pt complex has a different number of isomers due to varying ligand arrangements.

Step-by-step solution

Analyzing Molecular Geometry

Both \( [\mathrm{IF}_5]^{2-} \) and \( [\mathrm{XeF}_5]^{-} \) ions adopt a pentagonal planar geometry due to the arrangement of five fluorine atoms around the central atom. A central atom with five ligands and extra lone pairs can create a planar geometry due to steric repulsion and minimization of electron-pair repulsion, as per VSEPR theory.

Identifying Shapes and Isomers in Compounds

In \( [\mathrm{PCl}_4] \), phosphorus is at the center of a tetrahedral structure because it forms four single bonds with chlorine. In \( [\mathrm{PCl}_3 \mathrm{F}_3] \), phosphorus is surrounded by three chlorines and three fluorines, following VSEPR, leading to an octahedral shape. The \( [\mathrm{PCl}_3\mathrm{F}_3] \) can exhibit isomerism due to the different possible arrangements (facial or meridional) of Cl and F atoms around the phosphorus.

Comparing Isometries in Complex Series

The complexes \( [\mathrm{PtCl}_4]^{2-}, \) \( [\mathrm{PtCl}_3(\mathrm{PMe}_3)]^{-}, \) \( [\mathrm{PtCl}_2(\mathrm{PMe}_3)_2] \), and \( [\mathrm{PtCl}(\mathrm{PMe}_3)_3]^{+} \) have different substitution patterns affecting isomer count. \([\mathrm{PtCl}_4]^{2-}\) allows only one structure, due to all positions being identical. \([\mathrm{PtCl}_3(\mathrm{PMe}_3)]^{-}\) has more possibilities due to tram/cis configurations. Each new substitution increases possible geometries, affecting the number of isomers.

Key concepts

VSEPR Theory

The Valence Shell Electron Pair Repulsion (VSEPR) Theory helps predict the shape of molecules based on electron pair repulsions. According to this theory, electron pairs around a central atom arrange themselves as far apart as possible to minimize repulsion, leading to specific geometric structures. For example, in the case of the ions \( [\mathrm{IF}_5]^{2-} \) and \( [\mathrm{XeF}_5]^{-} \), each has five fluoride ligands. The central atoms utilize VSEPR principles to arrange these fluorine atoms in a pentagonal planar shape. If there are lone pairs around the central atom, they occupy space and influence the geometry. The ultimate geometry ensures that the repulsion between electron pairs is minimized, thus accurately predicting the distribution and arrangement of atoms in a molecule.

Molecular Geometry

Molecular geometry is the three-dimensional arrangement of atoms in a molecule. It's heavily influenced by VSEPR theory. The geometry dictates how atoms are positioned relative to one another and depends on the type and number of bonds and the presence of lone pairs. In molecules such as \( [\mathrm{PCl}_3 \mathrm{F}_3] \), the phosphorus atom is at the center with a distinctive octahedral shape due to its bonding with three chlorine and three fluorine atoms. This octahedral geometry is common when six substituents or electron pairs surround a central atom. Different arrangements of these substituents can result in various molecular geometries and can impact the physical and chemical properties of the compound.

Isomerism

Isomerism describes the phenomenon where compounds with the same molecular formula exhibit different structural arrangements and properties. In coordination complexes, isomerism is common and can be structural or stereoisomeric. Taking \( [\mathrm{PCl}_3 \mathrm{F}_3] \) as an example, the molecule can show isomerism based on the arrangement of chlorine and fluorine atoms. It can form either a facial or meridional isomer. Facial isomerism involves three identical ligands occupying one face of an octahedron, while in meridional isomers, three identical ligands are in a plane passing through the center of the complex. This diversity in structure directly affects the molecule's reactivity and interaction with other compounds.

Complexes

Coordination complexes form when a central atom or ion is bonded to surrounding molecules or ions known as ligands. These ligands donate electron pairs to the central atom, forming complex structures. For instance, the complexes \( [\mathrm{PtCl}_4]^{2-} \) and \( [\mathrm{PtCl}_3(\mathrm{PMe}_3)]^{-} \) showcase different coordination environments. In \( [\mathrm{PtCl}_4]^{2-} \), a platinum center is surrounded by four chloride ions, forming a square planar shape. Substitution of chloride ions with \( \mathrm{PMe}_3 \) ligands leads to changes not only in geometry but also in the number and type of isomers that can form. Each ligand introduces potential variations in coordination number, symmetry, and isomer formation.

Ligands

Ligands are atoms, ions, or molecules that bind to a central atom in coordination complexes, typically donating electron pairs to form covalent bonds. They are essential in defining the shape and stability of a complex. Ligands can be mono-, bi-, or polydentate, depending on the number of donor atoms they possess. In the study of complexes such as \( [\mathrm{PtCl}_2(\mathrm{PMe}_3)_2] \), \( \mathrm{PMe}_3 \) acts as a monodentate ligand, replacing chloride ions to modify the complex's structure. The choice and nature of ligands significantly influence the chemical behavior and reactivity of coordination complexes. They determine properties such as solubility, magnetic behavior, and both thermodynamic and kinetic stability.