Question

(a) Draw possible stereoisomers for the trigonal bipyramidal \(\left[\mathrm{SiF}_{3} \mathrm{Me}_{2}\right]^{-}\) anion \(\left(\mathrm{Me}=\mathrm{CH}_{3}\right) .\) An X-ray diffraction study of a salt of \(\left[\mathrm{SiF}_{3} \mathrm{Me}_{2}\right]^{-}\) shows that two \(\mathrm{F}\) atoms occupy axial sites. Suggest why this stereoisomer is preferred over the other possible structures that you have drawn. (b) Account for the fact that members of the series of complexes \(\left[\mathrm{PtCl}_{4}\right]^{2-},\left[\mathrm{PtCl}_{3}\left(\mathrm{PMe}_{3}\right)\right]^{-},\left[\mathrm{PtCl}_{2}\left(\mathrm{PMe}_{3}\right)_{2}\right]\) and \(\left[\mathrm{PtCl}\left(\mathrm{PMe}_{3}\right)_{3}\right]^{+}\) do not possess the same number of stereoisomers.

Short answer

(a) Two stereoisomers for [SiF_3Me_2]^- are possible, with the preferred having two F atoms axially. (b) Pt complexes vary in stereoisomer number due to differing ligand arrangements.

Step-by-step solution

Recall the shape and geometry

The [SiF_3Me_2]^- anion has a trigonal bipyramidal geometry. The central silicon atom is surrounded by three fluorine atoms and two methyl groups. In a trigonal bipyramidal structure, there are two types of positions: axial and equatorial.

Identifying Possible Stereoisomers

In trigonal bipyramidal structures, steric and electronic factors influence which ligands occupy axial or equatorial positions. You can have different stereoisomers by interchanging the positions of F atoms and Me groups. Possible configurations are: 1. Two F atoms in axial positions and one F in an equatorial position with two Me groups in the other equatorial positions. 2. One F atom in an axial position and two Me groups in the other axial position with the remaining F atoms in equatorial spots.

Determine the Preferred Stereoisomer

The X-ray diffraction study shows that the preferred structure has two F atoms in axial positions. This preference is due to the smaller size and higher electronegativity of F atoms, which minimizes steric hindrance and maximizes stability when occupying positions with more space (axial). The bulkier CH_3 groups fit better in equatorial positions, where there is less spatial competition.

Introduction to Pt Complexes

The series [PtCl_4]^{2-}, [PtCl_3(PMe_3)]^-, [PtCl_2(PMe_3)_2], and [PtCl(PMe_3)_3]^+ involves square planar coordination by platinum. This coordination allows for a variety of arrangements depending on the number of PMe_3 ligands replacing chloride ions (Cl^-).

Evaluate the Stereoisomers for Pt Complexes

[PtCl_4]^{2-} has no stereoisomers because all ligands are identical, leading to one distinct arrangement. [PtCl_3(PMe_3)]^- can have two isomers as PMe_3 can be positioned in either of two adjacent or opposite sites. [PtCl_2(PMe_3)_2] can have cis- and trans-isomers depending on the positioning of PMe_3 ligands with respect to each other. [PtCl(PMe_3)_3]^+ has one stereoisomer because once PMe_3 ligands dominate (3 out of 4 positions), the geometry restricts variability.

Key concepts

Trigonal Bipyramidal Geometry

In inorganic chemistry, understanding molecule geometry is essential. One common geometry is the trigonal bipyramidal shape. This structure is characteristic of certain central atoms bonded to five ligands, such as in the anion \([\text{SiF}_3\text{Me}_2]^−\). Here, Si is central, surrounded by three fluorine atoms and two methyl groups. A trigonal bipyramidal geometry has

• Two axial positions: one above and one below the central atom
• Three equatorial positions: forming a horizontal triangle

The geometry allows for dynamic arrangements of atoms, resulting in different stereoisomers. It's the specificity of which atoms similarly occupy these positions that determines stability. Understanding these nuances helps explain molecular behavior and stability.

Steric Hindrance

Steric hindrance is a key factor in determining which stereoisomers of a trigonal bipyramidal geometry are more stable. It's all about the spatial arrangement of atoms and how their sizes influence possible configurations. Smaller atoms can fit in constrained spaces more easily, reducing steric hindrance.For \([\text{SiF}_3\text{Me}_2]^−\), fluorine atoms are smaller and more electronegative than the bulkier methyl groups. Therefore,

• The F atoms prefer occupying axial positions, where there is more space available
• This minimizes steric clashes and maximizes stability

Methyl groups, being bulkier, are better suited to the equatorial positions where spatial competition is reduced. This arrangement aligns with the results seen through X-ray diffraction.

X-ray Diffraction

X-ray diffraction is a powerful tool used to determine the three-dimensional arrangement of atoms within compounds. By focusing X-rays on a crystalline material and observing the diffraction patterns, scientists gain insights into molecular geometry and atomic arrangements. In the case of \([\text{SiF}_3\text{Me}_2]^−\), X-ray diffraction studies were pivotal in identifying the preferred stereoisomer. They

• Revealed F atoms in the expansive axial positions, confirming reduced steric hindrance
• Grooved an understanding of electronic effects that stabilize this configuration

This technique thus confirms theoretical predictions about the most stable structural arrangements.

Square Planar Coordination

Square planar coordination is a common geometric arrangement in metal complexes, particularly those involving platinum. In this structure, the central metal atom, like platinum in our example, is surrounded by ligands positioned at the corners of a square. For the \([\text{PtCl}_n(\text{PMe}_3)_{4-n}]\) series, square planar coordination gives rise to different isomers:

• \([\text{PtCl}_4]^{2-}\): All identical ligands mean no stereoisomers
• \([\text{PtCl}_3(\text{PMe}_3)]^-\): The placement of non-identical ligands can result in two possible isomers
• \([\text{PtCl}_2(\text{PMe}_3)_2]\): Exhibits cis and trans forms, based on \text{PMe}_3's positioning
• \([\text{PtCl}(\text{PMe}_3)_3]^+\): Overwhelmed by \text{PMe}_3, resulting in only one isomer possibility

Understanding these variations helps in predicting reactivity and interaction patterns in platinum-based complexes.

Platinum Complexes

Platinum complexes, especially those with square planar coordination, are fascinating in inorganic chemistry due to their diverse applications and unique properties. The series \([\text{PtCl}_4]^{2-},[\text{PtCl}_3(\text{PMe}_3)]^-,[\text{PtCl}_2(\text{PMe}_3)_2],[\text{PtCl}(\text{PMe}_3)_3]^+\) showcases how the number of stereoisomers changes based on ligand variation.This is because:

• The number and type of ligands attached to the platinum influence geometry and stereochemistry
• The balance of electron donation and steric demands between \text{Cl} and \text{PMe}_3 influences complex behavior and stability
• Stereoisomers arise from the different possible arrangements of these ligands

Platinum complexes are pivotal not just for academia, but also for industries including pharmaceuticals and catalysts, thanks to these versatile properties.