Question

How many stereoisomers are possible for each of the following (not all species possess isomers)? (a) Square planar \(\left[\mathrm{PdCl}_{2}\left(\mathrm{PPh}_{3}\right)_{2}\right] ;\) (b) octahedral \(\mathrm{WCl}_{3} \mathrm{F}_{3}\) (c) octahedral \(\mathrm{WCl}_{2} \mathrm{F}_{4} ;\) (d) \(\mathrm{CCl}_{3} \mathrm{H}\) (e) octahedral \(\left[\mathrm{Cr}\left(\mathrm{H}_{2} \mathrm{O}\right)_{5} \mathrm{Cl}\right]^{2+} ;(\mathrm{f}) \mathrm{BBrCl}_{2}\) \((\mathrm{g})\) trigonal bipyramidal \(\mathrm{PCl}_{3} \mathrm{F}_{2} ;\) (h) trigonal bipyramidal \(\mathrm{PClF}_{4}\). Draw structures of the isomers and give them distinguishing labels where appropriate.

Short answer

(a) 2; (b) 2; (c) 2; (d) 0; (e) 1; (f) 0; (g) 2; (h) 2 stereoisomers possible.

Step-by-step solution

Understand the Type of Complex

Before determining the number of stereoisomers for each species, first identify the geometry of each complex or molecule based on the coordination number and the ligands involved. This will form the basis for analyzing possible isomers.

Analyze (a) Square Planar [PdCl2(PPh3)2]

Square planar complexes can have cis-trans isomers if there are two different types of ligands. Given \[\text{[PdCl}_2\text{(PPh}_3\text{)}_2]\], Pd has a square planar geometry with two Cl and two PPh3 groups. They can either be across from each other forming a trans isomer, or adjacent forming a cis isomer. Thus, there are two stereoisomers: cis and trans.

Analyze (b) Octahedral WCl3F3

An octahedral complex \[\text{WCl}_3\text{F}_3\] can have facial (fac) and meridional (mer) isomers. In the fac-isomer, three identical ligands occupy one face of the octahedron. In the mer-isomer, they occupy a meridian plane. Two stereoisomers are possible: fac and mer.

Analyze (c) Octahedral WCl2F4

For the molecule \[\text{WCl}_2\text{F}_4\] in an octahedral arrangement, the two Cl atoms can either be adjacent to form a cis isomer or opposite each other to form a trans isomer. This results in two stereoisomers: cis and trans.

Analyze (d) CCl3H

CCl3H is a tetrahedral molecule around carbon with no stereogenic center, as all surrounding groups are not unique. Thus, there are no stereoisomers possible for this compound.

Analyze (e) Octahedral [Cr(H2O)5Cl]2+

The \[\text{[Cr(H}_2\text{O})_5\text{Cl]}^{2+}\] complex is octahedral. Since only one Cl- is present among five water molecules, no cis-trans isomerism is possible around the Cr center. Therefore, there is only one isomer.

Analyze (f) BBrCl2

BBrCl2 is a trigonal planar molecule around boron. Trigonal planar molecules with different ligands might have enantiomers if they have a chiral center, but BBrCl2 doesn't have such center. Thus, no stereoisomers exist.

Analyze (g) Trigonal Bipyramidal PCl3F2

In trigonal bipyramidal \[\text{PCl}_3\text{F}_2\], the two fluorines can either be positioned in axial-equatorial relation or directly opposite each other in an axial-axial relation. This leads to two isomers: axial-equatorial and axial-axial.

Analyze (h) Trigonal Bipyramidal PClF4

The \[\text{PClF}_4\] molecule has one Cl and four F in a trigonal bipyramidal shape. Here, placing the lone Cl in either an equatorial or an axial position can result in two stereoisomers, based on axial or equatorial positioning.

Key concepts

Square Planar Complexes

In square planar complexes, the metal center is surrounded by four ligands which form a square around it in the same plane. Some square planar complexes are capable of forming cis and trans isomers. The presence of two different types of ligands makes this isomerism possible. One common example of a square planar complex is \([\text{PdCl}_2\text{(PPh}_3\text{)}_2]\). In this particular complex, with two chlorine (Cl) and two triphenylphosphine (PPh3) ligands, the complex can organize itself into two configurations:

• **Cis-isomer:** where similar ligands are next to each other
• **Trans-isomer:** where similar ligands are opposite each other

This gives rise to the set of stereoisomers that are labeled cis and trans, based on ligand placement.

Octahedral Geometry

Octahedral complexes have a metal center surrounded by six ligands arranged at the corners of an octahedron. This type of geometry is common in transition metal complexes. Octahedral complexes such as \([\text{WCl}_3\text{F}_3]\) and \([\text{WCl}_2\text{F}_4]\) exhibit **fac** and **mer** isomerism or cis-trans isomerism respectively.

• **Facial (fac)-isomer:** occurs when three identical ligands are adjacent, forming a face of the octahedron.
• **Meridional (mer)-isomer:** occurs when the three identical ligands form an arc across the octahedron.
• **Cis-isomer:** occurs when two identical ligands share a common edge in the polyhedron.
• **Trans-isomer:** occurs when two identical ligands are directly across from each other.

In the case of \([\text{WCl}_2\text{F}_4]\), it can specifically show cis-trans isomerism when chloride ions are placed adjacent or opposite to each other.

Trigonal Bipyramidal Molecules

Trigonal bipyramidal molecules have five ligands; three are placed in an equatorial plane, and the remaining two are located axially. This geometry offers interesting stereoisomerism due to the special positions the ligands can occupy.
Consider the example of \([\text{PCl}_3\text{F}_2]\). The two fluorines can occupy different positions:

• **Axial-equatorial isomer:** where one fluorine is axial and the other is equatorial.
• **Axial-axial isomer:** both fluorines occupy the axial positions.

Another example is \([\text{PClF}_4]\), where the chlorine can be either in an axial or equatorial position, leading to different stereochemical forms. This arrangement causes variations in the molecule's symmetry, resulting in different observable isomers.