Question

(a) Which of the following octahedral complexes are chiral: \(\operatorname{cis}-\left[\mathrm{CoCl}_{2}(\mathrm{en})_{2}\right]^{+},\left[\mathrm{Cr}(\mathrm{ox})_{3}\right]^{3-},\) trans \(\left[\mathrm{PtCl}_{2}(\mathrm{cn})_{2}\right]^{2+},\left[\mathrm{Ni}(\mathrm{phen})_{3}\right]^{2+},\left[\mathrm{RuBr}_{4}(\mathrm{phen})\right]^{-},\) cis- \(\left[\mathrm{RuCl}(\mathrm{py})(\mathrm{phen})_{2}\right]^{+} ?\) (b) The solution \(^{31}\) P NMR spectrum of a mixture of isomers of the square planar complex \(\left[\mathrm{Pt}(\mathrm{SCN})_{2}\left(\mathrm{Ph}_{2} \mathrm{PCH}_{2} \mathrm{PPh}_{2}\right)\right]\) shows one broad signal at \(298 \mathrm{K} .\) At \(228 \mathrm{K},\) two singlets and two doublets \((J=82 \mathrm{Hz})\) are observed and the relative integrals of these signals are solvent-dependent. Draw the structures of the possible isomers of \(\left[\mathrm{Pt}(\mathrm{SCN})_{2}\left(\mathrm{Ph}_{2} \mathrm{PCH}_{2} \mathrm{PPh}_{2}\right)\right]\) and rationalize the \(\mathrm{NMR}\) spectroscopic data.

Short answer

(a) Chiral complexes: \( cis-\left[\mathrm{CoCl}_{2}(\mathrm{en})_{2}\right]^{+}, \left[\mathrm{Cr}(\mathrm{ox})_{3}\right]^{3-}, \left[\mathrm{Ni}(\mathrm{phen})_{3}\right]^{2+}.\) (b) Square planar isomers: cis and trans forms, differentiated by their \(^{31}\)P NMR data.

Step-by-step solution

Analyze Chirality of Octahedral Complexes

Chirality in octahedral complexes depends on the symmetry of their configuration. The complexes that can exist as non-superimposable mirror images are chiral. For example, \( cis-\left[\mathrm{CoCl}_{2}(\mathrm{en})_{2}\right]^{+} \) is chiral because the presence of \(\mathrm{en}\) (ethylenediamine) ligands introduces chirality by creating non-superimposable enantiomers. In contrast, \(trans\) complexes or complexes with high symmetry do not exhibit chirality.

Identify Possible Isomers in Square Planar Complex

Square planar complexes can have geometric isomers due to different possible ligand arrangements. The complex \(\left[\mathrm{Pt}(\mathrm{SCN})_{2}\left(\mathrm{Ph}_{2} \mathrm{PCH}_{2} \mathrm{PPh}_{2}\right)\right]\) can exist in two dominant isomeric forms: cis or trans. These isomers differ by the relative positions of the \(\mathrm{SCN}\) ligands.

Interpret \(^{31}\)P NMR Spectroscopic Data

The \(^{31}\)P NMR spectrum at 298 K shows one broad signal, indicating a dynamic interchange between isomers that is fast on the NMR timescale. At 228 K, the slower dynamic process reveals distinct signals for each isomer: two singlets correspond to different phosphorus environments in their respective isomers, and two doublets result from coupling interactions (\(J=82 \mathrm{Hz} \)) between phosphorus atoms. Solvent-dependent integrals suggest the relative population of isomers varies with solvent.

Key concepts

Octahedral Complexes

Octahedral complexes are fascinating structures that arise in coordination chemistry, where a central metal ion is surrounded symmetrically by six ligands forming an octahedron. These ligands can be ions or neutral molecules that donate a pair of electrons to the metal ion. Chirality in these complexes is determined by their symmetry. If they can exist as non-superimposable mirror images, they are chiral, meaning they have enantiomers. An example of this is the complex

• cis- \( \left[ \mathrm{CoCl}_{2}(\mathrm{en})_{2} \right]^{+} \)

Here, ethylenediamine (\(\mathrm{en}\)) ligands contribute to its chirality. On the other hand, complexes with symmetrical arrangements, such as trans configurations, do not exhibit chirality, as they cannot have distinct mirror images. Recognizing the spatial arrangement and considering the symmetry of the ligands are crucial in determining the chirality of octahedral complexes.

NMR Spectroscopy

Nuclear Magnetic Resonance (NMR) Spectroscopy is a powerful technique used for determining the structure of compounds. In the context of coordination chemistry, \(^{31}\mathrm{P}\) NMR spectroscopy can provide insights into the structure and dynamics of phosphorus-containing ligands in complexes. The key to interpreting the NMR data is understanding the electronic environment surrounding the nuclei.In the exercise, we see different behaviors at two temperatures:

• At 298 K, a single broad signal indicates rapid isomer interconversion, meaning the isomers are exchanging positions quickly.
• At 228 K, the process slows down, distinguishing individual isomer signals, observed as two singlets and two doublets.

The number of signals and their splitting pattern arise from the different environments of the phosphorus atoms in each possible isomer. The coupling constant \(J=82 \mathrm{Hz}\) offers additional details about the interaction between phosphorus atoms present, such as how they are coupled or split. Additionally, solvent-dependent integrals inform us about how different isomer populations might vary with the polarity or environment of the solvents. Interpreting these facets of the NMR spectrum allows chemists to deduce the different isomers present and their proportions.

Square Planar Complexes

Square planar complexes are another important class of coordination compounds. In these complexes, the metal and four ligands occupy the corners of a square plane. Reactions and properties of square planar complexes often differ from other geometries due to their unique arrangement. One notable aspect is their ability to form isomers based on ligand positions.There are two basic types of geometric isomers:

• **Cis**: Ligands are adjacent, forming one corner of the square.
• **Trans**: Ligands are opposite each other, forming linear sides of the square.

For example, the complex\(\left[ \mathrm{Pt} (\mathrm{SCN})_{2} (\mathrm{Ph}_{2} \mathrm{PCH}_{2} \mathrm{PPh}_{2}) \right]\)can form both cis and trans isomers. These configurations have consequences on the chemical and physical properties, influencing not only stability but also how these complexes interact in chemical reactions. Often, geometric isomer differences can be subtle but significant, especially in catalytic or biochemical applications. Understanding the spatial arrangement of ligands around the central metal can provide insights crucial for differentiating between these isomers.