Question

Which of the following will have three stereo-isomeric forms? (1) \(\left[\mathrm{Cr}\left(\mathrm{NO}_{3}\right)_{3}\left(\mathrm{NH}_{3}\right)_{3}\right]\) (2) \(\mathrm{K}_{3}\left[\mathrm{Co}\left(\mathrm{C}_{2} \mathrm{O}_{4}\right)_{3}\right]\) (3) \(\mathrm{K}_{3}\left[\mathrm{Co}\left(\mathrm{C}_{2} \mathrm{O}_{4}\right)_{2} \mathrm{Cl}_{2}\right]\) (4) \(\left[\mathrm{Co}\left(\mathrm{en}_{2}\right) \mathrm{ClBr}\right]\) (Here, en = ethylene diamine) (a) 1 and 2 (b) 1 and 3 (c) 1 and 4 (d) 3 and 4

Short answer

Option (d) 3 and 4 have three stereo-isomeric forms.

Step-by-step solution

Understanding Stereo-isomeric Forms

Stereo-isomers are compounds that have the same molecular formula but differ in the spatial arrangement of atoms. For a compound to have three stereo-isomeric forms, it generally needs to exhibit two enantiomeric (mirror image) forms and one meso form. This often happens in coordination complexes where there are chiral centers or particular geometric arrangements.

Analyze Compound (1)

The compound \( \left[\mathrm{Cr}\left(\mathrm{NO}_{3}\right)_{3}\left(\mathrm{NH}_{3}\right)_{3}\right] \) is a complex with three bidentate ligands (\(\mathrm{NO}_{3}^{-}\)) and three monodentate ligands (\(\mathrm{NH}_{3}\)), but it lacks the chiral centers necessary for multiple stereo-isomers. Thus, it will not have three stereo-isomeric forms.

Analyze Compound (2)

The compound \( \mathrm{K}_{3}\left[\mathrm{Co}\left(\mathrm{C}_{2}\mathrm{O}_{4}\right)_{3}\right] \) contains three bidentate oxalate ligands. Such a complex can form optical isomers due to its ability to create chiral centers, potentially generating pairs of enantiomers. However, it will not have a meso form due to the lack of symmetry required for a non-chiral, inactive structure. This results in only two isomeric forms.

Analyze Compound (3)

The compound \( \mathrm{K}_{3}\left[\mathrm{Co}\left(\mathrm{C}_{2}\mathrm{O}_{4}\right)_{2} \mathrm{Cl}_{2}\right] \) consists of two bidentate oxalate ligands and two chloride ions. This configuration can produce two enantiomeric forms (due to the chiral arrangement of the chlorides opposite the oxalates) and one meso form (which is symmetric and doesn't have a plane of chirality). Therefore, it can have three stereo-isomeric forms.

Analyze Compound (4)

The compound \( \left[\mathrm{Co}\left(\mathrm{en}_{2}\right) \mathrm{ClBr}\right] \) is a coordination complex with two bidentate ethylene diamine (en) ligands creating a chiral center. This can produce optical isomers as there are two forms with enantiomeric differences, and additionally, a meso form due to the rest of its structure allowing for such isomers.

Determine Correct Choice

From the analysis, compounds (3) and (4) each have the potential to form three stereo-isomeric forms: two enantiomers and one meso isomer. Therefore, the correct option is (d) 3 and 4.

Key concepts

Chiral Centers

Chiral centers are crucial components in stereochemistry, particularly when examining coordination compounds. A chiral center typically refers to an atom in a molecule, usually carbon, that is bonded to four different substituents, leading to non-superimposable mirror images, also known as enantiomers. However, in coordination chemistry, the focus can shift slightly since the central metal atom and surrounding ligands dictate chirality. These arrangements create spatial configurations that can or cannot be mirror images of each other.
In coordination complexes, the geometry around the metal can lead to several chiral arrangements, depending on how ligands are attached. For a coordination compound to have chiral centers, it must allow for arrangements that can not be superimposed on their mirror images. This property is essential in determining the compound's ability to form different stereochemical isomers, including meso compounds and enantiomers.

• Central metal atoms with uneven ligand arrangements often contribute to chirality.
• Bidentate ligands, which attach at two points, can create complex spatial configurations that lead to chiral centers.
• Identifying these centers is crucial for predicting the number and types of isomers possible for a compound.

Optical Isomers

Optical isomers, or stereoisomers that differ only in the way they polarize light, are another fascinating facet of coordination chemistry. These isomers, being mirror images of one another, do not superimpose, much like left and right hands cannot overlay perfectly. This non-superimposability is due to the presence of chiral centers within the molecule.
When light passes through a solution of optical isomers, each isomer will rotate the plane of the polarized light differently. This property is marked as either dextrorotatory (rotating light clockwise) or levorotatory (counterclockwise). Such optical activity is a definitive test for these isomers.

• Key in separating optical isomers is focusing on their interaction with polarized light.
• They arise due to the three-dimensional arrangements that cannot replicate each other entirely.
• In coordination complexes, asymmetric metal centers and ligand arrangements lead to optical isomers.

Enantiomers

Enantiomers are specific types of optical isomers, known as non-superimposable mirror images. They are the 'left hand' and 'right hand' versions of a compound, having identical physical and chemical properties in most aspects except in how they interact with polarized light and other chiral environments.
In coordination compounds, enantiomers arise when a complex has a lack of a plane of symmetry. This absence creates an environment where different spatial forms, enantiomer pairs, are possible. For example, if a coordination compound has bidentate ligands, the specific spatial arrangement can lead to the formation of enantiomeric isomers if the complex itself is chiral.

• Enantiomers exhibit similar strengths in ionic or covalent bond formation, solubility in a chiral environment, and similar melting and boiling points.
• They are distinct when interacting with optically active substances or agents.
• In pharmacology, enantiomers can have drastically different biological activities, highlighting their importance.

Meso Compounds

Meso compounds present an intriguing case in stereochemistry. Unlike enantiomers, meso compounds are achiral even though they may contain multiple chiral centers. This non-chiral behavior is due to the internal plane of symmetry that allows the compound to be superimposable on its mirror image.
Meso compounds do not exhibit optical activity because the internal arrangement cancels out what would have otherwise been chirality. In coordination chemistry, identifying a meso compound involves finding a configuration that negates optical activity due to the symmetric arrangement of ligands or atoms.

• Meso compounds will often break typical trends seen in optical activity due to their symmetric nature.
• Numerous chiral centers can exist within meso compounds, which interestingly results in a net optical inactivity.
• Despite complex structures, meso compounds represent an excellent example of balance between symmetry and chirality.