Which of the following complex ions will not show optical activity? (a) \(\left[\mathrm{Co}(\mathrm{en})\left(\mathrm{NH}_{3}\right)_{2} \mathrm{Cl}_{2}\right]^{+}\) (b) \(\left[\mathrm{Cr}\left(\mathrm{NH}_{3}\right)_{4} \mathrm{Cl}_{2}\right]^{+}\) (c) \(\left[\mathrm{Pt}(\mathrm{Br})(\mathrm{Cl})(\mathrm{I})\left(\mathrm{NO}_{2}\right)(\mathrm{Py}) \mathrm{NH}_{3}\right]\) (d) cis-[Co (en) \(\left._{2} \mathrm{Cl}_{2}\right]^{+}\)

Chirality is a fundamental concept in stereochemistry, where the spatial arrangement of atoms in certain molecules or ions imbues them with unique properties. A molecule or ion is said to be chiral if it cannot be superimposed on its mirror image, similar to how your left hand is not superimposable on your right hand. Complex ions, which form part of coordination compounds, can exhibit chirality when their ligands are arranged in a non-superimposable, asymmetric manner around the central metal ion.

Understanding chirality is crucial, particularly in coordination chemistry, as it determines the optical activity of compounds. In solutions, chiral molecules rotate the plane of polarised light, a property measured by a polarimeter. The degree of rotation can vary based on the structure of the chiral molecule or ion and is a distinguishing feature for many biological molecules, pharmaceuticals, and synthesized chemical compounds.

When discussing optical activity and chirality, it is important to consider the concept of superimposable mirror images, or enantiomers. If a molecule or its complex ion has a non-superimposable mirror image, it exhibits optical isomerism, where each enantiomer will rotate polarized light to an equal degree but in opposite directions (known as dextrorotatory (+) or levorotatory (-)).

To visually identify potential enantiomers, one must inspect the spatial arrangement of atoms or ligands. If rotating or flipping one structure cannot make it look identical to its mirror image, then the molecule or ion is chiral, and these non-superimposable forms are enantiomers. However, if the structure can be superimposed over its mirror image, it is considered achiral, and optical activity is not present.

Preparation for the National Eligibility cum Entrance Test (NEET) requires a thorough understanding of coordination chemistry, which includes topics such as coordination compounds, their nomenclature, isomerism, and the bonding in such compounds. Optical activity in complex ions is commonly tested, as it requires integrating knowledge of stereochemistry with the intricacies of coordination bonds.

Prospective medical students need to recognize how the spatial arrangement and types of ligands surrounding a central metal ion can influence the optical activity of a coordination complex. Understanding the principles of chirality and the conditions under which a molecule may exhibit optical activity can aid students in answering questions accurately, as well as in grasping the broader implications in pharmaceutical sciences, where enantiomers can have dramatically different biological effects.

Isomerism is the phenomenon where two or more compounds have the same formula but different structures or spatial orientations of atoms, leading to different properties. In coordination compounds, two primary types of isomerism are observed: structural isomerism and stereoisomerism, which includes geometrical and optical isomerism.

Geometrical isomers differ in the arrangement of ligands around the central atom, often resulting in 'cis' and 'trans' forms. Optical isomers, on the other hand, pertain to the non-superimposability of molecules on their mirror images, as seen with 'chiral' coordination complexes. Isomerism in coordination compounds is an essential topic in chemistry that not only explains the variations in physical and chemical properties but also provides insight into the reactivity and functionality of these complex ions in biological systems and industrial applications.