Question

Of the following complex ions, the one that is optically active is (a) \(\operatorname{cis}-\left[\operatorname{CoCl}_{2}(\text { en })_{2}\right]^{+} ;\) (b) \(\left[\operatorname{CoCl}_{2}\left(\mathrm{NH}_{3}\right)_{4}\right]^{+}\) (c) \(\left[\mathrm{CoCl}_{4}\left(\mathrm{NH}_{3}\right)_{2}\right]^{-} ;(\mathrm{d})\left[\mathrm{CuCl}_{4}\right]^{-}\).

Short answer

Among the given complex ions, only the ion (c) \(\left[\operatorname{CoCl}_4(\operatorname{NH}_3)_2\right]^-\) is optically active.

Step-by-step solution

Analysis of Complex ion (a)

Consider \(\operatorname{cis}-[\operatorname{CoCl}_2(\operatorname{en})_2]^+\), the structure of this complex ion is that of a square planar, with two en (ethylenediamine) and two Cl- ions surrounding the Co ion. In the cis isomer, the same type of ligands are adjacent to each other. This makes the ion symmetrical and hence it is optically inactive.

Analysis of Complex ion (b)

Consider \(\left[\operatorname{CoCl}_2(\operatorname{NH}_3)_4\right]^+\), the structure of this complex ion is that of an octahedral, with four ammonia and two Cl- ions surrounding the Co ion. Again, it is a symmetric structure and thus is optically inactive.

Analysis of Complex ion (c)

Investigating the ion \(\left[\operatorname{CoCl}_4(\operatorname{NH}_3)_2\right]^-\), this also has an octahedral structure with four chloride ions and two ammonia ions around the central Co ion. An asymmetric structure forms as the two different types of ligands (NH3 and Cl-) do not all occupy equivalent positions. This makes the ion optically active.

Analysis of Complex ion (d)

Finally, looking at \(\left[\operatorname{CuCl}_4\right]^-\), here the tetrahedral shape is formed with four identical ligands (Cl-) surrounding the Cu ion. Therefore, it is symmetrical and does not have the characteristic of chiral structure, hence optically inactive.

Key concepts

Coordination Chemistry

Coordination chemistry is a branch of chemistry that deals with the study of compounds formed between metal ions and other molecules, termed as 'ligands', that can donate pairs of electrons. Coordination compounds, also known as complex compounds, consist of a central metal atom or ion that is surrounded by a number of ligands.

Ligands are usually organic or inorganic ions or molecules that bind to the central atom by donating electron pairs to form coordination bonds. The number of ligands attached to the central metal is called the coordination number, which can vary typically from two to six or even higher, depending on the metal and the size of the ligands.

The resulting coordination compounds can exhibit a wide variety of geometries, such as linear, square planar, tetrahedral, and octahedral. These geometries significantly impact the physical and chemical properties of the compound, making coordination chemistry a key topic in the creation of new materials, catalysis, and the understanding of biological processes involving metal ions.

Chirality in Complexes

Chirality in complexes refers to the property of a complex having a non-superimposable mirror image. This means that, just as with hands, there exists a left- and a right-handed version of the complex, known as enantiomers. For a complex to be chiral and thus, optically active, it must lack an internal plane of symmetry and cannot be superimposed on its mirror image.

The presence of chirality in a coordination complex can arise from several scenarios, such as having different ligands around the central metal in a specific arrangement. Chiral complexes are of great interest not only in coordination chemistry but also in pharmacology and biochemistry, where the chirality of a molecule can dramatically affect its biological activity.

A classic example of a chiral ligand is ethylenediamine (en), which can produce chiral complexes when bound to a central metal ion in a certain manner. However, chirality is not only determined by the ligand but also by the overall geometry and configuration of the complex.

Geometric Isomerism

Geometric isomerism, also known as configurational isomerism, is a type of stereoisomerism where isomers have the same connectivity between atoms but different spatial arrangements of the atoms. In coordination complexes, this primarily arises in square planar and octahedral complexes, where ligands can be arranged differently around the central metal ion.

In square planar complexes, geometric isomerism can be observed as cis and trans isomers, where cis indicates that identical ligands are adjacent to each other, while trans indicates they are opposite to each other. For octahedral complexes, more possibilities arise such as cis, trans, fac (facial), and mer (meridional) isomerism.

Isomers can have significantly different chemical and physical properties, including color, reactivity, and biological activity. Understanding geometric isomerism is crucial for synthesizing specific compounds with desired properties and elucidating the behavior of complex ions in various chemical environments.