Question

The amino acid glycine \(\left(\mathrm{NH}_{2} \mathrm{CH}_{2} \mathrm{CO}_{2} \mathrm{H}\right.,\) denoted Hgly) binds as an anion and is a bidentate ligand. Draw and name all possible isomers of \(\left[\mathrm{Co}(\mathrm{gly})_{3}\right]\) How many isomers are possible for the compound $$\left[\mathrm{Co}(\mathrm{gly})_{2} \mathrm{Cl}\left(\mathrm{NH}_{3}\right)\right]\left[\text {Hint:} \mathrm{NH}_{2} \mathrm{CH}_{2} \mathrm{CO}_{2}^{-}\text {is }\right.$$ the glycinate anion.]

Short answer

For the compound \(\left[\mathrm{Co}(\mathrm{gly})_{3}\right]\), two isomers can be drawn, whereas for the compound \(\left[\mathrm{Co}(\mathrm{gly})_{2} \mathrm{Cl}\left(\mathrm{NH}_{3}\right)\right]\), four isomers can be drawn.

Step-by-step solution

Understanding Bidentate Ligand

Bidentate ligands can bind to a central metal atom or ion through two pairs of donor electrons. In this case, the glycinate anion is the bidentate ligand which binds once through the nitrogen atom in the amine group (NH2) and once through the oxygen atom in the carboxyl group (CO2). Since the anion can attach to the cobalt in two different ways, it will form different isomers.

Drawing Isomers for \( \left[\mathrm{Co}(\mathrm{gly})_{3}\right]\)

Because the glycinate anion is a bidentate ligand, it can attach to the cobalt atom in two different ways: (a) via the nitrogen of the amine group, and (b) via the oxygen of the carboxyl group. Although these arrangements may appear similar, they are considered different because changing the attachment point from nitrogen to oxygen or vice versa would involve breaking and forming of bonds, not merely a simple rotation. Hence, two isomers are formed: one where all three glycinate anions are attached via the nitrogen atom (amine group), and another where these anions are attached via the oxygen atom (carboxyl group). Because there’s no other ligand attached to the central metal atom, so no cis-trans isomerism will be observed.

Drawing Isomers for \(\left[\mathrm{Co}(\mathrm{gly})_{2} \mathrm{Cl}\left(\mathrm{NH}_{3}\right)\right]\)

The \(\left[\mathrm{Co}(\mathrm{gly})_{2} \mathrm{Cl}\left(\mathrm{NH}_{3}\right)\right]\) complex has multiple types of ligands, so cis-trans (geometric) isomerism is possible. Cis-isomers occur when similar ligands are adjacent to each other, and trans-isomers occur when similar ligands are opposite to each other. Here, cis-isomers could occur when the two glycine ligands are adjacent to each other, and the Cl and NH3 ligands are opposite each other. Alternatively, if the two glycine units are opposite each other (trans configuration), it would also lead to a different unique isomer. In addition, we have two more optical isomers where one glycinate molecule attaches at nitrogen and the other at oxygen, which are enantiomers of each other. So a total of four isomers can be drawn.

Key concepts

Bidentate Ligand

A bidentate ligand, such as the glycinate anion in the given exercise, has the unique ability to form two coordinate bonds with a central metal atom or ion. These ligands possess two sets of donating atoms, which in the case of glycinate are the nitrogen in the amine group and the oxygen in the carboxyl group.

Understanding bidentate ligands is crucial because they introduce complexity into the structure of coordination compounds. Specifically, they allow for the possibility of chelation, where the ligand forms a 'ring' of sorts with the central metal atom. This ring structure adds stability to the compound and can also contribute to the formation of isomers. It's essential for students to recognize the two bonding sites of bidentate ligands because this directly influences the number and types of isomers that can exist for any given coordination compound.

Geometric Isomerism

Geometric isomerism in coordination chemistry is akin to taking a group photo with friends. Just as the arrangement of people in the photo can vary, ligands around a central metal ion can arrange in different spatial configurations resulting in distinct isomers.

In coordination compounds, geometric isomerism often presents itself as cis and trans isomers.

Cis-isomers

have identical or similar ligands on the same side of the central metal ion, while

trans-isomers

have them on opposite sides. In the exercise's compound \(\left[\mathrm{Co}(\mathrm{gly})_{2} \mathrm{Cl}\left(\mathrm{NH}_{3}\right)\right]\), the opportunity for these isomers arises from the presence of different ligands bond to cobalt. This type of isomerism has profound effects on the chemical properties and biological activities of coordination compounds, making its understanding essential for students in fields ranging from medicinal chemistry to materials science.

Optical Isomerism

Optical isomerism occurs when a compound can exist in two non-superimposable mirror image forms, much like a pair of hands. These mirror image forms are called enantiomers and have the unique property of rotating plane-polarized light in opposite directions. This characteristic is pivotal in fields like drug development since the two enantiomers can have different biological activities.

In the context of the exercise, the coordination compound \(\left[\mathrm{Co}(\mathrm{gly})_{2} \mathrm{Cl}\left(\mathrm{NH}_{3}\right)\right]\) can exist as enantiomers due to the arrangement of the bidentate ligands. If one glycinate ligand binds through nitrogen and the other through oxygen, it creates a chiral center at cobalt, leading to optical isomers. It's essential for students to grasp that even slight changes in the orientation of ligands, especially bidentate ones, can lead to compounds with vastly different optical properties.

Coordination Chemistry

Coordination chemistry explores how central metal atoms or ions combine with ligands to form coordination compounds. These compounds are the cornerstone of many biological processes and are at the heart of catalyst design, metal extraction, and the field of supramolecular chemistry.

In understanding the exercise on the coordination compound involving cobalt and the bidentate ligand glycinate, students must recognize the significance of the coordination number, the types of ligands involved, and the spatial arrangement of these ligands around the central metal ion. It's pivotal to appreciate the versatility and complexity of these compounds, as they can show a variety of isomerisms and varying physical and chemical properties based on their structure. The understanding of coordination compounds extends beyond academic interest and has practical implications in designing new drugs, catalysts, and metal-containing materials.