Question

Many trace metal ions exist in the blood complexed with amino acids or small peptides. The anion of the amino acid glycine (gly), can act as a bidentate ligand, coordinating to the metal through nitrogen and oxygen atoms. How many isomers are possible for (a) \(\left[\mathrm{Zn}(\mathrm{gly})_{2}\right]\) (tetrahedral), \((\mathbf{b})[\mathrm{Pt}(\mathrm{g}] \mathrm{y})_{2} ]\) (square planar), \((\mathbf{c})\left[\operatorname{Cog}(\mathrm{gly})_{3}\right](\) octahedral)? Sketch all possible isomers. Use the symbol to represent the ligand.

Short answer

For the given complexes: (a) The tetrahedral complex \(\left[\mathrm{Zn}(\mathrm{gly})_{2}\right]\) has only 1 possible isomer: ``` N | O--Zn--O | N ``` (b) The square planar complex \([\mathrm{Pt}(\mathrm{gly})_{2}]\) has 2 possible isomers: cis and trans. (c) The octahedral complex \(\left[\mathrm{Co}(\mathrm{gly})_{3}\right]\) has only 1 possible isomer: ``` N | O--Co--O | | N--O N | | O--Co--O | N ```

Step-by-step solution

(a) Tetrahedral Complex: \(\left[\mathrm{Zn}(\mathrm{gly})_{2}\right]\)

Since glycine is a bidentate ligand, it will occupy two coordination sites on the metal ion. There are four coordination sites in total for a tetrahedral complex. Therefore, we can only fit one glycine ligand within the complex, making it \(\left[\mathrm{Zn}(\mathrm{gly})\right]\). The \(\left[\mathrm{Zn}(\mathrm{gly})\right]\) complex has only one possible isomer because the ligands are symmetric, and all coordination sites will end up being occupied by a ligand. Therefore, there is just one isomer possible for \(\left[\mathrm{Zn}(\mathrm{gly})_{2}\right]\) as shown below: ``` N | O--Zn--O | N ```

(b) Square Planar Complex: \([\mathrm{Pt}(\mathrm{gly})_{2}]\)

For a square planar complex, we have four coordination sites on the metal ion. Since glycine acts as a bidentate ligand, taking up two sites, only two glycine ligands can be attached to the metal ion. There are two possible arrangements for the glycine ligands: 1. The first isomer's nitrogen and oxygen atoms are at adjacent positions (cis): ``` N | O--Pt--O | N--O ``` 2. The second isomer's nitrogen and oxygen atoms are opposite each other (trans): ``` N N | | O--Pt--O or O--Pt--O | | N N ``` Hence, there are two possible isomers for the square planar complex \([\mathrm{Pt}(\mathrm{gly})_{2}]\).

(c) Octahedral Complex: \(\left[\mathrm{Co}(\mathrm{gly})_{3}\right]\)

For an octahedral complex, there are six coordination sites on the metal ion. Since glycine acts as a bidentate ligand, only three glycine ligands will be attached to the metal ion. In this case, all three glycine ligands must be connected, and the only possible arrangement is: ``` N | O--Co--O | | N--O N | | O--Co--O | N ``` There is only one possible isomer for the octahedral complex \(\left[\mathrm{Co}(\mathrm{gly})_{3}\right]\).

Key concepts

Trace Metal Ions

Trace metal ions are essential components in various biological functions. They are present in very small amounts (often in the microgram range) but are crucial for processes like enzyme function, transport of oxygen, and regulation of metabolic pathways. In the context of coordination chemistry, these ions often form complexes with ligands to gain stability and functionality. For example, in the human bloodstream, trace metal ions such as Iron (Fe), Zinc (Zn), and Copper (Cu) can form complexes with amino acids or peptides. This is important because the formation of stable complexes helps in the transport and storage of these metals within the body, enhancing their bioavailability. Thus, understanding the role of trace metal ions in coordination chemistry helps bridge the gap between chemistry and biology, offering insights into their roles in health and disease.

Bidentate Ligands

A bidentate ligand is a type of ligand that can attach to a central metal atom at two points. The term "bidentate" comes from "bi" meaning two, and "dentate" meaning teeth or bite. Think of it as a molecule that bites onto the metal ion with two 'teeth'. An example is the amino acid glycine, which can bind through its nitrogen atom and one of its oxygen atoms (typically from the carboxylate group). These two points of attachment make bidentate ligands especially stable, due to the creation of a chelate ring when they form complexes with metal ions. This increased stability is known as the chelate effect, which is why bidentate ligands are often found in biological systems, such as enzymes or other metal-containing biomolecules.

Isomerism

Isomerism in coordination chemistry refers to the existence of compounds that have the same chemical formula but differ in the arrangement of atoms. This can lead to distinct physical and chemical properties. In coordination complexes, we often encounter two main types of isomerism: geometric isomerism and optical isomerism.

• Geometric isomerism occurs when ligands can assume different positions around the central metal ion, such as cis (adjacent) and trans (opposite) arrangements in square planar complexes like \([\text{Pt(gly)}_2]\).
• Optical isomerism arises when a complex is non-superimposable on its mirror image, much like left and right hands. However, this is rarer for complexes discussed here, such as octahedral complexes of cobalt.

Understanding isomerism is crucial as different isomers can exhibit vastly different biological activities, reactions, and solubility.

Tetrahedral Complexes

Tetrahedral complexes have a distinct shape where four ligands symmetrically surround a central metal ion. This complex resembles a three-dimensional pyramid and often does not exhibit geometric isomerism due to its symmetry. For instance, in the tetrahedral complex \([\text{Zn(gly)}_2]\), the bidentate ligand glycine occupies specific coordination sites symmetrically due to the tetrahedron's structure. This symmetry results in only one possible isomer in tetrahedral complexes, making them relatively simple compared to other geometric structures. Though simpler, they are energetically stable and can be found in various metal ion complexes across biological systems.

Square Planar Complexes

Square planar complexes consist of a central metal atom surrounded by ligands positioned at the corners of a square in a single plane. This geometry is common for complexes involving metals such as platinum (Pt), and allows for two types of geometric isomerism: cis and trans.

• In the cis configuration, similar ligands are adjacent to each other, leading to a potential for interactions between them.
• In the trans configuration, similar ligands are opposite each other, which can lead to distinct chemical behavior compared to the cis form.

Such isomer configurations heavily influence the chemical and biological activities of the complexes. In pharmacology, for example, the cis form of a drug can exhibit therapeutic effects, whereas the trans form might be inactive.

Octahedral Complexes

Octahedral complexes are characterized by six ligands symmetrically arranged around a central metal ion, forming an octahedral structure. With eight faces and twelve edges, this geometry provides a versatile structure that can hold a variety of ligands in different arrangements. In the complex \([\text{Co(gly)}_3]\), glycine, acting as a bidentate ligand, fills these positions efficiently, creating a stable arrangement. While octahedral complexes can have multiple isomers, in certain cases with symmetric ligands like glycine, only one configuration may be possible. This symmetry ensures a high level of stability and can be crucial in systems such as those involved in catalysis, signaling, and enzyme function, where specific spatial arrangements of atoms are needed for functionality.