Question

Many trace metal ions exist in the blood complexed with amino acids or small peptides. The anion of the amine acid glycine (gly). NCC(=O)[O-] can act as a bidentate ligand, coordinating to the metal through nitrogen and oxygen atoms. How many isomers are possible for (a) $[\mathrm{Zn}(\mathrm{gly}{)}_2]$ (tetrahedral), (b) [ $\mathrm{Pt}(\mathrm{gly}{)}_2]$ (square planar), (c) [Co(gly) 3$]$ (octahedral)? Sketch all possible isomers. Use the symbol to represent the ligand.

Short answer

In summary, the complexes $[\mathrm{Zn}(\mathrm{gly}{)}_2]$, $[\mathrm{Pt}(\mathrm{gly}{)}_2]$, and $[\mathrm{Co}(\mathrm{gly}{)}_3]$ have 1, 2, and 2 possible isomers respectively. For the tetrahedral complex $[\mathrm{Zn}(\mathrm{gly}{)}_2]$, there is only 1 symmetric isomer. For the square planar complex $[\mathrm{Pt}(\mathrm{gly}{)}_2]$, there are 2 isomers: cis and trans. Finally, for the octahedral complex $[\mathrm{Co}(\mathrm{gly}{)}_3]$, there are 2 isomers: fac (facial) and mer (meridional).

Step-by-step solution

Isomers of Tetrahedral $[\mathrm{Zn}(\mathrm{gly}{)}_2]$ (a)

As the coordination number is 4 (2 glycine ligands coordinating with 2 sites each), and the complex is tetrahedral, there is only 1 possible isomer. It is symmetric and no other geometries can be formed because the ligands are identical. Sketch of the isomer: Here, Zn is the central atom, and each glycine ligand is coordinating through its nitrogen and oxygen atoms. ``` O \ C / N--CH2-C-O-ZN \ C / O ```

Isomers of Square Planar $[\mathrm{Pt}(\mathrm{gly}{)}_2]$ (b)

In the case of a square planar complex, we have two possible isomers: cis and trans. The cis-isomer has both glycine ligands adjacent to each other, while the trans-isomer has glycine ligands opposite each other. Sketch of isomers: Cis-isomer: ``` O / C / N---CH2-C-O--Pt \ C \ O ``` Trans-isomer: ``` O / C / N---CH2-C-O--Pt--O-C--CH2-N ```

Isomers of Octahedral $[\mathrm{Co}(\mathrm{gly}{)}_3]$ (c)

In the case of an octahedral complex with tris-glycine ligands, we have two possible isomers: fac (facial) and mer (meridional). In the fac-isomer, all three glycine ligands occupy the three coordination sites that form a face of the octahedral geometry. In mer-isomer, each glycine ligand occupies adjoining coordination sites that form an equator around the central metal ion. Sketch of the isomers: Fac-isomer: ``` O \ C \ N---CH2-C-O--Co / C / O O \ C / N---CH2-C-O O / C / N---CH2-C ``` Mer-isomer: ``` O / C / N---CH2-C-O--Co--O-C---CH2-N \ C \ O \ N---CH2-C / C / O ``` In conclusion, the numbers of possible isomers for complexes (a), (b), and (c) are 1, 2, and 2, respectively.

Key concepts

Trace Metal Ions in Blood

Trace metal ions play critical roles in various biological processes within our bodies. These ions, such as iron, copper, zinc, and cobalt, assist in enzyme activity, electron transport, and oxygen transportation. In the blood, they often exist in a complex form with amino acids or small peptides to stabilize them and to facilitate their biological functions.

For example, iron is typically found in the form of hemoglobin, a complex that carries oxygen in the blood. The precise balance of these trace metals is vital for health, as imbalances can lead to multiple disorders and diseases. Understanding the coordination chemistry of these ions, including how they interact with ligands like glycine, is essential in biochemistry and medicine.

Bidentate Ligand Glycine

• A bidentate ligand is a molecule that forms two bonds with a central metal ion.
• Glycine, the simplest amino acid, acts as a bidentate ligand by coordinating through its amine nitrogen and carboxylate oxygen atoms.
• The ability of glycine to form two bonds with a metal ion makes it an excellent stabilizer for metal complexes in biological systems, such as those found in blood.

The dual bonding sites of bidentate ligands like glycine can significantly influence the structure and isomerism of the metal complexes they form. These interactions are crucial for the functionality of the complexes in biological systems.

Coordination Number

The coordination number of a central metal ion in a complex refers to the number of ligand attachment points to the metal. It's a significant factor that dictates the geometry and possible isomers of the metal complex.

For instance, a coordination number of four can lead to a tetrahedral or square planar structure, whereas six coordination points typically result in an octahedral complex. These structural frameworks are foundational for understanding the properties and reactivity of coordination compounds in various fields, including medicinal chemistry and metalloenzymology.

Tetrahedral Complex

In a tetrahedral complex, the central metal ion is bonded to four ligands positioned at the corners of a tetrahedron. This type of geometry affords no distinction between ligand positions for a symmetrical tetrahedral complex containing identical ligands, resulting in only one possible isomer.

The significance of tetrahedral geometry lies in its prevalence in bioinorganic molecules, such as zinc enzymes, where it is often critical for catalytic activity. The tetrahedral structure provides a specific spatial arrangement essential for the biological function of these enzymes.

Square Planar Complex

The square planar complex is typified by four ligands symmetrically arranged around a central metal ion at the corners of a square plane. This arrangement leads to distinct positional relationships between ligands, enabling the formation of cis and trans isomers.

The positional isomerism in square planar complexes is a key concept in coordination chemistry and has implications for the activity and properties of metal-containing drugs, such as certain anticancer agents where the alteration of ligand positions can significantly affect therapeutic efficacy.

Octahedral Complex

An octahedral complex features six ligands symmetrically arranged around a central metal ion, occupying the corners of an octahedron. This geometry can give rise to several isomer types, including facial (fac) and meridional (mer) isomers, depending on the relative positions of the ligands.

The two types of isomers arise because ligands can either occupy adjacent faces or spread out across the center in an octahedral complex. The octahedral geometry is crucial in bioinorganic chemistry, particularly for metal ions with a coordination number of six, which includes many metalloproteins and metalloenzymes.

Cis and Trans Isomers

Cis and trans isomers are types of geometrical isomers that can occur in coordination compounds with square planar and octahedral geometries. In cis isomers, two identical ligands are adjacent to each other, while in trans isomers, they are opposite one another.

These differences in ligand positions can significantly alter the physical and chemical properties of the compound. Such isomerism is especially important in the pharmaceutical industry, as it can affect how a drug interacts within biological systems. A prime example is the drug cisplatin, used in cancer therapy, where the 'cis' geometry is critical for its activity.

Facial and Meridional Isomers

Facial (fac) and meridional (mer) isomers are specific to octahedral complexes where three bidentate ligands are involved. The fac isomer has all three ligands orienting towards the same face of the octahedron, forming a triangle. In contrast, the mer isomer presents the ligands around the central ion like a meridian, with each ligand spanning from the top to the opposite bottom edge of the octahedron.

This isomerism is not just an academic interest; it can influence the reactivity and biological activity of coordination compounds. Understanding this concept is important in the synthesis of chiral compounds and the development of coordination compounds in medicinal chemistry.