Problem 90 - Chemistry The Central Science In Si Units

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), (b) \(\left[\mathrm{Pt}(\mathrm{gly})_{2}\right]\) (square planar), (c) \(\left[\mathrm{Co}(\mathrm{gly})_{3}\right]\) (octahedral)? Sketch all possible isomers. Use the symbol 6 to represent the ligand.

Step-by-Step Solution

Answer

(a) No isomers. (b) No isomers. (c) Two isomers (fac and mer).

Step 1: Understanding the Structure of Complexes

The problem relates to coordination chemistry, where we determine the possible isomers of coordination complexes given a certain geometry. The glycine anion acts as a bidentate ligand, meaning it coordinates to a metal at two points. Let's examine each complex and determine their isomeric forms based on their specified geometry.

Step 2: Determining Isomers for Tetrahedral Complex \([\mathrm{Zn}(\mathrm{gly})_2]\)

For a tetrahedral complex like \([\mathrm{Zn}(\mathrm{gly})_2]\), isomerism is not possible generally because all positions around the metal center are identical. So, there are no geometrical isomers for this compound.

Step 3: Determining Isomers for Square Planar Complex \([\mathrm{Pt}(\mathrm{gly})_2]\)

In a square planar geometry, the ligands are arranged in a square around the central metal. With two identical bidentate ligands, there are no distinct positions to differentiate them between cis and trans. Therefore, there are no isomers for \([\mathrm{Pt}(\mathrm{gly})_2]\) either.

Step 4: Determining Isomers for Octahedral Complex \([\mathrm{Co}(\mathrm{gly})_3]\)

Octahedral complexes with three bidentate ligands can have two types of isomerism: facial (fac) and meridional (mer). - **Facial (fac) Isomer:** All three ligands coordinate to form a face of the octahedron.- **Meridional (mer) Isomer:** The ligands are meridionally disposed around the metal center, forming a three-plane slice through the center.Since \([\mathrm{Co}(\mathrm{gly})_3]\) is an octahedral complex, both fac and mer configurations are possible.

Key Concepts

Bidentate Ligand

A bidentate ligand is a molecule that can attach to a central metal atom or ion at two separate binding sites. This property increases the stability of the coordination complex. Glycine, when it loses a hydrogen ion, forms a negatively charged anion and acts as a bidentate ligand. It uses both its nitrogen atom and its oxygen atom to bind to the metal.

• "Bi" indicates two points of attachment.
• Enhances complex stability through chelation.
• Commonly found in biological and synthetic systems.

Understanding the characteristics of bidentate ligands is essential in predicting and explaining the behavior and properties of coordination complexes.

Geometrical Isomerism

Geometrical isomerism occurs when the same set of atoms can be placed in different spatial arrangements around a central atom, leading to different physical and chemical properties. Not all geometries support geometrical isomerism.

• Common in square planar and octahedral complexes.
• No significant geometrical isomerism in tetrahedral complexes.

Geometrical isomerism helps chemists understand variations in reactivity and interaction with other molecules, important for both biological systems and industrial applications.

Tetrahedral Complex

A tetrahedral complex involves a central metal atom surrounded by four ligands at the corners of a tetrahedron. This geometry is prevalent in coordination chemistry due to its simplicity and commonality.

• Ligands are symmetrically arranged.
• No geometrical isomerism due to the equal positioning of ligands.
• Common with time variable complexes.

In the example of \([Zn(gly)_2]\), the symmetry ensures no isomerism, making it straightforward yet crucial in conceptual learning.

Square Planar Complex

A square planar complex geometry involves four ligands arranged at the corners of a square plane around a central metal atom. This structure is particularly prevalent in d8 metal centers like Pt(II).

• Planar arrangement of ligands.
• Typically no geometrical isomers with two identical bidentate ligands.
• Relevance in medicinal chemistry for drug complexes.

In \([Pt(gly)_2]\), with identical ligands, no cis/trans differentiation is possible, highlighting a unique aspect of square planar complexes.

Octahedral Complex

An octahedral complex structure is where a central atom is surrounded by six ligands equally spaced around it, forming an octahedron. This is the most common geometry in coordination compounds.

• Can exhibit both facial and meridional isomerism.
• Diverse configurations lead to interesting reactivity.
• Facial (fac) and meridional (mer) configurations provide different interaction potentials.

In the \([Co(gly)_3]\) complex, both fac and mer isomers are possible, providing variety and expanding the study of isomer effects on properties.