Question

How many EDTA (ethylenediamine-tetraacetic acid) molecules are required to make an octahedral complex with a \(\mathrm{Ca}^{2+}\) ion? [2006] (a) \(\operatorname{six}\) (b) three (c) one (d) two

Short answer

One EDTA molecule is needed.

Step-by-step solution

Understanding the Problem

The problem requires us to determine the number of EDTA molecules needed to form an octahedral complex with a \( \text{Ca}^{2+} \) ion. An octahedral complex means there are six coordination sites around the metal ion.

Structuring EDTA

EDTA is a hexadentate ligand, meaning it can form up to six bonds with a metal ion at once. This makes it capable of occupying all six coordination sites in one complex.

Forming an Octahedral Complex

Since EDTA can form six bonds, just one EDTA molecule can fully coordinate with a \( \text{Ca}^{2+} \) ion, forming an octahedron alone.

Conclusion

The number of EDTA molecules required is one, as a single EDTA molecule can occupy all six coordination sites in the octahedral complex.

Key concepts

EDTA

EDTA, short for ethylenediamine-tetraacetic acid, is a well-known example of a chelating agent used in coordination chemistry. This organic compound is equipped to form multiple bonds with a metal ion. This capability arises from its multiple donor atoms—nitrogen and oxygen—which are situated in a manner to effectively latch onto metal ions.

EDTA is widely utilized in various applications such as reducing metal ion concentrations, water softening, and preserving the stability of cosmetics and pharmaceuticals. In coordination chemistry, EDTA's role is significant as it can fully envelop a metal ion. This full coordination is crucial in creating stable complex structures with metals, such as calcium or lead.

To remember, when you come across EDTA in coordination chemistry, think of it as a multi-armed molecule capable of grabbing a metal ion in a bear hug.

Hexadentate Ligand

The term 'hexadentate' is derived from the Latin 'hexa,' meaning six, and 'dentate,' which refers to teeth. In the context of ligands, it refers to those that can form six covalent bonds with a single metal ion simultaneously. EDTA, as a hexadentate ligand, forms these six bonds through its four carboxyl groups and two amine groups.

This ability to coordinate through six distinctive points makes hexadentate ligands incredibly efficient at forming complexes, as they can satisfy all of the coordination sites on a metal ion by themselves. This often leads to the formation of highly stable complexes, as the ligand encapsulates the metal like a cage.

Understanding hexadentate ligands is important as they play a critical role in immobilizing metal ions, which prevents those ions from participating in unwanted reactions or biological activities.

Octahedral Complex

In coordination chemistry, an octahedral complex forms when six ligands bind symmetrically around a central metal ion. The geometric arrangement resembles an octahedron, with the metal ion at the center and each ligand located at one of the six vertices.

This arrangement allows for strong and efficient metal-ligand bonding, maximizing the stability of the complex. Octahedral complexes are common in transition metal chemistry and can be formed with a variety of ligands, from monodentate ligands like water to polydentate ligands like EDTA.

Octahedral geometry is important to understand because it represents one of the most straightforward and prevalent shapes in coordination chemistry, providing insights into both bonding patterns and molecular geometry.

Coordination Number

The coordination number is a fundamental concept in coordination chemistry, referring to the number of ligand donor atoms attached to the central metal ion. In simpler terms, it's a count of how many bonds are "biting" the metal center at any given time.

In an octahedral complex, the coordination number is typically six, reflecting the six ligands that surround the metal ion. Different metals and different ligands can result in varying coordination numbers, offering a diversity of complex shapes and stabilities.

By understanding coordination numbers, students can predict the likely structure and stability of a given metal complex. It allows chemists to engineer specific reactions and products, especially when designing catalysts or studying biological systems.