Which will form the most stable complex?

\(C_2O_4^{2-}\)

The correct answer is option 4.\(C_2O_4^{2-}\).

Let us take a detailed look at why \(C_2O_4^{2-}\) (oxalate ion) forms the most stable complex compared to the other ligands.

Key Concepts:

Chelation: Ligands that can bind to a metal ion at more than one site form a chelate, which enhances the stability of the complex. This effect is called the chelate effect and results from the entropic advantage of binding a metal ion at multiple points with one ligand.

Ligand field strength: Some ligands create a stronger field around the metal ion, leading to greater stability by maximizing the crystal field stabilization energy (CFSE). This is crucial for stability but is often outweighed by the chelate effect.

Let us break down the ligands:

1. \(NCS^-\) (thiocyanate):

Monodentate ligand: Thiocyanate can only form one bond with a metal ion at a time.

Binding ability: It can bind either through nitrogen or sulfur, but this flexibility does not contribute significantly to stability because it's only bound at one site.

Field strength: It is a relatively weak field ligand and doesn’t provide much crystal field stabilization energy.

2. \(CN^-\) (cyanide):

Monodentate ligand: Cyanide binds to the metal through only one site (the carbon atom).

Field strength: Cyanide is a strong field ligand, which means it causes a large splitting of the metal’s d-orbitals and provides high crystal field stabilization energy (CFSE). This can lead to very stable complexes.

Example: \([Fe(CN)_6]^{4-}\) is extremely stable.

However, it is still monodentate, and the lack of chelation limits its ability to form the most stable complex when compared to bidentate ligands like oxalate.

3. \(H_2O\) (water):

Monodentate ligand: Water can form only one bond with a metal ion.

Field strength: Water is a weak field ligand and does not provide significant crystal field stabilization.

Complex stability: Water forms relatively weak complexes due to its neutral charge and weak ligand field strength. It's unlikely to form the most stable complex compared to stronger, charged ligands.

4. \(C_2O_4^{2-}\) (oxalate ion):

Bidentate ligand: The oxalate ion can form two bonds with the metal ion simultaneously. This means it can “chelate” the metal, which significantly increases the stability of the complex due to the chelate effect.

When a ligand forms a ring with the metal ion by binding at more than one point, it leads to greater thermodynamic stability. This is because, in forming two bonds with one ligand, the system experiences a smaller decrease in entropy than if the two bonds were made with two separate ligands. In other words, fewer particles are involved in the overall reaction, which increases stability.

Negative charge: The oxalate ion has a 2- charge, which allows for stronger electrostatic attraction to the positively charged metal ion. This increases the bond strength and adds to the stability of the complex.

Chelate effect: The chelate effect makes complexes formed by oxalate more stable than those formed by monodentate ligands like cyanide or thiocyanate.

Example: Oxalate forms highly stable complexes with transition metals, such as \([Fe(C_2O_4)_3]^{3-}\), which are well-known for their stability.

Why \(C_2O_4^{2-}\) is the Most Stable:

Bidentate nature: The oxalate ion can form two simultaneous bonds with the metal ion. This creates a ring structure, enhancing the stability through the chelate effect.

Chelate effect: The stability of complexes formed by polydentate ligands (those that form multiple bonds) is generally much higher due to this effect. The more bonds a ligand can form with a metal ion, the harder it is to break those bonds.

Electrostatic attraction: The oxalate ion’s 2- charge increases its ability to form strong bonds with positively charged metal ions.

In contrast, while \(CN^-\) is a strong field ligand, it is monodentate, so it lacks the chelate effect. Ligands like \(NCS^-\) and \(H_2O\) are also monodentate, and their lower charge and weaker field strength result in less stable complexes.

Conclusion:

The oxalate ion \(C_2O_4^{2-}\) forms the most stable complex because: It is a bidentate ligand, allowing for chelation, which greatly enhances the stability of the complex. Its 2- charge increases the electrostatic interaction with the metal ion, further stabilizing the complex. This makes option 4: \(C_2O_4^{2-}\) the correct choice for forming the most stable complex.