Question

Which of the following does not exist? (a) \(\left[\mathrm{Cu} \mathrm{Cl}_{4}\right]^{2-}\) (b) \(\left[\mathrm{CuI}_{4}\right]^{2-}\) (c) \(\left[\mathrm{Zn}(\mathrm{CN})_{4}\right]^{2-}\) (d) \(\left[\mathrm{HgI}_{4}\right]^{2-}\)

Short answer

(b) \\([\mathrm{CuI}_{4}]^{2-}\\) does not exist.

Step-by-step solution

Understand the Coordination Complex

Coordination complexes consist of a central metal atom/ion bonded to a number of surrounding ligands. Ligands can be ions or molecules that donate at least one pair of electrons to the central metal.

Examine Each Option

Review each option, focusing on the central metal ion and its capability of forming stable coordination complexes with the given ligands.

Assess Stability of Each Option

(a) \([\mathrm{CuCl}_{4}]^{2-}\) forms a stable coordination complex with copper, as chlorine ions are common ligands.(b) \([\mathrm{CuI}_{4}]^{2-}\) needs to be checked for stability; copper typically doesn't form stable complexes with iodine.(c) \([\mathrm{Zn(CN)}_{4}]^{2-}\) is a stable complex, as cyanide forms very stable complexes with zinc.(d) \([\mathrm{HgI}_{4}]^{2-}\) may not be stable, as mercury does not form stable complexes with four iodides like this.

Identify the Unstable Complex

In \([\mathrm{CuI}_{4}]^{2-}\), copper typically forms complexes with smaller ligands and not with four large iodide ions, which makes it unstable or non-existent due to steric and electronic reasons.

Key concepts

Ligands

Ligands are crucial components in coordination complexes. They are typically ions or molecules that attach to a central metal ion through coordination bonds. Coordinating bonds form because ligands donate at least one pair of electrons to the central metal ion, thus establishing a stable interaction. Ligands can vary greatly, ranging from simple ions like chloride (Cl\(^-\)) to more complex organic compounds such as ethylenediamine.

In the context of coordination chemistry, the ability of a ligand to bind to a metal ion depends on its donor atoms which possess lone pairs of electrons. A few important properties of ligands include:

• Polydentate capacity: some ligands, known as chelating ligands, can attach at multiple binding sites on the metal ion, providing greater stability.
• Charge: Neutral ligands (like water) and charged ligands (like cyanide, CN\(^-\)) can both form stable complexes.
• Size: Larger ligands can lead to steric hindrance, affecting the stability of the complex.

Understanding the nature of a ligand is essential for predicting the stability and existence of coordination complexes.

Central Metal Ion

Central metal ions play a pivotal role in the structure and stability of coordination complexes. They act as the focal point to which ligands attach. These metals, often transition metals, have multiple oxidation states and can accommodate several ligands due to their d-orbitals.

The characteristics of central metal ions that impact complexes include:

• Oxidation State: It influences the number of ligands a metal ion can accommodate. Higher oxidation states often attract more ligands due to increased positive charge.
• Ionic Radius: Smaller ionic radii might hinder the binding of bulky ligands due to size constraints.
• Electronic Configuration: Determines the geometry and the ability to accept electron pairs from ligands.

In the case of the exercise, copper (Cu\(^{2+}\)) acts as the central metal ion. The tendency of Cu\(^{2+}\) to form complexes is high, but the combination with certain ligands, like iodide in \([\text{CuI}_4]^{2-}\), may not always lead to stable formations.

Stability of Complexes

Stability in coordination complexes is predominantly influenced by the interactions between the central metal ion and the ligands. Stability can be assessed through various factors such as the nature of the metal-ligand bond, electronic configurations, and entropic considerations.

Important factors influencing stability include:

• Ligand Field Strength: Strong field ligands, like cyanide (CN\(^-\)), create particularly stable complexes through strong interactions with the metal's d-orbitals.
• Chelate Effect: Ligands that form multiple bonds with a metal ion (polydentate ligands) enhance the complex's stability due to increased binding sites.
• Steric and Electronic Factors: Bulky ligands might destabilize a complex due to steric hindrance, as exhibited by \([\text{CuI}_4]^{2-}\).

During the evaluation of the complexes in the problem, it was found that \([\text{Zn(CN)}_4]^{2-}\) was stable due to the strong field strength of cyanide, whereas \([\text{CuI}_4]^{2-}\) was deemed unstable due to steric hindrance and weak field strength of iodide ligands.

Coordination Chemistry

Coordination chemistry is the study of compounds formed between metal ions and ligands, exploring their structure, stability, and reactivity. It combines aspects from different fields of chemistry, including inorganic chemistry, organic chemistry, and physics, to understand complex molecule interactions.

Key concepts in coordination chemistry encompass:

• Coordination Number: The number of ligand donor atoms bonded to the central metal ion, determining the geometry of the complex. Common geometries include tetrahedral, square planar, and octahedral.
• Crystal Field Theory: Offers an explanation for the bonding, color, magnetic properties, and stability of coordination complexes by considering the splitting of d-orbitals.
• Applications: Coordination complexes have applications ranging from industrial catalysis and material synthesis to biological systems and medicine.

Understanding coordination complexes, as seen in the exercise, provides foundational insight into the principles governing their formation and stability, essential for chemistry students and professionals alike.