Problem 50 - Chemistry The Molecular Science

Question

Give the coordination number of the central metal ion in (a) \(\left[\mathrm{Ni}(\mathrm{cn})_{2}\left(\mathrm{NH}_{3}\right)_{2}\right]^{2+}\). (b) \(\left[\mathrm{Fe}(\mathrm{cn})\left(\mathrm{C}_{2} \mathrm{O}_{4}\right) \mathrm{Cl}_{2}\right]^{-}\)

Step-by-Step Solution

Answer

(a) Coordination number is 4; (b) Coordination number is 5.

Step 1: Identify Coordination Compounds

The given exercise has two coordination complexes: (a) \(\left[\mathrm{Ni}(\mathrm{cn})_{2}\left(\mathrm{NH}_{3}\right)_{2}\right]^{2+}\) and (b) \(\left[\mathrm{Fe}(\mathrm{cn})\left(\mathrm{C}_{2} \mathrm{O}_{4}\right)\mathrm{Cl}_{2}\right]^{-}\). To find the coordination number of the central metal ion, we need to identify the number of ligands directly bonded to the metal.

Step 2: Determine Coordination Number for (a)

For \(\left[\mathrm{Ni}(\mathrm{cn})_{2}\left(\mathrm{NH}_{3}\right)_{2}\right]^{2+}\), the ligands present are two cyanide ions \((\mathrm{CN}^-)\) and two ammonia molecules \((\mathrm{NH}_3)\). Each ligand forms a single bond with the nickel \((\mathrm{Ni})\) atom, therefore making the coordination number equal to 4.

Step 3: Determine Coordination Number for (b)

In \(\left[\mathrm{Fe}(\mathrm{cn})\left(\mathrm{C}_{2} \mathrm{O}_{4}\right)\mathrm{Cl}_{2}\right]^{-}\), the ligands are one cyanide ion \((\mathrm{CN}^-)\), one oxalate ion \((\mathrm{C}_{2}\mathrm{O}_4^{2-})\), and two chloride ions \((\mathrm{Cl}^-)\). The cyanide ion is monodentate (1 bond), the oxalate ion is bidentate (2 bonds), and each chloride ion is monodentate (1 bond each). Thus, the total coordination number is \(1 + 2 + 1 + 1 = 5\).

Key Concepts

Coordination Number

In coordination chemistry, the coordination number refers to the total number of ligand atoms that are bonded to the central metal ion in a coordination complex. This number gives insight into the geometry and structure of the complex. To determine this number, simply count the bonds formed between the metal and its surrounding ligands.
For example:

• In the complex \(\left[\mathrm{Ni}(\mathrm{cn})_{2}\left(\mathrm{NH}_{3}\right)_{2}\right]^{2+}\), the nickel metal is surrounded by two cyanide ions \((\mathrm{CN}^-)\) and two ammonia molecules \((\mathrm{NH}_3)\), since each forms a single bond with nickel, the coordination number of nickel is 4.
• In the complex \(\left[\mathrm{Fe}(\mathrm{cn})\left(\mathrm{C}_{2} \mathrm{O}_{4}\right)\mathrm{Cl}_{2}\right]^{-}\), iron is bonded to a cyanide ion, an oxalate ion \((\mathrm{C}_{2}\mathrm{O}_4^{2-})\) which forms two bonds (bidentate), and two chloride ions \((\mathrm{Cl}^-)\). Thus, the coordination number is 5.

Understanding the coordination number is crucial because it affects the chemical properties and reactivity of the metal complex.

Ligands

Ligands are ions or molecules that can donate a pair of electrons to the central metal atom or ion in a coordination complex. These are crucial components, as they directly impact the structure and stability of the complex. Ligands can be categorized based on how many points of attachment, or donor atoms, they have:

• Monodentate ligands: These are ligands that bind through a single donor atom. Examples include \(\mathrm{CN}^-\) and \(\mathrm{Cl}^-\).
• Bidentate ligands: These have two donor atoms and can form two bonds with the metal center. An example is the oxalate ion \((\mathrm{C}_{2}\mathrm{O}_4^{2-})\).
• Polydentate ligands: These bind through multiple donor atoms beyond two, forming chelating rings with the metal.

Each ligand type affects the shape, valency, and ultimately the reactivity of the complex. Recognizing the type of ligand helps in predicting the coordination number and the potential geometry of the metal complex.

Metal Complexes

Metal complexes are formed when a central metal ion binds with surrounding ligands to form a stable structure. The metal center can be any transition metal, and it plays a significant role in defining the overall properties of the complex. The combination of different ligands around a metal center can lead to diverse geometries and chemical behaviors.
Key aspects of metal complexes include:

• Geometry: Depending on the coordination number and types of ligands, complexes can have different shapes like tetrahedral, square planar, or octahedral. For instance, a coordination number of 4 can lead either to a tetrahedral or a square planar geometry, whereas 6 generally results in an octahedral shape.
• Ligand Field Theory: This theory explains how the arrangement of ligands impacts the energy levels of the d-orbitals of the metal ion, which can affect magnetic and color properties.
• Stability: The type of bond between the metal and ligands, along with factors like the chelate effect, can strongly influence the stability of metal complexes.

Metal complexes are foundational in many fields, from catalysis to medicine, making an understanding of their formation and properties incredibly important.