Problem 49 - Chemistry The Molecular Science

Question

Give the coordination number of the central metal ion in (a) \(\left[\mathrm{Pt}(\mathrm{en})_{2}\right]^{2+}\) (b) \(\left[\mathrm{Cu}\left(\mathrm{C}_{2} \mathrm{O}_{4}\right)_{2}\right]^{2-}\).

Step-by-Step Solution

Answer

(a) 4 (b) 4

Step 1 - Understand Coordination Number

The coordination number of a central metal ion in a complex is determined by counting the number of ligand atoms that are bonded directly to the metal ion.

Step 2 - Recognize Ligands in [Pt(en)2]2+

The complex \[\left[\mathrm{Pt}(\mathrm{en})_{2}\right]^{2+}\]contains ethylenediamine (en) ligands. Each ethylenediamine ligand is a bidentate ligand, meaning it forms two bonds with the central metal ion, Pt.

Step 3 - Calculate Coordination Number for [Pt(en)2]2+

There are two ethylenediamine ligands present. Since each ligand forms 2 bonds, the coordination number is: \[2 \text{ ligands} \times 2 \text{ bonds per ligand} = 4\]. Thus, the coordination number of \(\left[\mathrm{Pt}(\mathrm{en})_{2}\right]^{2+}\) is 4.

Step 4 - Recognize Ligands in [Cu(C2O4)2]2-

The complex \[\left[\mathrm{Cu}\left(\mathrm{C}_{2}\mathrm{O}_{4}\right)_{2}\right]^{2-}\]contains oxalate (\(\mathrm{C}_{2}\mathrm{O}_{4}^{2-}\)) ligands. Each oxalate ligand is also a bidentate ligand, forming two bonds with the central metal ion, Cu.

Step 5 - Calculate Coordination Number for [Cu(C2O4)2]2-

There are two oxalate ligands present. Since each ligand forms 2 bonds, the coordination number is: \[2 \text{ ligands} \times 2 \text{ bonds per ligand} = 4\]. Thus, the coordination number of \[\left[\mathrm{Cu}\left(\mathrm{C}_{2}\mathrm{O}_{4}\right)_{2}\right]^{2-}\]is 4.

Key Concepts

Central Metal Ion

In the realm of coordination chemistry, the "central metal ion" holds a pivotal role in defining the structure and properties of a coordination complex. A coordination complex consists of a central metal ion surrounded by molecules or ions known as ligands.
These ligands bind to the metal ion through coordinate covalent bonds, where the ligand donates a pair of electrons to the metal for bonding.

• The central metal ion is often a transition metal, due to its ability to adopt various oxidation states and coordinate with different ligands.
• This central ion acts as an electron pair acceptor, playing a crucial role as a Lewis acid.
• The coordination number of a central metal ion signifies the total number of ligand points of attachment that are connected directly to it.

By examining the interaction of ligands with the central metal ion, one can determine the geometry and possible chemical reactivity of the complex.

Bidentate Ligand

A bidentate ligand is a type of ligand that has two "teeth" or atoms allowing it to bind to the central metal ion at two distinct points. This leads to the formation of a chelate ring, providing extra stability to the complex structure.
The prefix 'bi-' implies that there are two donor atoms available for bonding.

• Bidentate ligands possess two donor atoms, such as nitrogen or oxygen, capable of forming coordinate bonds with the same metal ion.
• The creation of chelate rings is significant as it increases the thermodynamic stability of the complexes compared to those with monodentate ligands.
• The presence of bidentate ligands can alter the electronic properties and geometry of the metal complexes, affecting color, magnetic properties, and reactivity.

Understanding bidentate ligands is essential for explaining why certain metal complexes exhibit enhanced stability and unique structural characteristics compared to their monodentate counterparts.

Ethylenediamine Ligand

Ethylenediamine, often abbreviated as "en," is a classic example of a bidentate ligand. It is a simple organic molecule composed of two amine groups connected by an ethylene (-CH2CH2-) bridge.
This configuration allows ethylenediamine to effectively coordinate with a central metal ion at two points.

• Ethylenediamine has two nitrogen atoms, each possessing a lone electron pair that can coordinate with a metal ion.
• In coordination complexes like \([\mathrm{Pt}(\mathrm{en})_{2}]^{2+}\), the ethylenediamine ligand binds to the metal at both nitrogen atoms, forming stable, five-membered rings.
• This particular ligand is frequently used in coordination chemistry due to its ability to form stable chelate complexes with a variety of metal ions.

Recognizing the role of ethylenediamine as a bidentate ligand helps in understanding the structural aspects of many coordination complexes and their resulting stability.