Problem 58 - Chemistry and Chemical Reactivity

Question

The glycinate ion, \(\mathrm{H}_{2} \mathrm{NCH}_{2} \mathrm{CO}_{2}^{-},\) formed by deprotonation of the amino acid glycine, can function as a bidentate ligand, coordinating to a metal through the nitrogen of the amino group and one of the oxygen atoms. (equation can't copy) A copper complex of this ligand has the formula \(\mathrm{Cu}\left(\mathrm{H}_{2} \mathrm{NCH}_{2} \mathrm{CO}_{2}\right)_{2}\left(\mathrm{H}_{2} \mathrm{O}\right)_{2} .\) For this complex, determine the following. (a) the oxidation state of copper (b) the coordination number of copper (c) the number of unpaired electrons (d) whether the complex is diamagnetic or paramagnetic

Step-by-Step Solution

Answer

(a) +2; (b) 6; (c) 1 unpaired electron; (d) Paramagnetic.

Step 1: Determine the oxidation state of copper

In the complex \( \text{Cu}\left(\text{H}_2\text{NCH}_2\text{CO}_2\right)_2\left(\text{H}_2\text{O}\right)_2 \), the glycinate ion \( \text{H}_2\text{NCH}_2\text{CO}_2^- \) acts as a bidentate ligand with a -1 charge per ligand. Since there are two glycinate ligands, the total charge contributed by these ligands is -2. The \( \text{H}_2\text{O} \) molecules are neutral. The whole complex is neutral, thus, the oxidation state of Cu must balance the -2 charge of the ligands, which makes the oxidation state of Cu \(+2\).

Step 2: Determine the coordination number of copper

The glycinate ion acts as a bidentate ligand, coordinating to the metal at two points (one with the nitrogen and one with the oxygen atom). Since there are two glycinate ions, they provide four coordination sites. Additionally, there are two \( \text{H}_2\text{O} \) molecules, each coordinated with one point. Thus, the total coordination number of copper is \(6\).

Step 3: Determine the number of unpaired electrons

Copper in the oxidation state \(+2\) has the electron configuration \([\text{Ar}] \, 3d^9\). For a \(3d^9\) configuration, there is one unpaired electron present.

Step 4: Determine the magnetic property of the complex

Since the copper complex has one unpaired electron, it exhibits paramagnetic properties, as paramagnetism is associated with unpaired electrons.

Key Concepts

Oxidation State

In coordination chemistry, understanding the oxidation state of a metal in a complex is crucial. It helps determine the element's overall charge and its interaction with the ligands bound to it. In our copper complex, \[ \mathrm{Cu}\left(\mathrm{H}_2 \mathrm{NCH}_2 \mathrm{CO}_2\right)_2\left(\mathrm{H}_2 \mathrm{O}\right)_2, \]the oxidation state of copper is deduced based on the charges of the ligands and the overall charge of the complex.

• The glycinate ion, functioning as a bidentate ligand, carries a \(-1\) charge.
• There are two such ligands, providing a total charge of \(-2\).
• The water molecules are neutral and do not add any charge.

The whole complex is neutral, meaning the oxidation state of the copper has to balance the \(-2\) from the ligands, making the copper's oxidation state \(+2\). Recognizing the oxidation state allows us to understand the electron configuration and magnetic properties of the metal center.

Coordination Number

The coordination number in a complex is the count of how many atoms are directly bonded to the central metal ion. Knowing this helps in predicting the geometry and structure of the complex. In our copper complex:
The glycinate ion acts as a bidentate ligand.

• Each glycinate ligand binds at two points through a nitrogen atom and an oxygen atom from the carboxylate group.

Since there are two glycinate ions, they provide four coordination sites to the copper,
In addition to two coordination sites from the water molecules,
Thus, the total coordination number of copper is 6, typically resulting in an octahedral geometry.

Bidentate Ligand

Bidentate ligands are unique because they can attach to a metal ion at two separate points. This creates a more stable complex by forming a chelate ring with the metal.
In the given exercise, the glycinate ion is bidentate.

• It uses the nitrogen atom from the amino group.
• And one oxygen atom from the carboxylate group to coordinate with the metal ion.

This dual attachment not only gives rise to its bidentate nature but also majorly contributes to the coordination number and overall stability of the complex. Bidentate ligands are especially significant in coordination chemistry for their tendency to form chelate rings, increasing the stability of the complex.

Paramagnetism

The magnetic properties of a coordination complex are largely dictated by the presence of unpaired electrons. Complexes with unpaired electrons exhibit paramagnetism,
which means they are attracted to a magnetic field. In our copper complex,
the copper ion has an oxidation state of \(+2\), leading to an electron configuration of \[ \text{[Ar]} \, 3d^9. \]With one unpaired electron present in this configuration, the complex exhibits paramagnetic behavior. This property can be measured using magnetic susceptibility techniques and provides insight into the electronic structure of the metal center.