Problem 42 - Chemistry An Atoms First Approach

Question

Amino acids can act as ligands toward transition metal ions. The simplest amino acid is glycine \(\left(\mathrm{NH}_{2} \mathrm{CH}_{2} \mathrm{CO}_{2} \mathrm{H}\right) .\) Draw a structure of the glycinate anion \(\left(\mathrm{NH}_{2} \mathrm{CH}_{2} \mathrm{CO}_{2}^{-}\right)\) acting as a bidentate ligand. Draw the structural isomers of the square planar complex \(\mathrm{Cu}\left(\mathrm{NH}_{2} \mathrm{CH}_{2} \mathrm{CO}_{2}\right)_{2}\)

Step-by-Step Solution

Answer

The glycinate anion has the structure \(\mathrm{NH}_{2} \mathrm{CH}_{2} \mathrm{CO}_{2}^{-}\), acting as a bidentate ligand through the nitrogen atom in the amino group and the oxygen atom in the carboxylate group. The square planar complex \(\mathrm{Cu}\left(\mathrm{NH}_{2} \mathrm{CH}_{2} \mathrm{CO}_{2}\right)_{2}\) has two structural isomers: the cis isomer, where the two nitrogen atoms and the two oxygen atoms are adjacent to each other, and the trans isomer, where the nitrogen atoms and oxygen atoms are opposite each other.

Step 1: Drawing the Glycinate Anion

For this step, we need to know the structure of glycine, which is given as \(\mathrm{NH}_{2} \mathrm{CH}_{2} \mathrm{CO}_{2}\mathrm{H}\). To convert glycine into glycinate anion, we need to remove a hydrogen atom from the carboxyl group (-COOH), creating the anionic form with a negative charge (-COO-). The resulting structure is now \(\mathrm{NH}_{2} \mathrm{CH}_{2} \mathrm{CO}_{2}^{-}\), which is the glycinate anion.

Step 2: Identifying the Bidentate Ligand Binding Sites

The glycinate anion acts as a bidentate ligand, meaning that it can bind to a metal ion through two donor atoms. In this case, it will bind to a transition metal ion through the nitrogen atom from the amino group (-NH2) and the oxygen atom from the carboxylate group (-COO-).

Step 3: Drawing the Square Planar Complex

Since the complex is square planar, the Cu(II) ion will be at the center with four coordination sites, two of which will be occupied by the glycinate anions acting as bidentate ligands. First, draw Cu(II) in the center, and then add the two glycinate anions coordinated through the nitrogen (from the -NH2) and one of the oxygen atoms (from the -COO-) to create the square planar complex \(\mathrm{Cu}\left(\mathrm{NH}_{2} \mathrm{CH}_{2} \mathrm{CO}_{2}\right)_{2}\).

Step 4: Drawing the Structural Isomers

In this step, we need to find the possible structural isomers for the square planar complex \(\mathrm{Cu}\left(\mathrm{NH}_{2} \mathrm{CH}_{2} \mathrm{CO}_{2}\right)_{2}\). There are two possible isomers due to the binding of the bidentate ligands, which are labeled as cis and trans isomers. - Cis isomer: The two nitrogen atoms (from the amino groups) are adjacent to each other, and the two oxygen atoms (from the carboxylate groups) are also adjacent to each other. - Trans isomer: The two nitrogen atoms (from the amino groups) are opposite to each other, and the two oxygen atoms (from the carboxylate groups) are also opposite to each other. Draw both cis and trans isomers of the complex \(\mathrm{Cu}\left(\mathrm{NH}_{2} \mathrm{CH}_{2} \mathrm{CO}_{2}\right)_{2}\).