Passage
Amino acids and copper both serve vital roles in many biological processes of living organisms. Beyond their separate biological roles, however, chemists have also discovered that amino acids are able to function as chelates with Cu²⁺ ions to form blue complexes with potential medicinal applications. One such complex, copper(II) glycinate, has shown to be effective against some oral diseases. The reaction to form copper(II) glycinate is shown in Figure 1.
Figure 1 Synthesis of two isomers of copper(II) glycinateTwo different isomers can form depending on the reaction conditions. Mixing hot aqueous glycine (70 °C) with a hot solution of copper(II) acetate dissolved in 50% aqueous ethanol, followed by cooling the mixture on ice, yields a precipitate of only the cis isomer. If a suspension of the cis isomer is held at boiling for an hour or longer in a solution of glycine and copper(II) acetate, the trans isomer is obtained.Because of the geometric differences between the two isomers, the stretching vibrations of the central Cu-N and Cu-O bonds in each isomer are different. Symmetric and asymmetric bond stretching give four possible vibrational modes for each isomer, as shown in Figure 2.
Figure 2 Vibrational modes of the coordinate bonds in copper(II) glycinateIf the stretching of the bonds during vibration causes a transient, unequal distribution of electron density between the atoms within the bond, an oscillating dipole will form. Bonds with dipoles that are not cancelled out by another dipole in the molecule will absorb infrared light at particular frequencies (often expressed in wave numbers) and are designated as IR-active. As a result, the vibrational differences between the two isomers can be analyzed by far-infrared (far-IR) spectroscopy, as shown in Figure 3.
Figure 3 Far-IR spectra of the cis and trans isomers of copper(II) glycinateBoth isomers show a skeletal absorbance (unrelated to coordinate bond vibration) near 385 cm⁻¹. Additional absorbance bands are seen that specifically relate to the vibrational modes of the bonds in each isomer.
-In the reaction to form the cis isomer shown in Figure 1, which of the following structural sites function(s) as a Lewis base?

Question

Passage
Amino acids and copper both serve vital roles in many biological processes of living organisms. Beyond their separate biological roles, however, chemists have also discovered that amino acids are able to function as chelates with Cu²⁺ ions to form blue complexes with potential medicinal applications. One such complex, copper(II) glycinate, has shown to be effective against some oral diseases. The reaction to form copper(II) glycinate is shown in Figure 1.

Figure 1 Synthesis of two isomers of copper(II) glycinateTwo different isomers can form depending on the reaction conditions. Mixing hot aqueous glycine (70 °C) with a hot solution of copper(II) acetate dissolved in 50% aqueous ethanol, followed by cooling the mixture on ice, yields a precipitate of only the cis isomer. If a suspension of the cis isomer is held at boiling for an hour or longer in a solution of glycine and copper(II) acetate, the trans isomer is obtained.Because of the geometric differences between the two isomers, the stretching vibrations of the central Cu-N and Cu-O bonds in each isomer are different. Symmetric and asymmetric bond stretching give four possible vibrational modes for each isomer, as shown in Figure 2.

Figure 2 Vibrational modes of the coordinate bonds in copper(II) glycinateIf the stretching of the bonds during vibration causes a transient, unequal distribution of electron density between the atoms within the bond, an oscillating dipole will form. Bonds with dipoles that are not cancelled out by another dipole in the molecule will absorb infrared light at particular frequencies (often expressed in wave numbers) and are designated as IR-active. As a result, the vibrational differences between the two isomers can be analyzed by far-infrared (far-IR) spectroscopy, as shown in Figure 3.

Figure 3 Far-IR spectra of the cis and trans isomers of copper(II) glycinateBoth isomers show a skeletal absorbance (unrelated to coordinate bond vibration) near 385 cm⁻¹. Additional absorbance bands are seen that specifically relate to the vibrational modes of the bonds in each isomer.
-In the reaction to form the cis isomer shown in Figure 1, which of the following structural sites function(s) as a Lewis base?

Quizplus · Accepted Answer

11ecba2d_120e_6326_a3bf_93b3c9aa0f77_MD0008_00 Coordination complexes consist of a central metal ion surrounded by one or more ions or molecules called ligands, which are bound to the metal center by coordinate bonds. The ligands act as a Lewis base (electron pair donor), and the metal center acts as a Lewis acid (electron pair acceptor). Ligand structures that can form more than one coordinate bond by donating more than one pair of electrons have more than one site that can function as a Lewis base. During the formation of copper(II) glycinate (Figure 1), glycine displaces acetate in Cu(C₂H₃O₂)₂ because stronger Lewis bases can displace weaker Lewis bases as ligands within a complex. Stronger Lewis bases tend to be those that have lone pair electrons on atoms with a net charge and/or a lower electronegativity. As a ligand, glycine has an available pair of electrons to donate on both the oxygen atom in the carboxylic group and on the nitrogen atom of the amino group, and both sites in the structure of glycine function as Lewis bases (electron pair donor) in the reaction shown in Figure 1. This is indicated by the formation of the Cu−N and Cu−O coordinate bonds in the complex. (Choice A) In the reaction, copper is present in the form of Cu²⁺ ions that are supplied by the Cu(C₂H₃O₂)₂ solution. The Cu²⁺ ion is electron-deficient and acts as a Lewis acid (electron pair acceptor). (Choices B and C) The oxygen atom in the carboxylic acid group and the nitrogen atom in the amino group in the structure of glycine can both donate a lone pair of electrons. As such, both sites can function as Lewis bases. Educational objective: Within coordination complexes, a ligand acts as a Lewis base, and the metal center acts as a Lewis acid. Ligand structures that can form more than one coordinate bond by donating more than one pair of electrons have more than one site that can function as a Lewis base.

Passage Amino Acids and Copper Both Serve Vital Roles in Many