Question

A compound related to acetylacetone is 1,1,1-trifluoroacetylacetone (abbreviated Htfa): Htfa forms complexes in a manner similar to acetylacetone. (See Exercise 49.) Both Be \(^{2+}\) and \(\mathrm{Cu}^{2+}\) form complexes with tfa - having the formula \(\mathrm{M}(\mathrm{tfa})_{2}\) . Two isomers are formed for each metal complex. a. The Be \(^{2+}\) complexes are tetrahedral. Draw the two isomers of \(\mathrm{Be}(\mathrm{tfa})_{2} .\) What type of isomerism is exhibited by \(\mathrm{Be}(\mathrm{tfa})_{2} ?\) b. The \(\mathrm{Cu}^{2+}\) complexes are square planar. Draw the two isomers of \(\mathrm{Cu}(\mathrm{tfa})_{2} .\) What type of isomerism is exhibited by \(\mathrm{Cu}(\mathrm{tfa})_{2} ?\)

Short answer

The tetrahedral complex Be(tfa)2 forms two isomers named meridional (mer) and facial (fac) based on the arrangement of the tfa ligands. The mer-isomer has the three fluorine atoms in a plane and the fac-isomer has the three oxygen atoms and the three fluorine atoms in the same plane. These isomers exhibit geometrical isomerism, also known as configurational isomerism due to the different geometry around the central Be atom. For the square planar complex Cu(tfa)2, two isomers called cis and trans are formed. The cis-isomer has the two tfa ligands adjacent to each other, whereas the trans-isomer has the tfa ligands opposite each other in the square plane. Similarly to Be(tfa)2, these isomers of Cu(tfa)2 also display geometrical isomerism, i.e., they have different arrangements of ligands around the Cu atom while the connections remain the same.

Step-by-step solution

Drawing the Isomers of Be(tfa)2

Be(tfa)2 has a tetrahedral geometry. Therefore, we will have two possible isomers called meridional (mer) and facial (fac) based on the arrangement of the tfa ligands around the Be atom. The mer-isomer has the three fluorine atoms from the tfa ligands in a plane, and the fac-isomer has all three ligands grouped together, with the three oxygen atoms and the three fluorine atoms in the same plane.

Identifying the Isomerism for Be(tfa)2

The two isomers of Be(tfa)2 exhibit geometrical isomerism, which is also known as configurational isomerism. This is because the atoms are connected in the same way, but their arrangement in space (the geometry around the central atom) is different.

Drawing the Isomers of Cu(tfa)2

Cu(tfa)2 has a square planar geometry. We will have two possible isomers called cis and trans, based on the arrangement of the tfa ligands around the Cu atom. The cis-isomer has the two tfa ligands adjacent to each other, whereas the trans-isomer has the two tfa ligands opposite each other in the square plane.

Identifying the Isomerism for Cu(tfa)2

Similar to Be(tfa)2, the two isomers of Cu(tfa)2 exhibit geometrical isomerism. They have different arrangements of ligands around the central metal atom, while the connections remain the same. In the case of Cu(tfa)2, the isomerism also lies in the arrangement of ligands in the square planar geometry.

Key concepts

Tetrahedral Geometry

Tetrahedral geometry is a molecular shape where a central atom is surrounded by four ligands positioned at the corners of a tetrahedron. This configuration is most commonly seen in coordination chemistry with complex ions. In a tetrahedral structure, because of the spatial arrangement, the bond angles are approximately 109.5 degrees.

Geometrical isomerism can arise in tetrahedral complexes when different ligands are present or due to unsymmetrical ligands. In the case of the \(\text{Be(tfa)}_2\) complex from our exercise, the isomers are identified as \text{meridional (mer)}\ and \text{facial (fac)}\ isomers:

• Meridional (mer) Isomer: The three identical ligands (in this case, the three fluorines) are aligned in a single plane.
• Facial (fac) Isomer: Here, the three identical ligands form a triangular face on the tetrahedral structure.

Both isomers demonstrate the significant geometric flexibility and spatial differences possible even when atoms are connected in the same order. This variation causes the difference in properties exhibited by the isomers.

Square Planar Geometry

Square planar geometry is prominent in coordination chemistry, especially with d8 metal ions such as \(\text{Cu}^{2+}\). In this geometry, a central metal atom is in the same plane as four equidistant ligands forming a square with 90-degree angles between them.

In \(\text{Cu(tfa)}_2\), which is square planar, two isomers are possible due to the geometrical arrangement of the ligands:

• Cis Isomer: The two tfa ligands are adjacent to each other.
• Trans Isomer: The two tfa ligands are opposite each other across the central metal.

Such isomerism demonstrates how spatial arrangements around the square planar center lead to different physical and chemical properties, even when connections between atoms remain unchanged. The rigidity of square planar complexes often leads to more pronounced isomerism compared to other geometries.

Coordination Chemistry

Coordination chemistry focuses on complex compounds composed of a central metal atom bonded to surrounding molecules or ions, known as ligands. These complexes are integral to molecular architecture and possess distinctive physical and chemical properties due to the specific arrangement of ligands.

Coordination complexes can exhibit several types of isomerism, including:

• Geometrical Isomerism: Different spatial arrangements of ligands can lead to isomers with distinct properties, as seen with tetrahedral and square planar complexes.
• Optical Isomerism: Occurs when a complex can exist in forms that are non-superimposable mirror images of each other, much like left and right hands.

In coordination chemistry, the ligand's denticity, the metal's oxidation state, and the overall coordination number determine the complex's geometry. For example, tetrahedral and square planar geometries often result from specific metal center preferences and ligand arrangements. This field provides insight into bonding theories and material properties, playing a crucial role in catalysis, medicine, and materials science.