Question

In which of the following complexes are geometric isomers possible? If isomers are possible, draw their structures and label them as cis or trans, or as fac or mer. (a) \(\left[\mathrm{Co}\left(\mathrm{H}_{2} \mathrm{O}\right)_{4} \mathrm{Cl}_{2}\right]^{+}\) (c) \(\left[\mathrm{Pt}\left(\mathrm{NH}_{3}\right) \mathrm{Br}_{3}\right]^{-}\) (b) \(\operatorname{Co}\left(\mathrm{NH}_{3}\right)_{3} \mathrm{F}_{3}\) (d) \(\left[\mathrm{Co}(\mathrm{en})_{2}\left(\mathrm{NH}_{3}\right) \mathrm{Cl}\right]^{2+}\)

Short answer

Geometric isomers are possible in (a) and (b): cis/trans for (a), fac/mer for (b).

Step-by-step solution

Identify Coordination Numbers and Geometries

Determine the coordination number and geometry of each complex. For (a), the coordination number is 6 (octahedral). For (b), it is also 6 (octahedral). In (c), the coordination number is 4 (square planar), and for (d), it is 6 (octahedral).

Evaluate for Geometric Isomers in Octahedral Complexes

In octahedral complexes, check for cis-trans or fac-mer isomerism. (a) \([\mathrm{Co}(\mathrm{H}_{2}\mathrm{O})_{4}\mathrm{Cl}_{2}]^{+}\) can show cis-trans isomerism, with Cl ligands either adjacent (cis) or opposite (trans).(b) \(\operatorname{Co}(\mathrm{NH}_{3})_{3}\mathrm{F}_{3}\) can exhibit fac-mer isomerism with three identical ligands, either forming a face (fac) or meridian (mer).

Consider Square Planar Geometry for (c)

Square planar complexes can show cis-trans isomerism if they have two different types of ligands. (c) \([\mathrm{Pt}(\mathrm{NH}_{3})\mathrm{Br}_{3}]^{-}\) cannot have geometric isomers because it has only one type of position relative to the others due to excess similarity.

Analyze Complex (d) for Isomerism Potential

Although \([\mathrm{Co}(\mathrm{en})_{2}(\mathrm{NH}_{3})\mathrm{Cl}]^{2+}\) is octahedral, it cannot show geometric isomerism. The ethylenediamine (en) ligands are bidentate, restricting possible isomers.

Key concepts

Coordination Number

In coordination chemistry, the "coordination number" of a complex indicates how many ligands are directly bonded to the central metal ion. It provides insight into the structure and geometry of the complex.

• For example, in complex (a) \([\mathrm{Co}\left(\mathrm{H}_{2}\mathrm{O}\right)_{4}\mathrm{Cl}_{2}]^{+}\), four water molecules and two chloride ions are attached to cobalt, yielding a coordination number of 6. This typically corresponds to an octahedral geometry.
• Complex (b) \(\operatorname{Co}\left(\mathrm{NH}_{3}\right)_{3} \mathrm{F}_{3}\) also has a coordination number of 6.
• For complex (c) \([\mathrm{Pt}\left(\mathrm{NH}_{3}\right) \mathrm{Br}_{3}]^{-}\), the coordination number is 4, which often results in a square planar configuration.
• Complex (d) \([\mathrm{Co}(\mathrm{en})_{2}\left(\mathrm{NH}_{3}\right) \mathrm{Cl}\right]^{2+}\) has a coordination number of 6, similar to complexes (a) and (b).

Understanding the coordination number is essential, as it influences the potential for isomerism within the complex.

Octahedral Complexes

Octahedral complexes are characterized by six ligands symmetrically surrounding the central metal ion. This six-ligand arrangement forms an octahedral shape, which can lead to isomerism if certain conditions are met.

• The presence of different types or arrangements of ligands within the complex can result in geometric isomers.
• In complexes (a) and (b), which have coordination numbers of six and adopt an octahedral geometry, potential geometric isomers exist.
• Complex (a) can demonstrate cis-trans isomerism if two of the ligands are different from the others, such as the water and chloride ions.
• Complex (b) can exhibit fac-mer isomerism because there are three identical ligands, either forming a triangular face or lining up along a meridian.

Identifying the shape and ligand arrangement helps in predicting geometric isomers.

Cis-Trans Isomerism

Cis-trans isomerism is a form of geometric isomerism where ligands in a complex differ in spatial arrangement around the central metal ion.

• In octahedral complexes, such as (a) \([\mathrm{Co}(\mathrm{H}_{2}\mathrm{O})_{4}\mathrm{Cl}_{2}]^{+}\), cis-trans isomers can occur if there are two identical ligands. The cis isomer has the identical ligands adjacent to each other, whereas in the trans isomer, they are opposite.
• This concept is also applicable to square planar complexes, provided that there are two different types of ligands. However, in complex (c) \([\mathrm{Pt}(\mathrm{NH}_{3})\mathrm{Br}_{3}]^{-}\), all positions are equivalent because of the similar types of positions occupied by the ligands, so it cannot demonstrate cis-trans isomerism.

Knowing how ligands can be arranged helps visualize potential isomers.

Fac-Mer Isomerism

Fac-mer isomerism occurs in octahedral complexes where three identical ligands are present around the central metal ion. This isomerism focuses on whether these identical ligands form a face (fac) or align along a meridian (mer) of the octahedron.

• In complex (b) \(\operatorname{Co}\left(\mathrm{NH}_{3}\right)_{3} \mathrm{F}_{3}\), the potential for fac-mer isomerism arises because it contains three identical \(\mathrm{NH}_{3}\) ligands.
• The fac configuration means that all three identical ligands are adjacent, forming one of the faces of the octahedron.
• The mer configuration occurs when the identical ligands span across a central meridian, forming a T-like arrangement.

This type of isomerism is unique to octahedral complexes with three identical ligands.