8.2 Isomerism in Coordination Compounds

Structural Isomerism

Structural isomers share the same molecular formula but differ in how their atoms are connected. In coordination chemistry, this means ligands, counter ions, or even solvent molecules can swap positions, producing compounds with genuinely different chemical behavior.

Linkage Isomerism

Linkage isomers arise when an ambidentate ligand (one that can bind through more than one donor atom) coordinates to the metal through different atoms. The classic examples involve $\text{NO}_2^-$ and $\text{SCN}^-$.

• $\text{NO}_2^-$ can bind through nitrogen (nitro, $\text{M–NO}_2$) or through oxygen (nitrito, $\text{M–ONO}$)
• $\text{SCN}^-$ can bind through sulfur (thiocyanato, $\text{M–SCN}$) or through nitrogen (isothiocyanato, $\text{M–NCS}$)

A well-known pair: $[\text{Co(NH}_3)_5\text{NO}_2]\text{Cl}_2$ (yellow, N-bound) and $[\text{Co(NH}_3)_5\text{ONO}]\text{Cl}_2$ (red, O-bound). Same formula, different donor atom, different color and stability.

Coordination Isomerism

This type only shows up in salts where both the cation and anion are complex ions. The ligands redistribute between the two metal centers.

• $[\text{Co(NH}_3)_6][\text{Cr(CN)}_6]$ vs. $[\text{Cr(NH}_3)_6][\text{Co(CN)}_6]$

The overall composition is identical, but the ligand environments around Co and Cr are swapped, which changes properties like color and magnetic behavior.

Ionization Isomerism

Here, a ligand in the inner coordination sphere trades places with a counter ion in the outer sphere. The key test: dissolve each isomer in water and they produce different ions.

• $[\text{Co(NH}_3)_5\text{Br}]\text{SO}_4$ releases $\text{SO}_4^{2-}$ in solution (precipitates with $\text{Ba}^{2+}$)
• $[\text{Co(NH}_3)_5\text{SO}_4]\text{Br}$ releases $\text{Br}^-$ in solution (precipitates with $\text{Ag}^+$)

Same empirical formula, but a simple qualitative test distinguishes them.

Hydrate (Solvate) Isomerism

A special case of ionization isomerism involving water. Water molecules can sit inside the coordination sphere (as ligands) or outside it (as lattice water). The classic example uses $\text{CrCl}_3 \cdot 6\text{H}_2\text{O}$, which exists in three forms:

You can distinguish these by adding excess $\text{AgNO}_3$ and counting how many equivalents of $\text{AgCl}$ precipitate.

Stereoisomerism

Stereoisomers have the same connectivity (same bonds) but differ in the spatial arrangement of their ligands. There are two main categories: geometric isomers and optical isomers.

Geometric Isomerism (Cis-Trans)

Geometric isomers differ in the relative positions of ligands around the metal center. This type requires at least two different kinds of ligands.

In square planar complexes (coordination number 4):

The textbook case is $[\text{Pt(NH}_3)_2\text{Cl}_2]$:

• Cis isomer: the two $\text{Cl}^-$ ligands sit adjacent (90° apart). This is cisplatin, a widely used anticancer drug.
• Trans isomer: the two $\text{Cl}^-$ ligands sit opposite (180° apart). This is transplatin, which is biologically inactive.

Same formula, dramatically different biological activity. Geometry matters.

In octahedral complexes (coordination number 6):

For complexes of the type $[\text{MA}_2\text{B}_4]$, cis and trans isomers exist just as in square planar cases. But octahedral geometry also introduces another possibility.

Fac-Mer Isomerism

When an octahedral complex has three of one ligand and three of another (type $[\text{MA}_3\text{B}_3]$), you get fac and mer isomers instead of cis/trans:

• Facial (fac): the three identical ligands occupy one triangular face of the octahedron. Each identical ligand is 90° from the other two.
• Meridional (mer): the three identical ligands span a plane that passes through the metal center. Two are trans to each other (180°), and the third is cis to both (90°).

$[\text{Co(NH}_3)_3(\text{NO}_2)_3]$ can exist in both fac and mer forms, and they have different physical properties (different dipole moments, different spectra).

Optical Isomerism

Optical isomers (enantiomers) are non-superimposable mirror images of each other. A complex is chiral when it lacks an improper rotation axis ($S_n$), which in practice often means it has no internal mirror plane.

Enantiomers share nearly all physical properties (melting point, solubility, IR spectra) but differ in one key way: they rotate plane-polarized light in equal and opposite directions. The two forms are labeled Δ (right-handed helix) and Λ (left-handed helix).

Diastereomers vs. Enantiomers

These terms describe relationships between pairs of stereoisomers:

• Enantiomers: mirror images of each other. Same physical properties except for optical rotation and interactions with other chiral species.
• Diastereomers: stereoisomers that are not mirror images. They have different physical and chemical properties (different solubilities, melting points, reactivities).

Cis- and trans-$[\text{Pt(NH}_3)_2\text{Cl}_2]$ are diastereomers. The Δ and Λ forms of $[\text{Co(en)}_3]^{3+}$ are enantiomers.

Chirality and Coordination Geometry

When Are Coordination Compounds Chiral?

Chirality in coordination compounds depends on both the geometry and the ligand arrangement. The most common chiral complexes are octahedral; square planar complexes are rarely chiral because the plane of the complex usually acts as a mirror plane.

Octahedral complexes that are typically chiral:

• Tris-bidentate complexes like $[\text{Co(en)}_3]^{3+}$ or $[\text{Ru(bpy)}_3]^{2+}$. Three bidentate chelate rings wrap around the metal in a helical fashion, and there's no mirror plane. These are the most straightforward examples of chiral octahedral complexes.
• Cis-bis-bidentate complexes such as cis-$[\text{CoCl}_2(\text{en})_2]^+$. The cis arrangement breaks the symmetry. (Note: the trans isomer of this complex does have a mirror plane and is achiral.)
• Complexes with unsymmetrical polydentate ligands can also be chiral, even with fewer chelate rings.

Square planar complexes are almost always achiral because the molecular plane serves as a mirror plane. Chirality in square planar geometry requires unusual ligands that are themselves asymmetric, such as chelates with a chiral carbon in the backbone.

Assigning Δ and Λ Configuration

For tris-bidentate octahedral complexes, you assign the absolute configuration by viewing the complex along the $C_3$ axis (looking down the axis that relates all three chelate rings):

1. Orient the complex so you're looking straight down the three-fold axis.
2. Identify the three chelate rings, each of which traces a short arc from one donor atom to the other.
3. If these arcs trace a right-handed (clockwise) helix as they go from the near end to the far end, the configuration is Δ.
4. If the arcs trace a left-handed (counterclockwise) helix, the configuration is Λ.

This distinction has real chemical consequences. Biological systems are chiral environments, so Δ and Λ enantiomers of metal complexes often interact differently with enzymes, DNA, and other biomolecules.