Questions: Isomerism in Coordination Compounds

Question 1

The compound [Co(NH₃)₄Cl₂]⁺ can exist as two geometric isomers. What are they, and how do their properties differ?

• A. cis (both Cl⁻ adjacent, violet) and trans (both Cl⁻ opposite, green) isomers with identical chemical reactivity
• B. cis (both Cl⁻ adjacent, violet) and trans (both Cl⁻ opposite, green) isomers with different colors, dipole moments, and reactivity
• C. fac and mer isomers, differing only in their NMR spectra
• D. d and l optical isomers that rotate plane-polarized light in opposite directions

Answer: (B)

In an octahedral complex with four NH₃ and two Cl⁻, the two chlorides can be adjacent (cis, 90° Cl-Co-Cl angle) or opposite (trans, 180° angle). These geometric isomers are physically distinct compounds: the cis isomer is violet and has a net dipole moment, while the trans isomer is green and has no dipole (the Cl-Co-Cl dipoles cancel). They also differ in reactivity — cis isomers undergo different substitution pathways. Option C describes fac/mer isomerism, which applies to MA₃B₃ octahedral complexes (three of each ligand), not MA₄B₂. Option D describes optical isomers, which are not possible for this compound.

Question 2

An octahedral complex [Co(en)₃]³⁺ (where en = ethylenediamine) has no geometric isomers because all donor atoms are nitrogen. However, it can exist as two optical isomers (enantiomers).

• T. True
• F. False

Answer: True

All six donor atoms in [Co(en)₃]³⁺ are equivalent nitrogen atoms, so there is no possibility of geometric isomerism — every arrangement of three identical bidentate ligands around an octahedron gives the same connectivity pattern. However, the three en ligands create a propeller-like arrangement that is non-superimposable on its mirror image, making the complex chiral. The two enantiomers (designated Δ and Λ) rotate plane-polarized light in equal and opposite directions. This is significant in bioinorganic chemistry because biological systems often interact selectively with one enantiomer.

Question 3

Linkage isomers like [Co(NH₃)₅(NO₂)]²⁺ and [Co(NH₃)₅(ONO)]²⁺ differ in which atom of the ambidentate ligand bonds to the metal.

• T. True
• F. False

Answer: True

Nitrite (NO₂⁻) is an ambidentate ligand — it can coordinate through nitrogen (nitro, -NO₂) or through oxygen (nitrito, -ONO). The two linkage isomers have the same molecular formula and overall charge but different metal-ligand bonds: Co-N in the nitro form and Co-O in the nitrito form. These isomers have different colors (yellow nitro vs red nitrito), different stabilities, and can interconvert — the nitrito isomer typically converts to the more stable nitro isomer over time or upon heating.

Question 4

Explain why square planar complexes of the type [MA₂B₂] exhibit geometric (cis/trans) isomerism, but tetrahedral complexes of the same type do not.

Model answer: In a square planar complex, the four coordination positions are not all equivalent with respect to each other: two positions are adjacent (90° apart, cis) and two are across from each other (180° apart, trans). Placing two B ligands in adjacent vs opposite positions creates geometrically distinct, non-interconvertible arrangements. In a tetrahedron, all four positions are equivalent — every pair of positions is separated by the same angle (109.5°). There is no 'opposite' position in a tetrahedron, so placing two B ligands in any two positions gives an arrangement that can be rotated to match any other placement. Therefore, only one isomer exists.

This geometric argument also explains why octahedral MA₂B₄ complexes show cis/trans isomerism (90° vs 180° positions exist) while trigonal bipyramidal complexes show axial/equatorial isomerism (distinct position types). Isomerism requires that the coordination geometry creates distinguishable positions.