Identify enantiomerism in octahedral complexes

Chelating ethane-1,2-diamine, abbreviated en, occupies two adjacent donor positions and can wrap around an octahedral metal centre.

GCE A-Level H3 Chemistry 9813-2027

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Learning objectives

Identify enantiomerism in octahedral complexes

Test the complete three-dimensional complex

Chelating ethane-1,2-diamine, abbreviated en, occupies two adjacent donor positions and can wrap around an octahedral metal centre.

Chirality is established only when a structure and its mirror image cannot be superimposed after any rotation.

Coordination enantiomers

Enantiomers are non-superimposable mirror images with opposite handedness; Δ and Λ label the right- and left-handed helical arrangements of three chelate rings.

A symmetry plane or inversion centre makes a structure achiral, while the absence of an obvious symmetry element is not by itself a complete proof of chirality.

Trace chelate connectivity

In [Ni(en)₃]²⁺, three bidentate en ligands occupy all six octahedral donor sites and form Δ and Λ arrangements that are mirror images but cannot be superimposed.

In cis-[Ni(en)₂(H₂O)₂]²⁺, the adjacent water positions leave a handed arrangement of two chelate rings; its mirror pair is enantiomeric, whereas a trans arrangement has a symmetry element and is achiral.

Do not try to judge chirality from one chelate ring in isolation. Trace all donor connections around the metal, construct the mirror image and test whether rotation can superimpose the entire connected structure.

Recognise a tris-chelate mirror pair

Draw one [Ni(en)₃]²⁺ structure with the three N–Ni–N chelate edges winding clockwise when viewed along the threefold axis.

Its reflected anticlockwise winding has identical bonds and formula but cannot be matched by rotation, so the pair is Δ/Λ enantiomers.

Try this

Classify the Δ and Λ forms of [Ni(en)₃]²⁺.

Check your answer

A strong answer gives non-superimposable mirror images, conserved connectivity and the term enantiomers.

Test the cis bis-chelate ion

Supplied structure: octahedral cis-[Ni(en)₂(H₂O)₂]²⁺, with the two water ligands adjacent and each en joined N to N through its carbon chain.

Draw the mirror image of the complete structure, keeping both chelate paths intact, then try to superimpose it on the original by proper rotation only.

Try this

Explain why the two reflected cis-[Ni(en)₂(H₂O)₂]²⁺ drawings form an enantiomeric pair.

Check your answer

With adjacent waters, the two intact en rings wrap the metal in a propeller sense. The mirror image reverses that sense, and no rotation superimposes it on the original. Mirror-related, non-superimposable structures are an enantiomeric pair (Δ and Λ). Moving donor labels without keeping the chelate connections is not evidence.

Use a symmetry control

Compare cis-[Ni(en)₂(H₂O)₂]²⁺ with trans-[Ni(en)₂(H₂O)₂]²⁺, where the two waters sit opposite each other.

For each isomer, inspect the complete chelate arrangement for a mirror plane or centre of symmetry before deciding whether it has a non-superimposable mirror image.

Try this

Contrast cis and trans [Ni(en)₂(H₂O)₂]²⁺ for chirality.

Check your answer

The cis isomer has no mirror plane or centre, so it is chiral and exists as an enantiomeric pair. The trans isomer has a mirror plane through the four nitrogen donors, so its mirror image is superimposable. It is achiral, not a second Δ/Λ pair.

Do not infer chirality from octahedral shape alone

An ideal octahedron is itself highly symmetric; chirality arises from the ligand identities, positions and chelate connections.

Fac/mer identification is outside this lesson and does not replace a mirror-image superimposability test.

Try this

Better reasoning: ‘Every octahedral complex containing en is optically active.’

Check your answer

Reject it and test the fully connected structure for a superimposable mirror image or symmetry element.

Make the chirality proof reproducible

Draw all six donor positions, label identical ligands, join both donor atoms of every en ligand and compare the reflected structure under rotation.

The next course step applies stereochemical reasoning to reaction transition states through the Hammond postulate.

For an enantiomerism answer, state the octahedral connectivity, identify the mirror relationship and explain why no rotation superimposes the pair. Use Δ/Λ only after the handed winding has been established.

Try this

State the evidence needed to claim complex enantiomerism.

Check your answer

Your answer should include correct octahedral geometry, chelate connectivity, a true mirror pair, unsuccessful rotational superposition and an appropriate Δ/Λ label where applicable.