Distinguishing cis and trans bis(ethylenediamine)dichlorocobalt(III) salts
A student prepares two samples of the complex [Co(en)2Cl2]Cl, where en = ethane-1,2-diamine, by two different routes. Sample A is optically inactive, has a dipole moment of approximately zero in a non-aqueous solvent, and shows no change in optical rotation on standing. Sample B has [α]D = +120° and a dipole moment of about 2.6 D under the same conditions. Both samples analyse for exactly one free chloride per formula unit: excess AgNO3 precipitates 1.00 mol AgCl per mole of complex at room temperature, and both behave as 1:1 electrolytes in water. Both are diamagnetic. On standing in water at 25 °C, the optical rotation of B slowly falls to zero over several hours, while its visible absorption spectrum is unchanged. The student must assign the isomers and decide what the rotation decay means.
1. Assign sample A and sample B as cis or trans. Explain the dipole moments, the optical activity and the diamagnetism using symmetry and crystal-field ideas.
Read the worked answer
Sample B is the cis isomer; sample A is the trans isomer.
In cis-[Co(en)2Cl2]⁺ the two chloride ligands are adjacent. The complex has no centre of symmetry and no mirror plane, so it is chiral and exists as Δ and Λ enantiomers. It also has a nonzero dipole moment because the ligand arrangement is not centrosymmetric; the observed 2.6 D is consistent with cis geometry.
The trans isomer has the two chlorides opposite each other. Its symmetry makes the two halves equivalent and gives zero dipole moment. It is achiral, so a pure sample is optically inactive.
Both samples are Co(III), a d⁶ ion. In an octahedral field Co(III) has a large Δo, so the configuration is low spin: t2g⁶ eg⁰, with two paired electrons in t2g and S = 0. Both isomers are therefore diamagnetic. Using CFSE relative to the barycentre, low-spin d⁶ has CFSE = −2.4Δo + 2P, whereas high-spin d⁶ would be −0.4Δo. Because Δo > P for Co(III), the low-spin form is strongly favoured, and the magnetism does not distinguish cis from trans.
2. Both samples give exactly 1 mol AgCl per mole with AgNO3 and conduct as 1:1 electrolytes. Explain why these observations are the same for both isomers and why they cannot be used to assign the geometry.
Read the worked answer
The formula is [Co(en)2Cl2]Cl. Two chlorides are covalently bound inside the coordination sphere and one chloride is the counter-ion outside it. Only the free counter-ion is rapidly precipitated by Ag⁺:
[Co(en)2Cl2]Cl + AgNO3 → [Co(en)2Cl2]NO3 + AgCl(s)
The two coordinated chlorides are not released rapidly at room temperature, so both cis and trans give exactly one mole of AgCl per mole of complex. The same logic explains the conductivity: the complex dissociates into one [Co(en)2Cl2]⁺ cation and one Cl⁻ anion, a 1:1 electrolyte, regardless of whether the two coordinated chlorides are cis or trans.
These measurements confirm the formula and the number of ionizable chlorides, but they are blind to the spatial arrangement of the coordinated ligands. The geometry assignment must come from symmetry-sensitive properties: dipole moment, optical activity, or spectroscopic/chromatographic discrimination of the enantiomers.
3. Interpret the decay of optical rotation of sample B. Is it racemization or cis–trans isomerization? Propose two experiments that would confirm the interpretation, and predict the effect of added chloride ion on the rate.
Read the worked answer
The rotation falls to zero while the visible spectrum is unchanged. That is the signature of racemization: Δ-[Co(en)2Cl2]⁺ is converting to Λ, and at equilibrium the two enantiomers are equal, so the net rotation is zero. The ligand field Chromium? No - the ligand field at cobalt is essentially unchanged during Δ ⇌ Λ interconversion, so the d–d absorption envelope stays the same.
Cis–trans isomerization would change the ligand field because a chloride would move from being trans to N (in cis) to being trans to Cl (in trans). That would normally shift the visible absorption spectrum, change the dipole moment, and give an achiral trans product with different chromatography. The unchanged spectrum therefore argues against isomerization.
Two confirming experiments:
- Circular dichroism: the CD signal should decrease to zero while the UV-visible absorbance remains constant.
- Chiral HPLC or polarimetry after spiking with authentic cis and trans samples: racemization should give only cis enantiomers, whereas isomerization would produce a trans peak.
Added Cl⁻ should slow the racemization if the mechanism involves Co–Cl bond rupture, because chloride is a common ion and suppresses dissociation. If the mechanism is an intramolecular twist, added Cl⁻ should have little effect. Measuring the rate at several chloride concentrations therefore tests the mechanism.