Which Cobalt Complex Is Which?
A student prepares two cobalt(III) ammine complexes. Sample X is made from CoCl3·5NH3 and sample Y from CoCl3·4NH3. Exactly 0.010 mol of each pure solid is treated with excess AgNO3(aq); X gives 2.87 g AgCl and Y gives 1.43 g AgCl (M(AgCl) = 143.5 g mol⁻¹). Conductivity measurements in water show that X gives three ions per formula unit and Y gives two. Both samples are diamagnetic. The student must assign a structural formula to each, decide whether either can exist as geometrical isomers, and explain why magnetic measurements do not help distinguish them. The teacher warns that the number of chloride ions precipitated is not the same as the total chloride in the formula.
1. Deduce the structural formula of X and Y and state how many chloride ions are outside the coordination sphere in each.
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For X: moles of AgCl = 2.87/143.5 = 0.020 mol. Since 0.010 mol of X gives 0.020 mol AgCl, two chloride ions per formula unit are ionic and precipitated by Ag⁺. The other chloride is covalently bound inside the coordination sphere. Formula: [Co(NH3)5Cl]Cl2. This gives one complex cation and two chloride ions in solution, i.e. three ions per formula unit, matching the conductivity.
For Y: moles of AgCl = 1.43/143.5 = 0.010 mol. Thus one chloride per formula unit is ionic. Formula: [Co(NH3)4Cl2]Cl. This gives one complex cation and one chloride ion, i.e. two ions per formula unit, again matching conductivity.
The AgCl test counts only the ionisable chloride, not the chloride ligands directly bonded to cobalt(III).
2. Which sample can show geometrical isomerism? Describe the isomers and explain how a physical measurement could distinguish them.
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Sample Y, [Co(NH3)4Cl2]Cl, can show geometrical isomerism because the octahedral complex [Co(NH3)4Cl2]⁺ has two Cl ligands that may be adjacent or opposite. In the cis isomer the two Cl ligands occupy adjacent positions, so the complex ion has a net dipole moment. In the trans isomer the two Cl ligands are opposite, and the symmetry cancels the dipole moment.
A dipole-moment measurement can therefore distinguish them: the cis form should have a nonzero dipole moment, while the trans form should have zero or near-zero dipole moment. Sample X, [Co(NH3)5Cl]Cl2, has only one Cl ligand in the coordination sphere, so it has no comparable cis/trans geometrical pair.
3. Explain why both samples are diamagnetic and why magnetic measurements cannot distinguish X from Y.
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Cobalt(III) is a d⁶ ion. In an octahedral field with NH3 and Cl⁻ ligands, the d⁶ configuration is low-spin, so all six d electrons are paired. The complex therefore has no unpaired electrons and is diamagnetic, with magnetic moment close to zero.
Both X and Y contain low-spin Co(III) centres, so both are diamagnetic. A magnetic moment measurement would give essentially the same result for both and cannot distinguish the number of coordinated chlorides or the cis/trans arrangement. The AgNO3 precipitation and conductivity data are much more useful here because they count the ionisable chloride ions and the number of ions per formula unit.