Co(III) Complex with an Unexpected Magnetic Moment
You isolate a purple crystalline complex from the reaction of CoCl2·6H2O with excess ammonia and ammonium chloride, followed by oxidation with 30% H2O2 and acidification with HCl. The proposed formula is [Co(NH3)5Cl]Cl2 (M = 250.4 g/mol). A 0.100 g sample dissolves in water to give a molar conductance of 260 S cm2 mol-1 at 25 °C, consistent with a 1:2 electrolyte. The magnetic moment is 4.8 μB per cobalt. The UV-Vis spectrum shows two d-d bands at 530 and 360 nm. The synthesis used 2.00 g CoCl2·6H2O (M = 237.9 g/mol), 5.0 g NH4Cl, 15 mL concentrated NH3, and 5 mL 30% H2O2; after isolation, the yield is 1.35 g. A colleague says the magnetic moment proves the product is Co(II) and should be discarded. Another says the conductance proves the chloride is coordinated and the product is fine. The lab needs the Co(III) complex for a kinetics study.
1. Interpret the magnetic and conductance data. What oxidation state and ion type are indicated, and are the two measurements consistent?
Read the worked answer
Co(III) is d6. In an octahedral field with strong-field ligands such as NH3, it is low-spin t2g6, with no unpaired electrons, so the magnetic moment should be essentially zero. A moment of 4.8 μB is far too high for low-spin Co(III). High-spin Co(II) is d7 with three unpaired electrons; its spin-only moment is sqrt(3 × 5) = 3.87 μB, and orbital contributions commonly raise the observed moment into the 4.4-5.2 μB range. Thus the magnetic data indicate a substantial Co(II) component.
The molar conductance of 260 S cm2 mol-1 is in the range expected for a 1:2 electrolyte, which fits [Co(NH3)5Cl]Cl2: the complex cation plus two chloride ions give three ions total. A 1:3 electrolyte such as [Co(NH3)6]Cl3 would give four ions and a higher conductance. So the conductance supports the desired ion type and supports chloride being coordinated in the inner sphere. The two measurements are not necessarily contradictory if the sample is a mixture: a Co(III) complex of the correct ion type plus a Co(II) impurity can show the expected conductance while the impurity dominates the magnetic moment. The data are therefore insufficient to accept the product as pure Co(III); the magnetism must be explained before the complex is used.
2. Calculate the theoretical yield and percent yield based on cobalt. Does the 1.35 g result change your interpretation?
Read the worked answer
Moles of CoCl2·6H2O = 2.00 g / 237.9 g/mol = 8.41 mmol. If all cobalt becomes [Co(NH3)5Cl]Cl2, theoretical mass = 8.41 mmol × 250.4 mg/mmol = 2.105 g. Percent yield = 1.35 g / 2.105 g × 100 = 64.1%.
A 64% yield is plausible for a coordination-complex preparation, so the mass alone does not prove a problem. However, a 64% yield is also consistent with partial oxidation or loss of a Co(II) fraction during workup. If the product were pure low-spin Co(III), the magnetic moment should be near zero regardless of yield. The high moment therefore points to incomplete oxidation, insufficient H2O2, or introduction of Co(II) during isolation. The yield calculation does not resolve the oxidation-state question; it only shows that the amount isolated is not abnormally high. The next step should be purification and re-measurement of magnetism, not acceptance based on yield.
3. What purification and verification plan would you use before releasing the complex for kinetics? Include a procedural change if the problem persists.
Read the worked answer
First, recrystallize the solid from warm dilute HCl or from water adjusted to low pH, in which the Co(III) complex is soluble but Co(II) impurities are more soluble in the mother liquor. Dry the crystals and re-measure the magnetic moment. If the moment falls toward zero, the impurity was Co(II) and the recrystallization was sufficient. Also run a Co(III) redox assay: Co(III) oxidizes iodide to iodine, which can be titrated with thiosulfate; compare the result with total cobalt by EDTA titration. A Co(III) fraction significantly below 100% indicates residual Co(II).
If the moment remains high after recrystallization, repeat the oxidation step. Use fresh 30% H2O2, add a small amount of activated charcoal as a catalyst if your procedure allows it, and heat at about 60-70 °C long enough to oxidize Co(II) to Co(III). Then boil to decompose excess H2O2 before adding HCl, because residual peroxide can interfere with crystallization and later redox assays. Add HCl only after oxidation is complete. Confirm the ion type again by conductance and confirm the inner-sphere chloride by the conductivity value and, if available, by chloride precipitation from the free chloride ions. Do not release the complex for kinetics until both the magnetic moment and the Co(III) assay agree with the proposed formula.