Is the purple solid [Co(phen)2Cl2] or [Co(phen)3]Cl2?
CoCl2·6H2O (1.00 g, 4.20 mmol) and 1,10-phenanthroline (1.51 g, 8.40 mmol) are refluxed in methanol (40 mL) for 30 min. On cooling, a purple crystalline solid forms. After filtration and vacuum drying at 60 °C, the yield is 1.86 g. Found: C 57.5, H 3.9, N 10.7%. The solid is a nonelectrolyte in dry nitromethane (ΛM = 16 Ω−1 cm2 mol−1 at 1.0 mM). Its magnetic moment is 4.75 μB per Co at 298 K. The UV–Vis spectrum in acetonitrile shows bands at 520 nm (ε ≈ 120 M−1 cm−1) and 610 nm (ε ≈ 60 M−1 cm−1). A colleague suggests the product is [Co(phen)3]Cl2 because phen was used in twofold excess; another suggests [Co(phen)2Cl2]·CH3OH.
1. Use the conductance and magnetic data to decide between [Co(phen)2Cl2] and [Co(phen)3]Cl2. Explain what each measurement rules out and what it cannot prove.
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ΛM = 16 Ω−1 cm2 mol−1 is in the nonelectrolyte range; [Co(phen)3]Cl2 would be a 1:2 electrolyte and should give roughly 150–250 Ω−1 cm2 mol−1 in nitromethane. Thus the chlorides are coordinated, not ionic. μeff = 4.75 μB is far above the low-spin Co(III) value (near 0) and is consistent with high-spin Co(II) (three unpaired electrons, spin-only 3.87 μB, with orbital contribution often 4.7–5.2). Conductance alone cannot distinguish [Co(phen)2Cl2] from a neutral seven-coordinate or dimeric species, and magnetism does not prove the coordination number. Together they favor [Co(phen)2Cl2].
2. Calculate the expected C, H, N percentages for [Co(phen)2Cl2] and its monomethanol solvate, and decide whether the combustion data support solvent inclusion. What additional experiment would confirm it?
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For [Co(phen)2Cl2], C24H16Cl2CoN4, M = 490.26: C 58.80, H 3.29, N 11.43%. Adding one CH3OH gives C25H20Cl2CoN4O, M = 522.30: C 57.49, H 3.86, N 10.73%. The found C 57.5, H 3.9, N 10.7% matches the monosolvate, not the unsolvated formula. The 60 °C vacuum drying did not remove all methanol, or the sample reabsorbed it. Confirm by TGA (expected mass loss 32.04/522.30 = 6.13%), by 1H NMR for a methanol singlet, or by drying a sample at a higher temperature under vacuum until constant mass and repeating CHN.