Magnetic Susceptibility Why are we doing this experiment? Magnetic Properties reveal numbers of unpaired electrons The number of unpaired electrons tell us about oxidation state, geometry, ligand field strength, etc… How are we doing this experiment? We are using a Johnson-Matthey MSB-Auto Magnetic Susceptibility Balance It uses the Evan’s detector which is a modified Gouy Method

How do we work up the data? The MSB-Auto output data is Volume Susceptibility = cV Example: [Mn(B13N4)Cl2]PF6 cV = 3.80 x 10-6 (unitless) average value We will need the volume of our sample: V = pr2l = (3.1416)(0.162cm)2(3.01cm) = 0.248 cm3 We will need the density of our sample: d = m/V = (0.1549g)(0.248cm3) = 0.624 g/cm3 We can calculate Mass Susceptibility = cg cg = cV/d = (3.80 x 10-6)/(0.624g/cm3) = 6.09 x 10-6 cm3/g Next we need to calculate Molar Susceptibility = cM cM = (cg)(Mol. Wt) = (5.61 x 10-6 cm3/g)(511.200 g/mol) = 3.11 x 10-3 cm3/mol We now need to correct for diamagnetic influences of the ligands We are wanting the metal only, but the ligand is interfering Ligand atoms (and even the metal core electrons) are diamagnetic We need to add back in the sum of the ligand diamagnetism observed paramag diamag.

(13)+(6)(13)+(28)(2.93)+(4)(4.61)+(2)(23.4) 8. Use the table in your notebook for diamagnetic corrections MnC13H28N4Cl2PF6 (13)+(6)(13)+(28)(2.93)+(4)(4.61)+(2)(23.4) +(26.3)+(6)(9.1) = -319 x 10-6 cm3/mol Calculate the corrected cM’ cM’ = cM - (diamag. Correc) cM’ = (3.11 x 10-3 cm3/mol) – (-319 x 10-6 cm3/mol) = 3.43 x 10-3 cm3/mol 10. Calculate the magnetic moment = meff meff = 2.83[(cM’)(T)]½ meff = 2.83[(3.43 x 10-3cm3/mol)(298K)]½ meff = 2.86 J/T = 2.86 Bohr Magnetons 11. Calculate the number of unpaired e- = n meff = [n(n+2)]½ (meff)2 = n(n+2) = (2.86)2 = 8.19 n ≈ 2

How do we interpret the results? We can compare the magnetic moment with literature values for that ion We can decide if the expected oxidation state of the metal matches We can decide what the geometry of the complex is We can decide if the complex is high spin or low spin We can decide if the ligand(s) is/are weak or strong field

Conclusions Our magnetic moment (ueff = 2.86) is a bit low compared to the table for low spin Mn3+ (3.18). Measurement error due to our inexperience could account for this. However, magnetic moments are variable, and this could be normal. A standard deviation of our 3 measurements might tell us how reproducible they were. The magnetic moment we found and the n = 2 calculated from it, are consistent with low spin Mn3+, which is the ion we were trying to make. Six coordinate Mn3+ is most likely octahedral, due to the usual preference for this ion. In a low spin configuration, n = 2 is consistent with this geometry. Since the complex appears to be low spin, the combination of the B13N4 ligand and the two chloride ligands must be strong field enough that Do is large enough to cause the low spin configuration.