Transcript Magnetic Moment

A.
B.
Magnetic Susceptibility
Why are we doing this experiment?
1) Magnetic Properties reveal numbers of unpaired electrons
2) The number of unpaired electrons tell us about oxidation state, geometry, ligand
field strength, etc…
How are we doing this experiment?
1) We are using a Johnson-Matthey MSB-Auto Magnetic Susceptibility Balance
2) It uses the Evan’s detector which is a modified Gouy Method
C.
How do we work up the data?
1. The MSB-Auto output data is Volume Susceptibility = cV
Example: [Mn(B13N4)Cl2]PF6 cV = 3.80 x 10-6 (unitless) average value
2.
We will need the volume of our sample:
V = pr2l = (3.1416)(0.162cm)2(3.01cm) = 0.248 cm3
observed
3.
We will need the density of our sample:
d = m/V = (0.1549g)(0.248cm3) = 0.624 g/cm3
paramag
diamag.
4.
We can calculate Mass Susceptibility = cg
cg = cV/d = (3.80 x 10-6)/(0.624g/cm3) = 6.09 x 10-6 cm3/g
5.
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
6.
We now need to correct for diamagnetic influences of the ligands
a. We are wanting the metal only, but the ligand is interfering
b. Ligand atoms (and even the metal core electrons) are diamagnetic
c. We need to add back in the sum of the ligand diamagnetism
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 106 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
9.
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
D.
How do we interpret the results?
1. We can compare the magnetic moment with literature values for that ion
2. We can decide if the expected oxidation state of the metal matches
3. We can decide what the geometry of the complex is
4. We can decide if the complex is high spin or low spin
5. We can decide if the ligand(s) is/are weak or strong field
E.
Conclusions
1. 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.
2.
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.
3.
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.
4.
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.