CHE 450G: Practical Inorganic Chemistry

U of KY Dept of Chemistry

Preparation and characterization of Mn(acac)₃

Addenda to Girolami et. al. Experiment #12

Introduction and Background

Acetylacetonate (acac) is a common monoanionic bidentate ligand derived from the monodeprotonation of acetylacetone (2,4-pentanedione). The binding of this ligand to a transition metal ion can be thought of as consisting of a covalent bond through one oxygen and a dative bond through the other oxygen. In actuality, the bonding is usually delocalized and is most correctly drawn using the delocalized resonance structure shown on the right.



As you should recall from Introductory or Inorganic chemistry, Ligand Field Theory explains that several factors, including the nature of the ligand and geometry of the complex, affect the relative energies of the d-orbitals in a metal complex. The number of unpaired spins in a complex will sometimes be differ depending on the geometry of the complex and whether the valence electrons occupy the d-orbitals with high versus low spin ordering. Therefore, in some instances, experimental determination of the number of unpaired spins (i.e. the magnetic susceptibility) may aid in assigning the structure and spin state of the complex ion.

In this experiment you will synthesize Mn(acac)₃ according to the following unbalanced equation:



Your job will be to synthesize this material, determine the number of unpaired electrons in this complex and (if possible) assign a structure and spin state.

Necessary Reading

Before doing the Evans method:
1. Errington, p 196 (or all of 11.2.1)
2. The NMR instruction sheet.

BEFORE performing the Guoy experiment:

1. The Guoy balance instruction sheet
2. Jolly, Chapter 25

Optional but useful information (for motivated students):

1. West, Chapter 16.1 and 16.2 (more on magnetism)

Synthesis and Characterization

Equipment and Chemicals required

• Potassium Permanganate (on the chemical shelf along the long bench).
• 2,4-pentandione (acetylacetone, found in the solvent cabinet).
• Distilled water (Use the good stuff in the large carboy).
• A fume hood
• Glassware found in your lab bench
• A sealed capillary of CDCl₃ (on chemical shelves). Return this when you are finished with the experiment.
• CDCl₃ (on the chemical shelf)
• HgCo(NCS)₄ (Obtain from your instructor). Please REUSE the standard.

General Considerations

• CAUTION: Potassium permanganate is a strong oxidant! If you spill some on your skin, wash it off immediately with copious amounts of water. Do not allow it to contact organic solvents (such as an organic waste bottle) as it could result in a fire and explosion.

• CAUTION: 2,4-pentanedione is a toxic and flammable liquid that should be dispensed in a fume hood.

• CAUTION: The HgCo(NCS)₄ standard contains toxic heavy metals.

• While your book gives you the precise amounts of each reagent, it is still up to you to determine the number of mmol etc. in your notebook and to balance the chemical equation provided.

Magnetic Susceptibility

Determine the magnetic susceptibility of your product using both the Evans and the Guoy method

NOTE!!!

• Our NMR spectrometer (like most modern ones) has a superconducting magnet. Therefore, follow the procedure in the third paragraph of the Evans Method instructions on page 126, not the second one. Be sure to use Q = 2 in Equation 10!

  

• Pay very careful attention to the units in equation 10. The line before the equation says the units will come out to be cm³ mol^-1, but it tells you to use mol/L (not mL) for the concentration. This is correct -- use M.

• TEXT ERROR: Footnote a of Table 12-1 is incorrect. The value should be -13 x 10^-6. The negative sign was omitted from the exponent by accident.

• Make sure you read the values in Table 12-1 correctly. For example, the value of chi for acac is -52 x 10^-6. The table is correct as written, but some people have been confused by the header notation.

Additional Characterization Data

1. Obtain an Infrared spectrum of your product (Nujol mull or KBr pellet. KBR is strongly recommended). Instructions for preparing IR samples can be found in Experiment 19 of your laboratory text. Hmmm, now that you have that data what should you do with it?

2. Obtain a melting point measurement of your material (Hint: get thee to the library to find some literature comparison for your lab report).

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