Determination Of Magnetic Moments In Metal-Metal Bonded Complexes

1
Determination Of Magnetic Moments In Metal-Metal
Bonded Complexes

• Daniel Villanueva1
• 1The Johns Hopkins University
• Department Of Chemistry
• Baltimore, MD

2
Background Information / Introduction

• The idea that metal atoms could individually bond
  to other metal atoms was one that arose
  comparatively late in the development of
  inorganic chemistry
• It was not until 1913 that a compound was
  discovered to have a metal-metal bond
• Compounds are currently known containing not only
  metal-metal single bonds, but also double,
  triple, and even quadruple bonds

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Problem / Hypothesis

• Rhodium(II) acetate ethanolate and copper(II)
  acetate monohydrate were synthesized and
  characterized using Infrared Spectroscopy (IR)
  and Nuclear Magnetic Resonance (NMR), to
  determine specific bonding and their magnetic
  moments respectively. These magnetic moments
  were used to determine if a metal-metal bond
  exists.

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Experimental Procedure

• Part A Rhodium(II) Acetate Ethanolate
• 50mg of rhodium(III) chloride hydrate, 100mg of
  sodium acetate trihydrate, 1mL of glacial acetic
  acid, 1mL of absolute ethanol were refluxed for
  1 hour
• Part B Copper(II) Acetate Monohydrate
• 160mg of copper(II) sulfate, 1mL of ammonia,
  80mg of sodium hydroxide flakes were stirred and
  heated for 25 minutes
• IR NMR spectra were collected
• Coaxial NMR tube (See Figure 3) ? Inner Tube
  sample DMSO Outer Tube only DMSO

Figure 3 Coaxial NMR Tube
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Results

• Rhodium(II) Acetate Ethanolate
• Blue-green crystals
• IR Spectrum C-H bond (2904cm-1), O-C-O
  bond (1720cm-1)
• Copper(II) Acetate Monohydrate
• Deep blue crystals
• IR Spectrum C-H bond (2950cm-1), OC
  (1736cm-1), C-O bond (1260cm-1)

Figure 1 Structure of rhodium(II) acetate
alcoholate L ethanol
Figure 2 Structure of copper(II) acetate
monohydrate
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Results
Signal of solvent protons in outer tube
NMR Spectrum Of Rhodium(II) Acetate Ethanolate
Signal of solvent protons in inner tube
NMR Spectrum Of Copper(II) Acetate Monohydrate
Signal of solvent protons in outer tube
Signal of solvent protons in inner tube
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Discussion Of Results

• The nuclei of the NMR solvent in the two
  compartments are shielded differently due to the
  different volume susceptibility
• Resulting shift difference of the absorption
  signals is related to the magnetic moment, µ, of
  the paramagnetic substance in the inner tube
• µ av(T??/c) a constant (2522x10-4
  mol1/2K-1/2ml-1/2cps-1/2 T absolute
• temperature ?? shift difference
  (in cps) 1ppm 10cps c
• concentration of solute
• Rhodium(II) acetate ethanolate
• T 293.8K ?? 8.35cps c 4.1404x10-5mol/ml
• µ 1.941 Bohr magnetons
• Rh-Rh bond exists because the magnetic moment is
  small
• Copper(II) Acetate Monohydrate
• T 293.8K ?? 11.45cps c 2.5247x10-5mol/ml
• µ 2.599 Bohr magnetons
• Cu-Cu bond is absent because the magnetic moment
  is large

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Conclusion

• Rhodium(II) acetate ethanolate contains a
  metal-metal bond
• The unpaired electrons of rhodium(II) (d7) are
  strongly coupled and the complex is diamagnetic
• Copper(II) acetate monohydrate does NOT contain a
  Cu-Cu bond
• The unpaired electrons of copper(II) (d9) are
  weakly coupled and the highly populated low
  energy excited state is paramagnetic