Transcript Determination Of Magnetic Moments In Metal

Determination Of Magnetic
Moments In Metal-Metal
Bonded Complexes
Daniel Villanueva1
1The
Johns Hopkins University
Department Of Chemistry
Baltimore, MD
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
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.
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
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
Figure 1: Structure
of rhodium(II)
acetate alcoholate
[L = ethanol]
– Deep blue crystals
– IR Spectrum: C-H bond (2950cm-1),
O=C (1736cm-1), C-O bond (1260cm-1)
Figure 2: Structure
of copper(II)
acetate
monohydrate
Results
NMR Spectrum
Of Rhodium(II)
Acetate
Ethanolate
NMR Spectrum
Of Copper(II)
Acetate
Monohydrate
Signal of
solvent
protons in
outer tube
Signal of
solvent
protons in
inner tube
Signal of
solvent
protons in
outer tube
Signal of
solvent
protons in
inner tube
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 = a√(T∆ν/c) *a = constant (2522x10 mol K ml cps ; T = absolute
-4
1/2 -1/2
-1/2
-1/2
temperature; ∆ν = shift difference (in cps) [1ppm = 10cps]; c =
concentration of solute
• Rhodium(II) acetate ethanolate:
– T = 293.8K; ∆ν = 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; ∆ν = 11.45cps; c = 2.5247x10-5mol/ml
– μ = 2.599 Bohr magnetons
– Cu-Cu bond is absent because the magnetic moment is large
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