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

3. 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.

4. 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

5. 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), O=C (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

6. 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

7. 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 (2522x10-4 mol1/2K-1/2ml-1/2cps-1/2; T = absolute 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

8. 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