Cobalt-59 Nuclear Magnetic Resonance of Sulphur, Selenium and Arsenic Chelates of Cobalt (III)

Abstract

Freeman, Murray and Richards¹, following a suggestion of Orgel, have confirmed that for first period ligands a linear relation exists between the nuclear resonance frequency ν_j and the wavelength λ_j of the ¹T_1g←¹A_1g absorption maximum of octahedrally coordinated cobalt (III) complexes where γ is the magnetogyric ratio of the cobalt-59 nucleus and H₀ is the external field (fixed to ±0.1 oersted at 12,575·₆ oersted in the experiments described here). The diamagnetic contribution A_j to the shielding constant is expected to be largely independent of the ligand environment, for its magnitude is controlled by 〈r_k⁻¹〉 for 1s, 2s, 3s, 2p and 3p electrons: a reasonable estimate¹ leads to A_j = 0.0021. A high degree of paramagnetic shielding B₀/Δ_j arises from second order Zeeman effect paramagnetism. Crystal field calculations^2,3 give with 1/Δ_j being approximated by the wavelength in cm of the ¹T_1g←¹A_1g band. Griffith³ has noted that any deviation from pure 3d orbitals can be accommodated by introducing an orbital reduction factor kσπ (between t_2g and e_g orbitals), so that where β is the Bohr magneton. Accordingly, broad line ⁵⁹Co NMR spectra should provide a sensitive probe for variations in either kσπ and/or r_3d with the nature of the attached ligand.