Interpretation of 59Co NMR shielding using the hard and soft acid–base concept. Insight into the relative magnitude of the nephelauxetic and the spectrochemical effect

Abstract

A modified Freeman–Murray–Richards (FMR) correlation for the analysis of ⁵⁹Co NMR shielding parameters, δ(⁵⁹Co)vs.(Δv^*_1/2)^1/2/ΔE_av, is formulated by combining the Ramsey shielding theory and the Townes and Dailey theory for the calculation of the nuclear quadrupole coupling constant. It is demonstrated that the experimental ⁵⁹Co NMR half-height linewidth is a useful parameter to account for the nephelauxetic effect in the shielding variation in Co^III complexes. The correlation shows that the differences in the cobalt chemical shifts are characteristic of the hardness or softness of the metal–ligand interaction. While Bramley and Juranić modified the FMR plot by introducing a second optical parameter, β₃₅, to account for the nephelauxetic effect, we invoke the quadrupolar metal NMR linewidth to achieve the same degree of modification. This treatment provides a practical means of separating the spectrochemical effect from the nephelauxetic effect. The former is shown to span a range of ca. 3500 ppm and the latter ca. 800–1200 ppm covering the complexes in the series [Co(NH₃)₅X]^{(3 +n)+}, [Co(CN)₅X]^{(2 –n)–} and trans-[Co(en)₂X₂]^{(3 + 2n)+}. In total, the ⁵⁹Co NMR data of 37 diamagnetic hexacoordinated Co^III complexes in five different solvents are analysed. The model is verified independently using complexes from the series trans-[Co(acac)₂X₂]^{(1 + 2n)±} as well as trans-[Co(acac)₂(NH₃)(NO₂)].