Interpretation of ruthenium NMR chemical shifts in ruthenium(II) complexes

Abstract

Ruthenium(II) complexes belong to the class of octahedral spin-paired d6 transition metal complexes whose metal ion NMR chemical shifts span an extremely large range of values. From the results of the ligand field theory, an interpretation of these large chemical shifts in terms of Ramsey theory of paramagnetic shielding has been straightforward (1). A linear correlation between metal ion chemical shifts and wavelengths of the relevant d-d electronic transition absorption has been predicted and observed in cobalt(III) complexes (2). However, when more data were collected it became clear that the correlation does not hold for complexes differing substantially in the metalligand bond covalency (3-5). An empirical incorporation of the covalency into the expression for paramagnetic shielding, in terms of the nephelauxetic effect, reestablished linear correlation between metal ion chemical shifts and the ligand-field parameters (6-8). In this recent interpretation, the paramagnetic shielding term of the metal nuclei in octahedral spin-paired d6 complexes is expressed through the strong ligand-field