NMR of nuclei bound to a paramagnetic metal centre: the case of S = 1 square-planar ruthenium(ii).

A series of ruthenium(ii) halide complexes, with a carbazole based PNP pincer ligand, featuring an intermediate-spin (S = 1) ground state, seldomly adopted by 4d metals such as ruthenium, have been synthesized. This has been achieved using the planar tridentate ligand, which combines hard and soft donor atoms, giving rise to a very large zero-field splitting (D > 200 cm-1). The electronic structure and systematic influence of the fourth (halogenido) ligand have been explored theoretically via DFT and CASSCF calculations as well as experimentally via UV-vis spectroscopy and SQUID magnetometry. Analysis of the strikingly well-resolved pNMR spectra allowed the detection of the directly bound 31P nuclei at around -3900 ppm for all discussed complexes, as well as the 19F resonance of the fluorido complex at over +3900 ppm chemical shift, which are a direct probe for the sign and magnitude of the hyperfine coupling constants A iso. Simulation of the room temperature spectra based on DFT and CASSCF calculations allowed for full assignment of the observed resonances and experimental determination of the axial zero-field splitting parameter D from VT-NMR experiments.