Here we report a combined use of THz-EPR and paramagnetic NMR spectroscopy for obtaining a detailed electronic structure of a high-spin cobalt(ii) complex.

Here we report the first successful attempt to identify spin-crossover compounds in solutions of metal complexes produced by mixing different ligands and an appropriate metal salt by variable-temperature nuclear magnetic resonance (NMR) spectroscopy. Screening the spin state of a cobalt­(II) ion in a series of thus obtained homoleptic and heteroleptic compounds of terpyridines (terpy) and 2,6-bis­(pyrazol-3-yl)­pyridines (3-bpp) by using this NMR-based approach, which only relies on the temperature behavior of chemical shifts, revealed the first cobalt­(II) complexes with a 3-bpp ligand to undergo a thermally induced spin-crossover. A simple analysis of NMR spectra collected from mixtures of different compounds without their isolation or purification required by the current method of choice, the Evans technique, thus emerges as a powerful tool in a search for new spin-crossover compounds and their molecular design boosted by wide possibilities for chemical modifications in heteroleptic complexes.