Suturina, E.A.; Nehrkorn, J.; Zadrozny, J.; Liu, J.; Atanasov, M.; Weyhermueller, T.; Maganas, D.; Hill, S.; Schnegg, A.; Bill, E.; Long, J.R.; Neese, F.: Magneto-Structural Correlations in Pseudotetrahedral Forms of the [Co(SPh)4]2- Complex Probed by Magnetometry, MCD Spectroscopy, Advanced EPR Techniques, and ab Initio Electronic Structure Calculations. Inorganic Chemistry 56 (2017), p. 3102-3118

The magnetic properties of pseudotetrahedral Co(II) complexes spawned intense interest after (PPh4)2[Co- (SPh)4] was shown to be the first mononuclear transition-metal complex displaying slow relaxation of the magnetization in the absence of a direct current magnetic field. However, there are differing reports on its fundamental magnetic spin Hamiltonian (SH) parameters, which arise from inherent experimental challenges in detecting large zero-field splittings. There are also remarkable changes in the SH parameters of [Co(SPh)4]2− upon structural variations, depending on the counterion and crystal- lization conditions. In this work, four complementary experimental techniques are utilized to unambiguously determine the SH parameters for two different salts of [Co(SPh)4]2−: (PPh4)2[Co- (SPh)4] (1) and (NEt4)2[Co(SPh)4] (2). The characterization methods employed include multifield SQUID magnetometry, high-field/high-frequency electron paramagnetic resonance (HF-EPR), variable-field variable-temperature magnetic circular dichroism (VTVH-MCD), and frequency domain Fourier transform THz-EPR (FD-FT THz-EPR). Notably, the paramagnetic Co(II) complex [Co(SPh)4]2− shows strong axial magnetic anisotropy in 1, with D = −55(1) cm−1 and E/D = 0.00(3), but rhombic anisotropy is seen for 2, with D = +11(1) cm−1 and E/D = 0.18(3). Multireference ab initio CASSCF/NEVPT2 calculations enable interpretation of the remarkable variation of D and its dependence on the electronic structure and geometry.