Definitive spectroscopic determination of zero-field splitting in high-spin cobalt(II)

A high-spin Co(II) complex (3d(7), S = 3/2), Co(PPh3)(2)Cl-2 (Ph = phenyl), has been investigated in the solid state by both high-frequency and -field electron paramagnetic resonance (HFEPR) and by variable-temperature, variable-field magnetic circular dichroism (VTVH-MCD). In HFEPR spectroscopy, the combination of variable sub-THz frequencies generated by backward wave oscillators (150-700 GHz, corresponding to energy 5-23 cm(-1)) and high magnetic fields (0-25 T) constitutes a novel experimental technique allowing accurate determination of a complete set of spin Hamiltonian parameters for this complex: D = -14.76(2) cm(-1), E = 1.141(8) cm(-1), g(x) = 2.166(4), g(y) = 2.170(4), g(z) = 2.240(5). Independent VTVH-MCD studies on multiple absorption bands of the complex yield D = -14(3) cm(-1), E = 0.96(20) cm(-1) (\E/D\ = 0.08(2)), g(x) = 2.15(5), g(y) = 2.16(4), and g(z) = 2.17(3). This very good agreement between HFEPR and MCD indicates that there is no inherent discrepancy between these two quite different experimental techniques. Thus, depending on the nature of the sample, either can be reliably used to determine zero-field splitting parameters in high-spin Co(II), with the HFEPR being more accurate but VTVH-MCD being more sensitive.