A brief review is presented of a novel method of high-frequency magnetic resonance spectroscopy, which sweeps the frequency at a fixed magnetic field, including zero field. We describe the main features of this frequencydomain spectrometer which works in the spectral range from 30 GHz to 1.5 THz and at magnetic fields up to 8 T; the temperature can be as low as 0.4 K. The versatility of this technique is demonstrated by means of a number of examples from the field of molecular magnetism.

Novel far-infrared (FIR) absorption spectroscopy in conjunction with multiple, fixed external magnetic fields (FIR magnetic spectroscopy, FIRMS) has been used to investigate pseudotetrahedral complexes with the formula M(PPh(3))(2)Cl(2) (M = Ni, Zn; Ph = C(6)H(5)). Crystal structures have been reported for the Ni complex; we report the structure of the Zn complex. Transmission spectra at 5 K of Ni(PPh(3))(2)Cl(2) (S = 1) at zero magnetic field exhibit absorption bands at 11.41, 15.28, and 23.0 cm(-1). The two lower frequency bands show great sensitivity to external magnetic field, and their field dependence is as expected for electron spin transitions allowing precise determination of the following parameters: |D| = 13.35(1) cm(-1), |E| = 1.93(1) cm(-1), g(x,y) = 2.20(1), g(z) = 2.00(1). Corresponding spectra of Zn(PPh(3))(2)Cl(2) (S = 0) exhibit bands only at >20 cm(-1), which show no field dependence. FIRMS is a promising technique for direct investigation of the electronic structure of high-spin transition metal complexes.