EPR spectra from "EPR-silent" species: High-frequency and high-field EPR spectroscopy of pseudotetrahedral complexes of nickel(II)

High-frequency and high-field electron paramagnetic resonance (HFEPR) spectroscopy (using frequencies of similar to90-550 GHz and fields up to similar to15 T) has been used to probe the non-Kramers, S = 1, Ni2+ ion in a series of pseudotetrahedral complexes of general formula NiL2X2, where L = PPh3 (Ph = phenyl) and X = Cl, Br, and I. Analysis based on full-matrix solutions to the spin Hamiltonian for an S = 1 system gave zero-field splitting parameters: D = +13.20(5) cm(-1), \E\ = 1.85(5) cm(-1), g(x) = g(y) = g(z) = 2.20(5) for Ni(PPh3)(2)Cl-2. These values are in good agreement with those obtained by powder magnetic susceptibility and field-dependent magnetization measurements and with earlier, single-crystal magnetic susceptibility measurements. For Ni(PPh3)(2)Br-2, HFEPR suggested \D\ = 4.5(5) cm(-1), \E\ = 1.5(5) cm(-1), g(x) = g(y) = 2.2(1), and g(z) = 2.0(1), which are in agreement with concurrent magnetic measurements, but do not agree with previous single-crystal work. The previous studies were performed on a minor crystal form, while the present study was performed on the major form, and apparently the electronic parameters differ greatly between the two. HFEPR of Ni(PPh3)212 was unsuccessful; however, magnetic susceptibility measurements indicated \D\ = 27.9(1) cm(-1), \E\ = 4.7(1), g(x) = 1.95(5), g(y) = 2.00(5), and g(z) = 2.11(5). This magnitude of the zero-field splitting (similar to840 GHz) is too large for successful detection of resonances, even for current HFEPR spectrometers. The electronic structure of these complexes is discussed in terms of their molecular structure and previous electronic absorption spectroscopic studies. This analysis, which involved fitting of experimental data to ligand-field parameters, shows that the halo ligands act as strong pi-donors, while the triphenylphosphane ligands are pi-acceptors.