The EPR spectrum of the novel radical Mes*(CH₃)P—PMes* (Mes*=2,4,6-(tBu)₃C₆H₂) was measured in the temperature range 100-300 K, and was found to be drastically temperature dependent as a result of the large anisotropy of the ³¹P hyperfine tensors. Below 180 K, a spectrum of the liquid solution is accurately simulated by calculating the spectral modifications due to slow tumbling of the radical. To achieve this simulation, an algorithm was developed by extending the well-known nitroxide slow-motion simulation technique for the coupling of one electron spin to two nuclear spins. An additional dynamic process responsible for the observed line broadening was found to occur between 180 K and room temperature; this broadening is consistent with an exchange between two conformations. The differences between the isotropic ³¹P couplings associated with the two conformers are shown to be probably due to an internal rotation about the P—P bond.

Several radiation defects have been detected by EPR in a single-crystal of Pt(dmimt)4Cl2.4H2O (dmit = 1,3-dimethyl-imidazoline-2-thione). In order to identify these rediogenic species, the structure of the crystal has been resolved and the angular dependence of the EPR signals has been analysed. The resulting g tensors and 195Pt hyperfine tensors have been determined and the orientations of their principal axes have been compared to those of the bond directions of the precursor. It is shown that both Pt(I) and Pt(III) complexes are trapped, whereas Pd(dmimt)42+ present as an impurity, leads only to the Pd(I) species. The temperature dependende of the EPR spectra gives information about the relative stability of the paramagnetic species and shows that the formation of some species, especially the Pt(III) complexes, requires drastic modifications of the parent Pt(II) cation.