ELECTRON-PARAMAGNETIC-RES SPECTRUM OF THE ION-PAIR [CR(CO)5-]2 IN IRRADIATED CRYSTALS OF PPH4 + HCR(CO)5-
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UVirradiated crystals of PPh4+HCr(CO)5 exhibit anisotropic EPR spectra at room temperature and below which are characteristic of the presence of an electronic triplet (S = 1). From measurements of the angular dependence of the spectrum it is established that the carrier has axial symmetry with g = 2.0019, g = 2.0202, |D| = 163.0 MHz, and with the unique direction parallel to the c axis of the tetragonal (P4/n) crystal. The magnitudes of the zerofield parameter D and the gtensor components show that the carrier is a biradical consisting of a pair of Cr(CO)5 radicals separated approximately by one unit cell length along c. A weak singleline spectrum which accompanies the triplet spectrum has the identical gtensor, and is assigned to the Cr(CO)5 radical itself. A third spectrum with the same gtensor, which is present in irradiated crystals only, is ascribed to another electronic triplet with |D| = 84 and E 0 MHz. It is associated with a second pair of exchangecoupled Cr(CO)5 radicals whose interionic axis lies skew in the crystal axis system. Measurements of the signal intensities as a function of temperature show that both biradicals have singlet ground states and small isotropic exchange parameters, J 35 and 11 cm1, respectively. Copyright 1988 John Wiley & Sons, Ltd.