EPR spectrum of the ion pair [CR(CO) 5 − ] 2 in irradiated crystals of PPh 4 + HCr(CO) 5 −

UV‐irradiated crystals of PPh 4 + 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 ( P 4/ n ) crystal. The magnitudes of the zero‐field parameter D and the g‐tensor 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 single‐line spectrum which accompanies the triplet spectrum has the identical g‐tensor, and is assigned to the Cr(CO) 5 − radical itself. A third spectrum with the same g‐tensor, which is present in γ‐irradiated crystals only, is ascribed to another electronic triplet with | D | = 84 and E ≈ 0 MHz. It is associated with a second pair of exchange‐coupled 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 cm −1 , respectively.