EPR spectrum of the ion pair [CR(CO)5−]2 in irradiated crystals of PPh4+ HCr(CO)5−

Abstract

AbstractUV‐irradiated 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 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.