✍ Scribed by J. R. Morton; K. F. Preston; M. Y. Darensbourg
No coin nor oath required. For personal study only.
UV-irradiated crystals of PPh,+HCr(CO),-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 I = 163.0 MHz, and with the unique diection 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 biadical consisting of a pair of Cr(CO),-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),radical itself. A thud spectrum with the same g-tensor, which is present in y-irradiated crystals only, is ascribed to another electronic triplet with I D I = 84 and E z 0 MHz. It is associated with a second pair of exchange-coupled Cr(CO),-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 sz 35 and 11 cm-', respectively.
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