Electron paramagnetic resonance studies of radical pairs [M(CO)–5]2(M = Cr, Mo, W) trapped in single crystals of PPh+4HM(CO)–5

γ-Irradiated single crystals of PPh+4HM(CO)–5(M = Cr, Mo, W) exhibit powerful EPR spectra due to two distinct interacting pairs of M(CO)–5 radicals. g- and fine-structure (D) matrices were determined from measurements at 77 K of EPR spectra obtained as a function of angle in the ac and aa′ planes of crystallographically aligned single crystals of the Mo and W compounds. The data are compared with those obtained previously for the Cr compound. In all three cases, g and one of the D-tensors are aligned with the tetragonal c-axis of the crystal, and the principal g-values are appropriate for d7 square-pyramidal M(CO)–5 radicals. Classical dipolar calculations show that the two radical pairs correspond to the closest and next-closest pair of undamaged anions in the crystals, the former lying along c and the latter along a skew direction. The D-values for the Mo and W closest pairs indicate a substantial contribution from anisotropic exchange to the coupling of the two unpaired electrons.