Electron distribution in the Cr(CO)4(bpy)>·>>–>> (bpy = 2,2′-bipyridine) radical anion as revealed by EPR spectroscopy and IR spectroelectrochemistry of 13CO-enriched species
Samples of Cr(CO)4(bpy) (bpy = 2,2′-bipyridine) with natural abundance of 13CO and with 16 and 79% 13CO enrichment, respectively, were reduced to the corresponding radical anion, [Cr(CO)4(bpy)]˙– and studied by EPR and IR spectroscopies. Electron paramagnetic resonance showed a considerable coupling of the unpaired electron with 13C(CO) of 0.601 mT which occurs to the 13CO ligands bound in axial positions, i.e. cis to bpy. It indicates an interaction between the singly occupied π* orbital of the bpy˙– ligand and the σ and/or σ* orbitals of the axial C–Cr–C bonds. Coupling to the 53Cr nucleus is rather small, 0.128 mT. Infrared spectra in the ν(CO) region were used to calculate the CO stretching and interaction force constants within the energy factored CO force field approximation. The stretching force constants of the equatorial and axial CO ligands were found to decrease by 68 and 21 N m–1, respectively, on going from Cr(CO)4(bpy) to [Cr(CO)4(bpy)]˙–. It follows that the reduction strengthens significantly the π-back donation to the equatorial (trans to bpy) CO ligands.