ELECTRON-DISTRIBUTION IN THE CR(CO)(4)(BPY)(CENTER-DOT-)(BPY EQUALS 2,2'-BIPYRIDINE) RADICAL-ANION AS REVEALED BY EPR SPECTROSCOPY AND IR SPECTROELECTROCHEMISTRY OF (CO)-C-13-ENRICHED SPECIES

Samples of Cr(CO)(4)(bpy) (bpy = 2,2'-bipyridine) with natural abundan ce of (CO)-C-13 and with 16 and 79%;(CO)-C-13 enrichment, respectively , were reduced to the corresponding radical anion, [Cr(CO),(bpy)](.-) and studied by EPR and IR spectroscopies. Electron paramagnetic resona nce showed a considerable coupling of the unpaired electron with C-13( CO) of 0.601 mT which occurs to the (CO)-C-13 ligands bound in axial p ositions, i.e. cis to bpy. It indicates an interaction between the sin gly occupied pi orbital of the bpy(.-) ligand and the sigma and/or si gma orbitals of the axial C-Cr-C bonds. Coupling to the Cr-53 nucleus is rather small, 0.128 mT. Infrared spectra in the v(CO) region were used to calculate the CO stretching and interaction force constants wi thin the energy factored CO force field approximation. The stretching force constants of the equatorial and axial CO ligands were found to d ecrease by 68 and 21 N m(-1), respectively, on going from Cr(CO)4(bpy) to [Cr(CO),(bpy)](.-). It follows that the reduction strengthens sign ificantly the pi-back donation to the equatorial (trans to bpy) CO lig ands.