Structural and electronic changes accompanying reduction of Cr(CO)(4)(bpy)to its radical anion: a quantum chemical interpretation of spectroelectrochemical experiments

Optimised molecular structures and charge distributions within Cr(CO)(4)(bp y) and its radical anion were calculated using density functional theory (D FT). It was found that, although reduction predominantly concerns the bpy l igand, its structural and electronic effects extend to the Cr(CO)(4) fragme nt. Each equatorial and axial CO ligand was calculated to accept 7.1 and 4. 8%, respectively, of the extra electron density in Cr(CO)(4)(bpy)(.-). This is in accordance with the IR spectroelectrochemical results which show tha t the corresponding CO stretching force constants decrease by 68 and 21 N m (-1), respectively. The calculated spin density in Cr(CO)(4)(bpy)(.-) resid es predominantly on the bpy ligand which behaves spectroscopically as bpy(. -). The spin density is delocalised to both axial and equatorial pairs of C O ligands by mixing of pi*(C=O) orbitals with the, predominantly pi*(bpy), SOMO. In addition, part of the spin density is delocalised selectively to t he axial CO ligands by an admixture of their sigma orbitals into the SOMO. This sigma-pi* contribution is responsible for isotropic EPR hyperfine spli tting which was observed from the axial C-13(CO) atoms only. Accordingly, t he isotropic hyperfine splitting constants correlate with calculated Fermi contact terms instead of total spin densities. Complete active space self-c onsistent field (CASSCF)-calculated changes in charge distribution upon a C r --> bpy MLCT excitation show that the electron density localised on the b py ligand increases by about the same amount upon reduction or MLCT-excitat ion of Cr(CO)(4)(bpy). The axial CO ligands are depopulated by MLCT excitat ion ca. 1.6 times more than the equatorial ones. These conclusions can be g eneralised and applied to other coordination and organometallic complexes o f low-valent metals which contain a reducible or radical-anionic ligand.