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

Abstract

Optimised molecular structures and charge distributions within Cr(CO)₄(bpy) and its radical anion were calculated using density functional theory (DFT). It was found that, although reduction predominantly concerns the bpy ligand, its structural and electronic effects extend to the Cr(CO)₄ fragment. 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 that the corresponding CO stretching force constants decrease by 68 and 21 N m^–1, respectively. The calculated spin density in Cr(CO)₄(bpy)^˙– resides predominantly on the bpy ligand which behaves spectroscopically as bpy^˙–. The spin density is delocalised to both axial and equatorial pairs of CO ligands by mixing of π*(CO) orbitals with the, predominantly π*(bpy), SOMO. In addition, part of the spin density is delocalised selectively to the axial CO ligands by an admixture of their σ orbitals into the SOMO. This σ–π* contribution is responsible for isotropic EPR hyperfine splitting which was observed from the axial ¹³C(CO) atoms only. Accordingly, the isotropic hyperfine splitting constants correlate with calculated Fermi contact terms instead of total spin densities. Complete active space self-consistent field (CASSCF)-calculated changes in charge distribution upon a Cr→bpy MLCT excitation show that the electron density localised on the bpy ligand increases by about the same amount upon reduction or MLCT-excitation of Cr(CO)₄(bpy). The axial CO ligands are depopulated by MLCT excitation ca. 1.6 times more than the equatorial ones. These conclusions can be generalised and applied to other coordination and organometallic complexes of low-valent metals which contain a reducible or radical-anionic ligand.