BONDING PROPERTIES OF 3,5-DI-TERT-BUTYL-1,2-BENZOSEMIQUINONE RADICAL-ANIONIC LIGAND - RESONANCE RAMAN-SPECTRA OF RE(CO)4(DBSQ) AND RE(CO)3(PPH3)(DBSQ) COMPLEXES

Absorption spectra in the visible region and resonance Raman spectra of the spin-doublet Re(CO)4-n(PPh3)n(DBSQ) (n = 0,1) complexes were measured in order to obtain information on the nature of their electronic transitions and excited states. Qualitative Raman excitation profiles measured across the visible absorption band of both complexes point to the presence of two electronic transitions. Based on the rR data, both transitions are identified as d(pi) --> pi*SOMO of the DBSQ ligand. These transitions have essentially MLCT character, with only very small amount of mixing between the metal and ligand-SOMO orbitals. The spectral assignment is also supported by the shift of the spectral band going from the tetracarbonyl to the tricarbonyl-phosphine complex and by the solvatochromism observed for the latter one. From these spectral data, the DBSQ radical-anion emerges as a ligand lacking any pronounced pi-bonding properties toward the Re central atom. At best, it can be viewed as a very weak pi-acceptor. This bonding picture is also supported by the EPR and IR data.