SINGLE-CRYSTAL POLARIZED ABSORPTION SPECTROSCOPIC STUDY OF THE ELECTRONIC-STRUCTURE OF MU-1,2-PEROXO BINUCLEAR COBALT COMPLEXES

Single-crystal polarized absorption spectra of five trans planar mu-peroxo cobalt systems are presented and interpreted. Ligand field (LF) transitions are assigned at energies typical for Co(III) systems. Charge-transfer (CT) transitions from the peroxide pi*v orbital vertical with respect to the Co-O-O-Co plane to the cobalt d-sigma orbitals are found to be at lower energy than the LF transitions. This indicates that the highest occupied molecular orbital (HOMO) of planar monobridged cobalt peroxo complexes has peroxide pi*v character. Due to the limited overlap between the orbitals involved, the intensity of these CT transitions is low (epsilon almost-equal-to 100 M-1 cm-1). The overlap between the peroxide pi*(sigma) orbital within the Co-O-O-Co plane and the cobalt d(z)2 orbital is large. The intense band (epsilon = 14000 M-1 cm-1) at 300 nm is assigned to the pi*(sigma) --> d(z)2 CT transition, which is supported by the A-term Raman enhancement behavior observed for the intra-peroxide stretch. These results are related to the spectra of cis mu-1,2-peroxo cobalt the trans mu-1,2-superoxo cobalt complexes, and the photochemical implications of the different bonding schemes are discussed. A comparison of the spectra of the trans mu-peroxo cobalt and the trans mu-peroxo binuclear copper systems reveals characteristic differences with respect to dimer splitting in CT excited states.