Transannular effects in dicobalta-superphane complexes on the mixed-valence class II/class III interface: Distinguishing between spin and charge delocalization by electrochemistry, spectroscopy, and ab initio calculations

The oxidative electron-transfer properties of several superphane complexes consisting of cyclopentadienylcobalt cyclobutadiene moieties linked by eith er three (compounds 1 and 4) or five (compounds 2 and 5) bridging methylene groups have been studied by experimental and theoretical methods. In both cases two separate one-electron oxidations are found. The mixed-valent mono cations of the (-CH2-)(5)-bridged complexes are valence-trapped with very w eak interactions between metal centers. The (-CH2-)(3) complexes, however, have strong interactions between the two molecular halves. The intervalence transfer (IT) band of 1(+) has characteristics of both class II (localized ) and class III (delocalized) behavior, but the IR spectra of carboxy-label ed 4(+) clearly establish trapped valence for the monocations of the propan e-bridged systems. Photoelectron spectra and ab initio calculations at the UHF level show that, in the ground electronic state, 1(+) has a half-filled orbital (i.e., electron spin) that is essentially localized in one Co d(xy ), orbital, but that the charges on the two metals are unequal owing to ind uctive electronic effects which give unequal electron flow from the ligands to the two metal centers. Calculations and IR (carbonyl) spectral shifts s uggest about a 70:30 charge ratio between the two metal centers in 1(+) and 4(+), whereas both spin and charge localization is virtually complete in t he pentano-bridged complexes 2(+) and 5(+). The intervalence transition in 1(+) is proposed to proceed through a "hole"-transfer process mediated by a pi-cyclobutadiene MO, ultimately involving a through-bond transannular mec hanism.