Electron and energy transfer through bridged systems. 9. Toward a priori evaluation of the intermetallic coupling in bis-metal complexes

It is now commonplace to perform a priori calculations using either density functional or ab initio theory for intramolecular electron-transfer coupling strengths within organic molecules, but generally applicable a priori schemes for intermetallic couplings are yet to be determined. We examine the reasons for this, predominantly the need to treat solvent effects within the calculation, shortcomings of, or the unavailability of, Koopmans' theorem (i.e., the need for configuration interaction), and the difficulty of correctly determining ligand orbital band gaps and positioning metal orbitals within them. In particular, we model the observed intermetallic coupling in the alpha,omega-dipyridylpolyene-bridged ruthenium pentaammine series designed by Woitellier, Launay, and Spangler. After appropriately positioning the metal and ligand orbitals, couplings calculated using Fock matrices are found to be within 25% of experimental values. However,they attenuate too rapidly with increasing bridge length. Use of B3LYP Kohn-Sham matrices as replacements of Fock matrices yields much poorer results, however; the predicted coupling is too strong and even increases for very long bridge lengths.