ELECTRONIC-STRUCTURE OF RU2(O2CR)4+ AND RH2(O2CR)4+ COMPLEXES

SCF-Xα-SW calculations of Ru2(O2CH)4Cl2-, Ru2(02CH)4+, Ru2(O2CH)4, Rh2(O2CH)4(H2O)2+, and Rh2(02CH)4+are used to discuss the bonding, magnetic properties, stable oxidation level, axial ligand trans influence, electronic spectra, and ESR spectra of ruthenium and rhodium carboxylate dimers. The Ru(2.5) dimers are predicted to have a σ2π4δ2π*2δ*1 configuration, corresponding to three unpaired electrons as found by experiment. For the Rh(2.5) dimers, the predicted configuration is σ2π4δ2π*2δ;*1. Reasons for the close spacing of μ*and δ*orbitals, and the energy order μ*< δ*, are discussed. Favorable exchange energy and susceptibility of the Ru(ll) dimer to Jahn-Teller distortion are seen as factors stabilizing the Ru(2.5) oxidation level. An explanation is given for the observed changes in Rh-Rh, Rh-OH2, and Rh-0(acetate) distances between Rh2(O2CR)4(H2O)2and Rh2(O2CR)4(H2O)2+. Comparison of Ru2(O2CH)4Cl2-and Rh2(O2CH)4- (H2O)2+ indicates that Cl-is slightly better than H2O at weakening a trans metal-metal bond. The observed order of Rh-Rh distances in Rh2(O2CCH3)4L2, L = H2O, py, NHEt2, CO, PR3, is interpreted as consistent with the Xα-SW energy level diagram. The prominent band near 2 μm-1in the visible spectrum of Ru2(O2CR)4+complexes is assigned to an 0π →π* transition, where “Oπ;” is a mainly Ru-0 orbital, but with significant Ru-Ru π; character. The weak band near 1 μm-1in the aqueous solution spectrum is assigned to mainly the δ;→δ* transition. The bands near 1.9 and 2.5 μm-1in the acidic solution spectrum of Rh2(O2CCH3)4(H2O)2+are assigned to>π*→ σ*and π;*→ Rh-O π;* transitions, respectively, as for the analogous bands of Rh2(O2CCH3)4(H20)2- The extra band which appears for the cation near 1, 3 μm-1is tentatively assigned to the δ; → δ;* transition. Good agreement is obtained between calculated and experimental g values for the Ru(2.5) dimers. Problems in developing a suitable model for calculation of hyperfine coupling constants are discussed. © 1979, American Chemical Society. All rights reserved.