QUANTITATIVE STUDIES OF CHEMICAL-REACTIVITY OF TETRA-MU-BUTYRATO-DIRHODIUM(II) COMPLEXES

We have studied the thermodynamics of adduct formation for 1:1 and 2:1 adducts formed by Lewis bases with Rh2(C4H7O2)4 in benzene solutions. Electrochemical studies of these adducts were also carried out in CH2CI2. Corrections for a benzene-acid interaction were necessary to obtain solvent minimized enthalpies of acid-base adduct formation. The thermodynamic data clearly demonstrates substantial changes in the acidic and redox properties of the second metal as a result of base coordination to the first. The metal-metal bonding in the system causes this dimer to be a most unusual Lewis acid, as evidenced by deviations of the E and C predicted enthalpies from those observed. The unusual Lewis acid properties are attributed to the enhanced π-back-bonding capability of the rhodium(II) center as a result of extensive mixing of orbitals with symmetry on the two metal centers. This causes the rhodium(II) center to be very effective in π-back-bonding to the axial ligands. The reduction potentials of Rh2(C4H7O2)+, Rh2(C4H7O2)4B+, and Rh2(C4H7O2)4B2+ are analyzed, and provide further support for the extensive -back-bonding capabilities of this metal cluster. Copyright © 1979, American Chemical Society. All rights reserved.