DYNAMIC BEHAVIOR, REDISTRIBUTION REACTIONS, AND INTERMETALLIC DISTANCES OF DINUCLEAR BIS(MU-PYRAZOLATO)RHODIUM(I) COMPLEXES

Replacement of the olefin in the complexes [{Rh(mu-RPz)(C2H4)(2)}(2)] (RPz = pyrazolate (Pz) (1), 3-methylpyrazolate (MePz) (2), 3,5-dimethy lpyrazolate (Me(2)Pz) (3)) by tert-butyl isocyanide gives [{Rh(mu-RPz) (CNBu(t))(2)}(2)] (4, 5, 6) respectively. Complex 4 can alternatively be prepared from [{Rh(mu-Pz)(cod)}(2)] (cod = 1,5-cyclooctadiene) (7) leading to a chemical equilibrium between. 4, 7, and the intermediate [(cod)Rh(mu-Pz)(2)Rh(CNBu(t))(2)] (8), which has been also isolated. K inetic studies on this apparent ligand redistribution reaction, leadin g to 8, show that it follows a second order rate, giving the activatio n parameters Delta H-not equal = 21.8 kcal . mol(-1), Delta S-not equa l = -7.4 eu, and Delta G(not equal 298) = 24.0 kcal . mol(-1), suggest ing that the dinuclear complexes are the active species, and no fragme ntation seems to occur. The molecular structures of complexes 3, 4, 8, and [{Rh(mu-Pz)(CO)(2)}(2)] (9) determined by X-ray diffraction show discrete dinuclear complexes with the six-membered ''Rh(mu-Pz)(2)Rh'' ring showing a boatlike conformation. Complexes 3 and 4 present the sh ortest and the largest intermetallic nonbonding separations, 3.0961(2) and 3.8995(6) Angstrom, respectively, so far reported in di-mu-pyrazo lato-dirhodium(I) complexes. In addition, complex 3 shows the shortest olefinic C=C distance found in ethylene rhodium complexes and accordi ngly a very low activation energy, 10.0 kcal . mol(-1), for the rotati on of the ethylene ligands. Complex 8 undergoes two independent intram olecular fluxional processes associated to the ring inversion of the s ix-membered ''Rh(mu-Pz)(2)Rh'' metallacycle and to a sigma-1,2-metallo tropic shift showing activation parameters Delta H-not equal = 15.0 kc al . mol(-1), Delta S-not equal = -1.7 eu and Delta H-not equal = 19.7 kcal . mol(-1), Delta S-not equal = 4.8 eu, respectively. The first m ovement is influenced by medium effects and may be restricted by the a ncillary ligands and the substituents on the pyrazolate rings in other bis(mu-pyrazolato) complexes. Finally, the intermetallic distance in dinuclear pyrazolato complexes is analyzed in terms of steric and elec tronic factors.