Coordinatively Unsaturated [rhcp*rf(2)] (cp* = C5Me5; Rf = C6F3Cl2-3,5), General Precursor to Cp*-Diaryl and Cp*-Halo-Aryl Rh-III Complexes. Observing and Testing the Effect of Cp* As Electronic Buffer

The pentacoordinated [RhCp*Rf2] (Rf = C6F3Cl2-3,5) and the octahedral (μ-Cl)2[RhCp*Rf]2, obtained by stoichiometric rearrangement with (μ-Cl)2[RhCp*Cl]2, are general precursors of [RhCp*RfXL] (X = Rf, Cl; L = ligand) complexes, which were studied by NMR (L dissociation and fluxional processes) and X-ray diffraction (structural effects affecting the Rh–Cp* distances) techniques. The Rh–Cp*centroid distances decrease markedly for identical L in the order [RhCp*Rf2L] > [RhCp*RfClL] > [RhCp*Cl2L] and are further influenced regularly within each family by the trans influence of L (longer distances for higher trans influence of L). The structural effects observed reveal a remarkable capability of Cp* to act as an electron-density buffer, which attenuates the Rh electron density variations induced by the substituents in front of Cp* by releasing toward Rh or polarizing toward Cp*, on demand, the electron density of the Rh–Cp* bonds. This buffer effect explains the easy L dissociation from [RhCp*Rf2L] and the accessibility to formally 16e pentacoordinated [RhCp*Rf2].