METAL-METAL BONDING IN DIRHODIUM TETRACARBOXYLATES - STRUCTURE OF THE BIS(DIETHYLAMINE) ADDUCT OF TETRA-MU-ACETATO-DIRHODIUM(II) AND SYSTEMATICS OF THE BONDING IN TETRACARBOXYLATE-BRIDGED METAL DIMERS

The structure of the bis(diethylamine) adduct of dirhodium tetraacetate has been analyzed by X-ray crystallography and the results are compared with those for the structures of the diaquo and bis(pyridine) adducts. The Rh-Rh bond, at 2.4020 (7) Å, is greater than in the bis(pyridine) complex and 0.0165 (9) Å greater than in the diaquo complex, in accord with the ranked σ-donor abilities of these ligands. The Rh-N bond is about 0.2 Å longer than expected for mononuclear Rh(I) or Rh(III) complexes, consistent with the very strong Rh-Rh bond. The tetraacetate framework of the diethylamine adduct is very similar to that of the other adducts. Examination of the bonding parameters of more than 40 tetracarboxylate-bridged metal dimers reveals that, despite differences in the R group of the carboxylate and differences in the metals, the M-M-O angle is a remarkably linear function of the M-M distance. The O-C-O angles for each R depend linearly upon the M-M distance, and the slope of O-C-O vs. M-M is characteristic of R, decreasing in the order H ⨠ CH3 ~ Ph > CF3. The M-O-C angles appear to be the most flexible of the interbond angles, adjusting to accommodate the requirements of both the particular R group and the particular M-M distance. The O-C-O angles and C-O bond lengths for the dirhodium tetraacetates are found to be anomalous when compared with those of the other tetraacetate-bridged complexes. The tetraacetate framework could without difficulty accommodate a much longer Rh-Rh distance than is observed, suggesting that the bridging acetates do not constrain the Rh-Rh distance to be 0.3 Å less than expected for a single Rh-Rh bond. Bis(diethylamine)tetra-μ-acetato-dirhodium(II) crystallizes with four molecules per unit cell in the orthorhombic space group Pbcn. The molecule possesses a crystallographic center of symmetry. The cell constants are a = 16.329 (4) Å, b = 8.011 (4) Å, and c = 17.660 (6) Å for X(MoKa) 0.71069 A. Data were collected by automated diffractometer and corrected for Lorentz-polarization and absorption effects. The structure was solved by the heavy-atom Patterson method and refined by convential Fourier least-squares techniques to a final R factor of 0.066 for 3346 unique reflections. © 1979, American Chemical Society. All rights reserved.