METAL-METAL BONDING IN DIRHODIUM TETRACARBOXYLATES - TRANS INFLUENCE AND DEPENDENCE OF THE RH-RH BOND DISTANCE UPON THE NATURE OF THE AXIAL LIGANDS

The structures of metal-metal bonded dirhodium tetracarboxylate complexes, Rh2(OAc)4X2, where X = py, NHEt2, CO, P(OPh)3,PF3, and P(OMe)3, have been determined by three-dimensional X-ray crystallography. The rhodium-rhodium distances are found to vary with the nature of X from 2.3963 (2) to 2.4555 (3) Å, the latter being the longest yet observea in such tetracarboxylate dirhodium complexes. The variation of M-M distance is rationalized on the basis of a qualitative trans-influence theory which considers σ and π interactions and their mutual dependence upon M-X bond distance. The Rh-Rh distances are much shorter than the 2.7 Å expected for a single Rh-Rh bond, a result easily explained if a triple Rh-Rh bona is assumed. The details of the spectra, inconsistent with a triple bond formulation, have been found to be easily explained by a single bond formulation originally proposed by Dubicki and Martin and more recently supported by the theoretical work of Norman and Kolari. Although a simple interpretation of our structural results appears to favor the triple bond assignment, a more careful consideration of the metal-bridging ligand interactions shows that the data are also consistent with a single Rh-Rh bond of extraordinary strength. The shortness and high strength of the Rh-Rh bond are judged to be due in part to the extensive mixing of metal and bridging ligand orbitals and in part to Rundle-type interactions that stabilize the axially stacked arrangement of formally nonbonded Ni(II), Pd(II), and Pt(II) d8 complexes. As a consequence of the complicated orbital interactions, the comment is made that the formal bond order is not a particularly useful measure of the metal-metal interactions in these complexes. © 1979, American Chemical Society. All rights reserved.