PYRAMIDALITY EFFECT ON RH(II)-RH(II) SINGLE BONDS

A structural database and molecular orbital study of the binuclear Rh(II) compounds of the type Rh2X8Ln (n = 0-2), where X2 may be one bridging bidentate ligand or two monodentate ligands, is presented. The structures of 139 crystallographically independent molecules from 130 compounds were analyzed in search for a structural correlation between the Rh-Rh bond distance and the Rh-Rh-X pyramidality angle, and the effect of the axial ligands on both structural parameters was studied. The largest family, of the tetrakis(chelate) compounds with N or I donor atoms (113 molecules), and the much smaller one with two carboxylato ligands and two metallated phosphines (6 molecules), both show a strong susceptibility of the Rh-Rh bond strength to pyramidalization. No clear correlation between bond distance and pyramidality angle was found for the family of complexes with two diamines and two carboxylato ligands (eight molecules). A theoretical study using extended Huckel methodology was carried out on model compounds. The effect of the chelating nature of the equatorial X ligands was analyzed, as well as the influence of axial ligands. A simple orbital model based on the hybridization of the p(z) orbital of the RhX4 moieties accounts for the pyramidality effect on the Rh-Rh bond strength.