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 bridg ing bidentate ligand or two monodentate ligands, is presented. The str uctures of 139 crystallographically independent molecules from 130 com pounds were analyzed in search for a structural correlation between th e Rh-Rh bond distance and the Rh-Rh-X pyramidality angle, and the effe ct 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 l igands and two metallated phosphines (6 molecules), both show a strong susceptibility of the Rh-Rh bond strength to pyramidalization. No cle ar correlation between bond distance and pyramidality angle was found for the family of complexes with two diamines and two carboxylato liga nds (eight molecules). A theoretical study using extended Huckel metho dology was carried out on model compounds. The effect of the chelating nature of the equatorial X ligands was analyzed, as well as the influ ence of axial ligands. A simple orbital model based on the hybridizati on of the p(z) orbital of the RhX4 moieties accounts for the pyramidal ity effect on the Rh-Rh bond strength.