HOW A BIDENTATE LIGAND MAY FAVOR THE UNUSUAL CIS ISOMER IN DIOXO D(2)OCTAHEDRAL TRANSITION-METAL COMPLEXES - AN EHMO STUDY

d(2)cis-Octahedral dioxo transition metal complexes are much more rare than the corresponding trans complexes. Qualitative EHMO calculations show how a bidentate ligand could reverse the usual stability order o f the two isomers (Mingos's analysis) or, at least, make them closer i n energy. The geometrical and electronic properties of a bidentate lig and that work in favor of the cis isomer are a small bite angle and go od sigma donor and pi acceptor properties (through the intermediary of a vacant pi orbital symmetrical with respect to the plane bisecting t he bite angle). These calculations also indicate why the O=M=O equilib rium angle value in diamagnetic d(2) CiS complexes is larger than in t he much more usual d(0) complexes. This opening of the O=M=O angle res ults in a rather large HOMO-LUMO energy gap, which might explain the u nexpected diamagnetism of the characterized d(2) cis complexes. Finall y, opposite conformational preferences were found for single-faced pi donor and pi acceptor ancillary ligands in the cis complexes; pi donor s lie in the plane bisecting the O=M=O angle while pi accepters are pe rpendicular to that plane. These results are in agreement with the few experimental results available.