MOLECULAR MODELING OF DIMETAL SYSTEMS - DIMOLYBDENUM QUADRUPLE BONDS

The concept of quadruple bonding at a dimetal center has been used ver y successfully to account for the eclipsed conformation of unbridged d imers, contrary to steric demands. The situation is more complicated i n the case of bridged dimers with sterically dictated eclipsed structu res. Electronic factors operating through bridging, as well as axial, ligands have such a serious influence on observed bond lengths and con formations that, in this case, the effective bond order of the formall y quadruply-bonded systems is moot. A strategy is defined whereby diff erent factors contributing to the observed trends can be separately as sessed. This is achieved by systematic molecular modeling of a large n umber of Mo-2 and Cr-2 derivatives to establish a general mathematical relationship among strain-free bond lengths, force constants, and bon d orders. The modeling of compounds with Mo-Mo/4 bonds and well-refine d crystallographic structures is reported here. All structural details independent of the dimolybdenum bond are simulated in terms of a tran sferable force field. Simulation of the dimetal-bond properties is the n achieved by trial-and-error procedures, in terms of a family of matc hed pairs of harmonic force constant (k(r)) and characteristic bond le ngth (r(0)), for each bond. These solution curves have different slope s for bridged and unbridged compounds, and they intersect within a suf ficiently small region to define a characteristic solution pair of k(r ) = 4.07 mdyn Angstrom(-1) and r(0) = 2.02 Angstrom. A torsional twist through the angle chi, away from eclipsing, is calculated to reduce t he delta stabilization by a factor of cos 2(chi) from a maximum of 50 kJ mol(-1) at (chi 0).