MOLECULAR MECHANICS CALCULATIONS ON CARBORANES AND METALLOCARBORANES

A new, approach has been developed within the framework of the MM3 for ce field which allows us to calculate the molecular conformations of o rganometallic molecules containing 7-vertex carborane and metallocarbo rane residues. A carborane cage in the molecular mechanics approximati on is treated as a superimposition of a 5-membered ring and apical (ca p) groups. Chemical bonds between these fragments are described by a H ill-like potential. The fitting of the force field parameters was base d on the results of ab initio calculations of geometry and energy-dist ance dependences of several simple 7-vertex carborane molecules: 2,4-C 2B5H7, 2,3-C2B5H7, 1-(SiH2)-2,3-C2B4H6 and 1-Mg-2,3-C2B4H6. The elabor ated force field was applied in calculations of conformational feature s of a series of sandwich metallocarboranes of the [RR'C2B4H2](2)M typ e. It was shown that the bent sandwich structure of the Sn derivative is due to nonbonded interligand interactions, as in the case of the me tallocenes.