3-DIMENSIONAL HUCKEL THEORY FOR CLUSTER COMPOUNDS

Applications of Huckel molecular orbital theory to planar aromatic hyd rocarbons have been of enormous conceptual value to organic chemistry. The success of 2-dimensional Huckel theory is mainly a result of mole cular connectivity or topology. But most of chemistry is 3-dimensional . In the widely applied 3-dimensional extended Huckel method, much of the significance of topology is lost or at least obscured in choices o f bond distances, Coulomb integrals, and calibration parameters. We pr esent a 3-dimensional version of Huckel theory developed directly from the approximations of simple Huckel theory plus a few additional assu mptions. The significance of molecular topology is retained. The 3-dim ensional Huckel theory we describe is designed specifically for polyhe dral molecular clusters. Following an exposition of the method, we pre sent applications to specific classes of clusters for which results ca n be compared with those from experiment and from ab initio molecular orbital calculations. For the closo-boranes, BnHn2-, the 3-dimensional Huckel model correctly chooses experimentally observed structures fro m a large set of plausible polyhedral structures. The results also con firm the well-known rule of n + 1 skeletal electron pairs in the closo -boranes. For a group of transition metal clusters, the model provides a rationalization for the empirically observed numbers of cluster bon ding electrons.