SMALL HETEROBORANE CLUSTER SYSTEMS .1. THEORETICAL-STUDY OF BRIDGED AND CAGE-INSERTED PHOSPHABORANE CLUSTER COMPOUNDS WITH SEMIEMPIRICAL MOLECULAR-ORBITAL CALCULATIONS

The structural, thermodynamic, and electronic properties of 91 phosphaborane cluster compounds have been calculated by using the MNDO-SCF semiempirical molecular orbital calculational method. The geometry-optimized minimum energy structures for all the known, structurally characterized phosphaborane systems have been calculated, and exceptionally good agreement between experimentally determined and calculated structural parameters has been observed. Calculations for five classes of small phosphaborane clusters, with fewer than nine vertex atoms, have been completed and have been related to experimentally and spectroscopically proposed structural types. The relationship between bridged and inserted phosphaborane structures has been related to the variation in molecular orbital energies as a function of the (phosphorus-bridged boron atom plane)-(basal boron plane) dihedral angle. Complete MO correlation diagrams that relate the orbitals for bridged phosphaborane systems to inserted systems have been constructed. Linear relationships have been observed between the calculated charge on the phosphorus atom and both the phosphorus-apical boron bond distance and the bridgehead basal boron bond distance. The hydrogen atoms bridging basal boron atoms in the phosphorus-inserted clusters were found to be deflected toward the boron atoms bonded to the phosphorus. The boron atoms bonded to the phosphorus atom, therefore, more closely resemble BH2 units with two terminal protons rather than BH units that contain only one terminal proton each. Predictions concerning structural and chemical reactivities for unknown phosphaborane compounds have been made on the basis of these MO calculations.