THEORETICAL-STUDY OF MULTICENTER BONDING USING A DELOCALIZED MO APPROACH

The nature of the bonding in about thirty electron-deficient molecules has been studied on the basis of two-centre and multi-centre (mainly three-centre) bond indices and atomic valencies calculated from ab initio SCF density matrices using 4-31G (or 6-31G) and 6-31G* (or 6-31G**) basis sets. For comparison, the density matrices corresponding to non-orthogonal and symmetrically orthogonalised basis functions have been employed. The nature of the bonding deduced on the basis of bond indices and valencies has been confirmed by localised MO calculations. It has been observed that a three-centre bond is invariably associated with a positive and significantly high value (greater-than-or-equal-to 0.1) of the bond index. For a four-centre bonding interaction the corresponding bond index is generally positive, but rather small.