Pentacoordination of boron, carbon, aluminum, and silicon atoms in organiccompounds: an ab initio study

Pentacoordination of boron, carbon, aluminum, and silicon atoms in bicyclic organic compounds of the pentalene type was studied using the ab initio RH F/6-31G** and MP2(full)/6-31G** methods. It was shown that the ability of t he atom to form pentacoordinate structures increases on going from B to Al and from C to Si atom, i.e., as the number of the element of Groups IIIA an d IVA of the periodic system increases. At the same time, the reverse tende ncies are observed in the 2nd and 3rd periods of the periodic system, viz., the ability of the atom to form pentacoordinate structures increases on go ing from C to B and from Al to Si atom.