3-CENTER, 2-ELECTRON SYSTEMS - ORIGIN OF THE TILTING OF THEIR SUBSTITUENTS

The origin of the tilting of the H-substituents of 14 parent three-cen ter, two-electron (3c-2e) systems, C2H3+, C2H42+, C2H5+, B2H3-, B2H3-, B2H5-, Be2H42-, Si2H3+, Si2H42+, Si2H5+, Al2H3-, Al2H4, Al2H5-,and Mg 2H42- is analyzed. The mixing of the sigma and pi orbitals underlies t he upward tilt of these hydrogen substituents. Except for the ethyl ca tion and the ethylene dication, all 3c-2e sytems have all their hydrog ens on one side of a plane that contains both heavy elements. These el ements thus have inverted geometries. It is shown that the geometrical tilt angles between the X-Ht and the X-X bonds of all the 3c-2e elect ron systems (anions, cations, dianions, dications, and neutrals contai ning eight different elements of the first and second rows) correlate linearly with the electronegativities of the heavy elements. The elect ronic structures of these systems are also reported. They highlight th e convex curvature of the X-X bonds. All the investigated 3c-2e system s are either minima or transition structures. The minima associated wi th the transition structures are identified.