Three-center, two-electron systems. Origin of the tilting of their substituents

The origin of the tilting of the H-substituents of 14 parent three-center, two-electron (3c-2e) systems, C2H3+, C2H42+, C2H5+, B2H3-, B2H3-, B2H5-, Be2H42-, Si2H3+, Si2H42+, Si2H5+, Al2H3-, Al2H4, Al2H5-,and Mg2H42- is analyzed. The mixing of the sigma and pi orbitals underlies the upward tilt of these hydrogen substituents. Except for the ethyl cation and the ethylene dication, all 3c-2e sytems have all their hydrogens on one side of a plane that contains both heavy elements. These elements 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 electron systems (anions, cations, dianions, dications, and neutrals containing eight different elements of the first and second rows) correlate linearly with the electronegativities of the heavy elements. The electronic structures of these systems are also reported. They highlight the convex curvature of the X-X bonds. All the investigated 3c-2e systems are either minima or transition structures. The minima associated with the transition structures are identified.