Nonpolar dihydrogen bonds - On a gliding scale from weak dihydrogen interaction to covalent H-H in symmetric radical cations [H_nE-H-H- EH_n]⁺

The electronic structures and bonding patterns for a new class of radical cations, [H_nE-H-H-EH_n]⁺ (EH_n = element hydride, E = element of Groups 15-18), have been investigated by applying quantum-chemical methods. All structures investigated give rise to symmetric potential energy minimum structures. We envisage clear periodic trends. The H-H bond length is shorter for elements toward the bottom of the periodic table of elements, and a short H-H bond corresponds to accumulation of electron density in the central H-H region. All [H_nE-H-H-EH _n]⁺ of Groups 15-17 are thermodynamically unstable towards loss of either H₂ or H. The barriers for these dissociations are rather low. The Group 18 congeners, except E = Xe, appear to be global minima of the respective potential energy surfaces. The findings are discussed in terms of H₂ bond activation, and a general mechanistic scheme for the standard reduction process 2H⁺ + 2e^- → H₂ is given. Finally, it is proposed that some of the symmetric radical cations are likely to be observed in mass spectrometric or matrix isolation experiments.