Nature of the Three-Electron Bond in H2S∴SH2+

We have investigated the model system H2S∴SH2+, i.e., the sulfur−sulfur bound dimer radical cation of H2S, using both density functional theory (LDA, BP86, PW91) and traditional ab initio theory (up to CCSD(T)). Our purpose is to better understand the nature of the three-electron bond. The S−S bond length is 2.886 Å and the bond enthalpy (for 298.15 K) amounts to −40.7 kcal/mol at the BP86/TZ2P level. The best ab initio estimates for the S−S bond strength (our CCSD(T)/6-311++G(2df,2pd)//MP2(full) and literature values) are some 10 kcal/mol weaker than those from nonlocal DFT. It is shown, using an energy decomposition scheme for open-shell systems, that the sulfur−sulfur bond (ΔE = ΔE2c-3e + ΔEelst) is nearly 60% provided by the three-electron bond (ΔE2c-3e) between the unpaired sulfur 3px electron on H2S+• and the sulfur 3px lone pair on H2S; electrostatic attraction (ΔEelst) is important, too, with a contribution of somewhat more than 40%. We show furthermore that the three-electron bond (ΔE2c-3e = ΔE2c-1e + ΔEPauli) can be conceived as and quantitatively analyzed in terms of a one-electron bond (ΔE2c-1e), arising from the β-electron of the H2S lone pair interacting with the corresponding empty β-spin orbital of H2S+•, opposed by the Pauli repulsion (ΔEPauli) between the α-electrons of the H2S lone pair and H2S+• SOMO.