Two-center three-electron bonds involving selenium

Abstract

We report the first known experimental gas-phase study of two-center three-electron bonding involving selenium. Ions studied include: [(CH 3) 2Se∴Se(CH 3) 2] +, [(CH 3) 2Se∴Se(CH 3) 2] + and [(CH 3) 2Se∴ICH 3] +. We investigated these ions using metastable and collision-induced dissociation MS/MS methods. The metastable spectra of [(CH 3) 2Se∴Se(CH 3) 2] +, [(CH 3) 2Se∴S(CH 3) 2] + and [(CH 3) 2Se∴ICH 3] + show direct cleavage of the Se∴Se, Se∴S and Se∴I bonds. In addition, the metastable spectrum of [(CH 3) 2Se∴ICH 3] + shows fragmentation via a rearrangement path with ejection of the iodine atom. These metastable and collision-induced dissociation spectra are all consistent with Se∴Se, Se∴S and Se∴I hypervalent interactions. Spectra of [(CH 3) 2Se∴(CH 3) 2] + were also compared with spectra of [CH 3SeSeCH 3] +, these comparisons also very strongly support the two-center three-electron Se∴Se interaction in [(CH 3) 2Se∴Se(CH 3) 2] +. Experimental equilibrium studies of ( CH 3) 2 Se ++ Se( CH 3) 2⇌[( CH 3) 2 Se∴ Se( CH 3) 2] + resulted in Δ H° rxn=−95±3 kJ/mol and Δ S° rxn=−76±6 J/mol K. Since in this reaction, Δ H° rxn=−Δ H° bond, the experiment yields a bond enthalpy of 95 kJ/mol at 511 K. We believe that these are the first gas-phase experiments on 2c-3e bonding involving selenium and thus, this may be the first experimental determination of a Se∴Se bond enthalpy.