σ/σ* and π/π* Two-center three-electron bonding in complexes formed between charged thiocyanate and X radicals (X=Cl, Br, I and SCN)

Structure, bonding and spectra of various intermediates formed on the reaction of thiocyanate anion (SCN−) with Cl, Br, I and SCN radicals are investigated following ab initio electronic structure theory with 6-311+G(d) set of basis function. Visualization of relevant localized MOs in SCNX− systems (X=Cl, Br, I and SCN) depicts the presence of a σ type two-center three-electron (2c-3e) bond between two hetero atoms and the optical absorption is assigned to σ→σ* transition. However, dimer radical cation, (SCN)2·+ shows the formation of a π type 2c-3e bond between two hetero atoms and the corresponding optical absorption is assigned to π→π* transition.