Preference of np(pi)-np(pi) bonding (n = 3, 4) over purely sigma-bonded species in M-4(2+) (M = S, Se): Geometries, bonding, and energetics of several M-4(2+) isomers

The dimerization energies of 2M(2)(+) to give M-4(2+) (M = S, Se) were calc ulated as input into thermodynamic Born-Fajans-Haber cycle calculations to determine the relative stabilities of salts of these mono- and dications in the solid state. Computed dimerization energies showed a strong dependence on the basis set and correlated method utilized. Coupled cluster calculati ons, compound methods or hybrid HF/DFT methods employing large basis sets [ CCSD(T)/cc-pV5Z, CBS-Q or B3PW91/6-311 +G(3df)//B3PW91/6-311+G*] had to be used and showed an average dimerization energy of 258 (199) kJ/mol for sulf ur (selenium). Square planar M-4(2+) (M = S, Se) was fully optimized (B3LYP , B3PW91), and the calculated vibrational spectra of M-4(2+) were then comp ared to averaged experimental data to derive scaling factors. The structure , bonding, and energetics of seven starting geometries of the M-4(2+) (M = S, Se) dication were computed (B3PW91), as well as AIM and NBO analyses of these species. The global minimum of the examined sulfur (selenium) species is the planar, 6 pi-aromatic D-4h symmetric square, which is 76 (106) and 155 (115) kJ/mol more stable than a D-2h symmetric pi*-pi*-bonded rectangul ar (S-2(+))(2) [(Se-2(+))(2)] dimer and a classical, sigma-bonded, butterfl y-shaped isomer, respectively. This supports the thesis that the observed g eometries of the homopolyatomic cations of groups 16 and 17 and related spe cies maximize positive charge delocalization, resulting in thermodynamicall y stable np(pi)-np(pi) (n greater than or equal to 3) and pi*-pi* bonds. Th e formation of chain-like (Te-4(2+))(n), polymeric Te-8(4+), and square pla nar Te-4(2+) is accounted for semiquantitatively. The published, experiment al enthalpy of formation of gasous S-4(+) (1131 kJ/mol) was computationally shown to be due to a fragmentation of S-6 to give S-4(+) and S-2, confirmi ng earlier photoionization studies. An enthalpy of formation of 972 kJ/mol was then established for the gaseous S-4(+) cation, 159 kJ/mol lower than t he erroneously assigned published experimental value.