Preference of npp-npp Bonding (n = 3, 4) over Purely s-Bonded Species in M42+ (M = S, Se): Geometries, Bonding, and Energetics of Several M42+ Isomers

The dimerization energies of 2M2+ to give M42+ (M = S, Se) were calcd. as input into thermodn. Born-Fajans-Haber cycle calcns. to det. 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 calcns., compd. 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 av. dimerization energy of 258 (199) kJ/mol for S (Se). Square planar M42+ (M = S, Se) was fully optimized (B3LYP, B3PW91), and the calcd. vibrational spectra of M42+ were then compared to averaged exptl. data to derive scaling factors. The structure, bonding, and energetics of 7 starting geometries of the M42+ (M = S, Se) dication were computed (B3PW91), as well as AIM and NBO analyses of these species. The global min. of the examd. S (Se) species is the planar, 6p-arom. D4h sym. square, which is 76 (106) and 155 (115) kJ/mol more stable than a D2h sym. p*-p*-bonded rectangular (S2+)2 [(Se2+)2] dimer and a classical, s-bonded, butterfly-shaped isomer, resp. This supports the thesis that the obsd. geometries of the homopolyat. cations of groups 16 and 17 and related species maximize pos. charge delocalization, resulting in thermodynamically stable npp-npp (n >= 3) and p*-p* bonds. The formation of chain-like (Te42+)n, polymeric Te84+, and square planar Te42+ is accounted for semiquant. The published, exptl. enthalpy of formation of gaseous S4+ (1131 kJ/mol) was computationally shown to be due to a fragmentation of S6 to give S4+ and S2, confirming earlier photoionization studies. An enthalpy of formation of 972 kJ/mol was then established for the gaseous S4+ cation, 159 kJ/mol lower than the erroneously assigned published exptl. value. [on SciFinder (R)]