Crystal structures and ab initio calculations of new dicationic telluranes (λ4-tellane), [10-Te-4(C2X2)]2+ (X = S, Se):: Positively charged hypervalent bonding systems

The reactions of 2,6-bis[(phenylthio- or phenylseleno)methyl]phenyl phenyl telluride or the corresponding Te-oxide, with NOBF4 or trifluoromethanesulfonic anhydride afforded the new telluranes, [10-Te-4(C2X2)](2+.)2Y(-) (lambda(4)-tellane) (X = S or Se, Y = BF4 or CF3SO3). These compounds were characterized by elemental and spectroscopic (FABMS, H-1, C-13, and Te-125 NMR) analyses. X-ray structure determinations revealed that the hypervalent tellurium atoms are at the center of a distorted trigonal bipyramid, with two apical sulfonio or selenonio ligands connected via transannular bonds. The Te-S distances are in the range 2.652-2.706 Angstrom, the Te-Se distances in the range 2.759-2.807 Angstrom, and the X-Te-X bond angles in the range 160.41-163.92 degrees. Ab initio calculations indicate that the positive charges in the dications are exclusively on the three chalcogen atoms that farm three-center, four-electron bonds.