INFLUENCE OF SECONDARY BONDING ON THE INTRADIMER DISTANCE OF TRICHLORO(ETHANE-1,2-DIOLATO-O,O')TELLURATE(IV)

The structure of ammonium trichloro(ethane-1,2-diolato-O,O')tellurate( IV) was determined by single-crystal methods: monoclinic, space group P2(1)/c (no. 14), a = 10.779(1), b = 12.418(2), c = 6.915(1) angstrom, beta = 108.65(1)-degrees. The co-ordination around Te(IV) is distorte d psi octahedral with one of the two oxygen atoms lying opposite to th e lone pair of Te. Two identical anions form a dimeric unit via a cent re of symmetry. A short Te-O intradimeric distance of 2.764(6) angstro m is about 0.84 angstrom shorter than the sum of the corresponding van der Waals radii. The structure is compared with that of the correspon ding tetraphenylphosphonium salt where the intradimer distance is clea rly longer. The size of the cation and the amount of intermolecular in teractions seem to be decisive factors in secondary bonding between tw o anionic units. The electronic properties of the anions in the two sa lts were investigated by ab initio calculations and compared with thos e of the optimised structure of the corresponding monomeric anion. The positive charge of 1.36 electrons at Te is almost invariable. The mai n interaction within a dimeric unit in the ammonium salt is a donation of an electron pair from oxygen to the antibonding molecular orbital between Te-Cl in the neightbouring anion. The bond between Te and one of the equatorial Cl atoms (opposite to oxygen) is sensitive to dimeri sation. The charge density, ionic nature of the bond and bond length i ncrease when the degree of dimerisation increases, whereas the other t wo Te-Cl bonds are almost insensitive.