Metal Complexes of Biologically Important Ligands, CXII Bis(dipeptide ester-H+) Complexes of Palladium(II) Precursors for the Formation of Cyclotetrapeptides

Ta11Si2Se8 was synthesized from a pre-reacted mixture of the elements in a sealed tantalum tube at 1570 K. Mo-doped crystals were obtained in a welded molybdenum crucible by a chemical transport reaction using TeI4 as a transport agent. The structure of Mo0.11Ta10.89(2)Si2Se8 has been determined by single crystal X-ray means [Pnnm, Z = 2, a = 1184.4(1) pm, b = 1939.4 (1) pm, c = 346.48(2) pm, wR2 = 0.069]. The structure of the ternary phase was verified by means of a Rietveld profile fit of a powder X-ray diffractogram. The silicide is isotypic with Ta11Ni2Se8. The structure is composed of Si-centred, tricapped trigonal prismatic Ta9Si clusters. The tetrakaidecahedral Ta9Si clusters are fused into twin chains by sharing the Tapr atoms of facing triangulated prism faces and one out of three capping atoms Tac. Intercluster linkage of the remaining Tac atoms generates a microporous (infinity)3[SiTa2c Ta1/2c Ta6/2pr] substructure. The channels of the metal network are sheathed by three-, four-, five-, and sixfold coordinated Se atoms. Four-probe dc resistivity measurements revealed the Mo-doped phase to be a moderate metallic conductor. The bonding interactions contributing to the stability of Ta11Si2Se8 and distinctions in bonding for Ta11Si2Se8 and Ta11Ni2Se8 are analysed on the basis of semi-empirical extended Hückel calculations.