A survey of the structural chemistry of ternary and quaternary uranium (U) and thorium (Th) chalcogenides is presented. Specific attention is paid to the coordination environments of U and Th in a variety of classical and nonclassical structure types. The problems of defining the oxidation state of U from UQ (Q = S, Se, Te) bond lengths and magnetic susceptibility data are also discussed.

Time-resolved IR spectroscopy has been used to study the oxidative addition of H2 to Fe(CO)4 and its reverse reaction, the reductive elimination of H2 from Fe(CO)4H2, in the gas phase. The rate constant for oxidative addition of H2 shows little temperature dependence, indicating that if there is an activation barrier for this process it is small (<4 kcal mol-1). The activation barrier for the reductive elimination of H2 is 20.5 ± 2.1 kcal mol-1. From these measurements, the average of the dissociation energies for the two Fe−H bonds in Fe(CO)4H2 is calculated to be 62 ± 2 kcal mol-1. Kinetic measurements employing D2 indicate small kinetic isotope effects for both the oxidative addition and reductive elimination reactions.

Single crystals of CuTh(2)Te(6) form by a stoichiometric reaction of the elements at 1000 degrees C. The compound crystallizes in the space group -P2(1)/m with unit cell parameters a = 6.170(2) Å, b = 4.332(1) Å, c = 10.424(3) Å, beta = 98.85(1) degrees, and Z = 1 at 113 K. The structure was solved from single-crystal X-ray data. It consists of layers of [Th(2)Te(6)(-)] double chains joined by Cu(+) cations. Each Th atom is coordinated to eight Te atoms in a bicapped trigonal prismatic arrangement. There are three crystallographically unique Te atoms. Each ThTe(8) unit is bridged through one distinct Te atom, such that the capping Te atom of one unit forms the vertex of its neighbor. The two nonbridging Te atoms form infinite Te-Te chains along the exterior of the [Th(2)Te(6)(-)] layer. Copper atoms are coordinated to these Te atoms in a tetrahedral arrangement. Owing to the existence of Te-Te bonds of intermediate length, the assignment of formal oxidation states in this compound is not possible. Four-probe dc electrical conductivity measurements of a single crystal of CuTh(2)Te(6) indicate the compound is a semiconductor along [010]. Magnetic susceptibility measurements in the range 2-300 K show CuTh(2)Te(6) to be paramagnetic with &mgr;(eff) = 2.06 &mgr;(B) at 300 K. Single crystals of SrTh(2)Se(5) form from the reaction SrSe + Th + 3Sn + 3Se at 1000 degrees C. EDAX experiments show no tin present in several crystals analyzed. Transparent red blocks of SrTh(2)Se(5) crystallize in the space group -P2(1)/c with unit cell parameters a = 8.704(2) Å, b = 7.861(2) Å, c = 12.458(4) Å, beta = 90.00(2) degrees, and Z = 4 at 113 K. The structure, which is related to that of U(3)S(5), is a three-dimensional framework with Sr cations located in one-dimensional channels. There are two distinct Th environments, bicapped trigonal prismatic and distorted monocapped octahedral. There are no Se-Se bonds and so formal oxidation states of Sr(2+), Th(4+), and Se(2)(-) may be assigned.

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Single crystals of BaLn Te (Ln5Gd-Tm, Y) form as black needles from the stoichiometric reaction BaTe12Ln13Te at 10008C. 2 4 BaSm Te forms as a by-product in the reaction BaTe1Sm1Zn1Te at 8508C with the aid of a BaBr / KBr flux. The compounds 2 4 2 16 crystallize with four formula units in the space group D -Pnma (No. 62) in the CaFe O (calcium ferrite) structure type. Unit cell 2h 2 4˚˚constants range from a513.6883(10) A, b54.5148(3) A, and c516.3427(12) A in the Sm compound to a513.5677(10) A, b54.4058(3)Å , and c515.9989(12) A for Tm (t521208C). Ln atoms are coordinated by six Te atoms in an octahedral arrangement. These octahedra share corners and edges to form a three-dimensional channel structure. Ba atoms occupy the bicapped trigonal-prismatic sites within these channels. Magnetic susceptibility data for two representative compounds (Ln5Er and Tb) show paramagnetic behavior in the region 5-300 K.