The title compounds crystallize with a very pronounced subcell structure that has been determined from single-crystal X-ray di4ractometer data of all four compounds. Only subcell (and no superstructure) re6ections have been observed for Pr 3 Si 2 C 2 : space group Cmmm, a β«Ψβ¬ 396.7(1) pm, b β«Ψβ¬ 1645.2(3) pm, c β«Ψβ¬ 439.9(1) pm, R β«Ψβ¬ 0.019 for 309 structure factors and 20 variable parameters. In this subcell structure there are C 2 pairs with split atomic positions. This structure may be considered the thermodynamically stable form of these compounds at high temperatures. Two di4erent superstructures with doubled a or c axes, respectively, of the subcell have been observed, where the C 2 pairs have di4erent orientations. In the structure of Tb 3 Si 2 C 2 the a axis of the subcell is doubled. The resulting superstructure in the standard setting has the space group Pbcm: a β«Ψβ¬ 423.6(1) pm, b β«Ψβ¬ 770.7(1) pm, c β«Ψβ¬ 1570.2(3) pm, R β«Ψβ¬ 0.031 for 1437 structure factors and 22 variable parameters. Dy 3 Si 2 C 2 has the same superstructure: a β«Ψβ¬ 420.3(1) pm, b β«Ψβ¬ 767.5(1) pm, c β«Ψβ¬ 1561.1(3) pm, R β«Ψβ¬ 0.045, 801 F values, 22 variables. In the structure of Y 3 Si 2 C 2 the c axis of the subcell is doubled, resulting in a body-centered space group with the standard setting Imma: a β«Ψβ¬ 842.6(2) pm, b β«Ψβ¬ 1563.4(2) pm, c β«Ψβ¬ 384.6(1) pm, R β«Ψβ¬ 0.035, 681 F values, 15 variables.
In all of these structures the rare earth atoms form two-dimensionally in5nite sheets of edge-sharing octahedra that contain the C 2 pairs. In between these sheets there are zig-zag chains of silicon atoms with Si+Si distances varying between 246.2(3) and 253.6(3) pm, somewhat longer than the two-electron bonds of 235 pm in elemental silicon, suggesting a bond order of 0.5 for the Si+Si bonds. The C+C distances in the C 2 pairs vary between 127(1) and 131(1) pm, corresponding to a bond order of approximately 2.5. Hence, using oxidation numbers, the compounds may to a 5rst approximation be represented by the formula (R Ψ 3 ) 3 (Si Ψ 3 ) 2 (C 2 ) Ψ 3 . A more detailed analysis of the interatomic distances shows that the shortest R+R distances are comparable with the R+R distances in the structures of the rare earth elements, thus indicating some R+R bonding. Therefore, the oxidation numbers of the rare earth atoms are slightly lower than Ψ 3, in agreement with the metallic conductivity of these compounds. As a consequence, considering the relatively short Si+Si bonds, the absolute value of the oxidation number of the silicon atoms may be lower than 3, resulting in a Si+Si bond order somewhat higher than 0.5. 2001