New ternary rare-earth metal boride carbides RE 25 B 14 C 26 (RE¼ Pr, Nd) and Nd 25 B 12 C 28 were synthesized by co-melting the elements. Nd 25 B 12 C 28 is stable up to 1440 K. RE 25 B 14 C 26 (RE¼ Pr, Nd) exist above 1270 K. The crystal structures were investigated by means of single-crystal X-ray diffraction. Nd 25 B 12 C 28 : space group P1, a¼ 8.3209(7) Å, b¼ 8.3231(6) Å, c¼ 29.888(2) Å, a¼83.730(9)1, b¼83.294(9)1, g¼89.764(9)1.
Pr 25 B 14 C 26 : space group P2 1 /c, a¼ 8.4243(5) Å, b¼8.4095(6) Å, c¼ 30.828(1) Å, b¼105.879(4)1, V¼ 2100.6(2) Å3 , (R1¼ 0.048 (wR2¼ 0.088) from 2961 reflections with I o 42s(I o )); for Nd 25 B 14 C 26 space group P2 1 /c, Z¼ 2, a¼ 8.3404(6) Å, b¼ 8.3096(6) Å, c¼30.599(2) Å, b¼106.065(1)1. Their structures consist of a three-dimensional framework of rare-earth metal atoms resulting from the stacking of slightly corrugated and distorted square nets, leading to cavities filled with cumulene-like molecules [B 2 C 4 ] 6 À and [B 3 C 3 ] 7À , nearly linear [BC 2 ] 5À and bent [BC 2 ] 7À units and isolated carbon atoms. Structural and theoretical analysis suggests the ionic formulation for RE 25 B 14 C 26 :
) 2 (C 4À ) 4 Á 7e À . Accordingly, extended H ückel tight-binding calculations indicate that the compounds are metallic in character.