New members of ternary rare-earth metal boride carbides containing finite boron–carbon chains: RE25B14C26 (RE=Pr, Nd) and Nd25B12C28

New ternary rare-earth metal boride carbides RE 25B14C26 (RE =Pr, Nd) and Nd25B12C28 were synthesized by co-melting the elements. Nd25B12C28 is stable up to 1440 K. RE 25B14C26 (RE =Pr, Nd) exist above 1270 K. The crystal structures were investigated by means of single-crystal X-ray diffraction. Nd25B12C28: space group P View the MathML source1¯, a=8.3209(7) Å, b=8.3231(6) Å, c=29.888(2) Å, α=83.730(9)°, β=83.294(9)°, γ=89.764(9)°. Pr25B14C26: space group P21/c, a=8.4243(5) Å, b=8.4095(6) Å, c=30.828(1) Å, β=105.879(4)°, V=2100.6(2) Å3, (R1=0.048 (wR2=0.088) from 2961 reflections with Io>2σ(Io)); for Nd25B14C26 space group P21/c, Z=2, a=8.3404(6) Å, b=8.3096(6) Å, c=30.599(2) Å, β=106.065(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 [B2C4]6− and [B3C3]7−, nearly linear [BC2]5− and bent [BC2]7− units and isolated carbon atoms. Structural and theoretical analysis suggests the ionic formulation for RE25B14C26: (RE3+)25[B2C4]6−([B3C3]7−)2([BC2]5−)4([BC2]7−)2(C4−)4·5e− and for Nd25B12C28: (Nd3+)25([B2C4]6−)3([BC2]5−)4([BC2]7−)2(C4−)4·7e−. Accordingly, extended Hückel tight-binding calculations indicate that the compounds are metallic in character.