Ternary Alkali Metal Transition Metal Acetylides A2MC2 (A=Na, K; M=Pd, Pt)

Abstract

Ternary transition metal acetylides A2MC2 (A = Na, K; M = Pd, Pt) can be synthesised by reaction of the respective alkali metal acetylide A2C2 with palladium or platinum in an inert atmosphere at about 350 degrees C. The crystal structures are characterised by (infinity)1[M(C2)(2/2)2-] chains, which are separated by the alkali metals (P3m1, Z = 1). The refinement of neutron powder diffraction data gave C-C = 1.263(3) A for Na2PdC2 (Na2PtC2: 1.289(4) A), which is distinctively longer than the expected value for a C-C triple bond (1.20 A). On the basis of band-structure calculations this can be attributed to a strong back-bonding from the metal into the anti-bonding orbitals of the C2 unit. This was further confirmed by Raman spectroscopic investigations, which showed that the wavenumbers of the C-C stretching vibrations in Na2PdC2 and Na2PtC2 are about 100 cm(-1) smaller than in acetylene. 13C MAS-NMR spectra demonstrated that the acetylenic C2 units in the title compounds are very different from those in acetylene. Electrical conductivity measurements and band-structure calculations showed that the black title compounds are semiconductors with a small indirect band gap (approximately 0.2 eV).

Ternary Alkali Metal Transition Metal Acetylides

“…< Figure 2 > 6 K 2 PdC 2 and K 2 PtC 2 were prepared from K 2 C 2 and palladium/platinum sponge at similar conditions like the sodium compounds [15]. The synthesis of rubidium and cesium compounds led to some problems, as the starting materials Rb 2 C 2 and Cs 2 C 2 were not characterized completely.…”

“…All crystal structures of ternary alkali metal palladium and platinum acetylides were solved and refined from X-ray powder diffraction data. To get more reliable interatomic distances additional neutron diffraction experiments were performed on Na 2 PdC 2 and Na 2 PtC 2 [15]. Most interestingly, the C-C distances are 126.3(3) and 128.9(4) pm.…”

“…Similar syntheses of the other ternary palladium and platinum acetylides are currently performed. From the color it can be presumed that these compounds are semiconductors, which is confirmed by temperature-dependent measurements of electrical conductivity [15]. But no linear behavior is found in ln(1/R) = f(1/T) plots (R = resistance) to calculate the band gap from these measurements.…”

“…The shielding anisotropies ∆σ and asymmetry parameters η are discussed elsewhere [15]. In summary, 13 C NMR spectroscopy has turned out to be a very helpful tool for structural verification even at natural 13 C abundances.…”

Ternäre Alkalimetall-Übergangsmetall-Acetylide der Zusammensetzung A2MC2 mit A = Rb, Cs und M = Pd, Pt

“…Ihre Kristallstrukturen sind durch 1 1 [M(C 2 ) 2/2 2± ]-Ketten charakterisiert, die durch die Natriumionen separiert werden. Diese Synthese konnten wir ku È rzlich auf die analogen Kaliumverbindungen u È bertragen [2]. Ferner zeigten wir, daû Na 2 PdC 2 , Na 2 PtC 2 , K 2 PdC 2 und K 2 PtC 2 Halbleiter mit einer sehr kleinen Bandlu È cke sind.…”