Ternary alkali metal transition metal acetylides A2MC2 (A = Na, K; M = Pd, Pt)

Ternary transition metal acetylides A(2)MC(2) (A = Na, K: M = Pd, Pt) can be synthesised by reaction of the respective alkali metal acetylide A(2)C(2) with palladium or platinum in an inert atmosphere at about 350 degreesC. The crystal structures are characterised by (1)(x)[M(C-2)(2/2)(2-)] chains. which are separated by the alkali metals (P(3) over bar m1, Z=1). The refinement of neutron powder diffraction data gave C-C=1.263(3)Angstrom for Na2PdC2 (Na2PtC2: 1.289(4) Angstrom). which is distinctively longer than the expected value for a C-C triple bond(1.20 Angstrom). On the basis of band-structure calculations this can be attributed to a strong back-bonding from the metal into the antibonding orbitals of the C-2 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. C-13 MAS-NMR spectra demonstrated that the acetylenic Ct 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).