Ternary alkali metal transition metal acetylides A(2)MC(2) (A = Na, K; M =Pd, Pt)

Ternary transition metal acetylides A(2)MC(2) (A = Na, K: M = Pd, Pt) can b e 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. T he 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 refin ement of neutron powder diffraction data gave C-C=1.263(3)Angstrom for Na2P dC2 (Na2PtC2: 1.289(4) Angstrom). which is distinctively longer than the ex pected value for a C-C triple bond(1.20 Angstrom). On the basis of band-str ucture calculations this can be attributed to a strong back-bonding from th e metal into the antibonding orbitals of the C-2 unit. This was further con firmed by Raman spectroscopic investigations, which showed that the wavenum bers 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 t he acetylenic Ct units in the title compounds are very different from those in acetylene. Electrical conductivity measurements and band-structure calc ulations showed that the black title compounds are semiconductors with a sm all indirect band gap (approximately 0.2 eV).