ORGANOMETALLIC STRUCTURAL UNITS IN SOLID-STATE TERNARY TRANSITION-METAL CARBIDES

Solid state ternary transition metal carbides containing carbon, a tra nsition metal (Cr to Ni and their heavier congeners), and a highly ele ctropositive multivalent metal such as a lanthanide (Ln), Sc, Y, or Th , exhibit a number of structural features resembling those in metal ca rbonyls and other transition metal derivatives of ir-acceptor hydrocar bon ligands. Complete ionization of the electropositive metal to the s table ions Ln(3+), Sc3+, Y3+, and Th4+ leads to a negatively charged t ransition metal-carbon subnetwork, which may be regarded as an organom etallic net. Many such carbides contain discrete C-2 units with C=C di stances in the range 1.32 to 1.47 Angstrom suggestive of double bondin g and the C-2(4-) tetra-anion obtained by the complete deprotonation o f ethylene. Thus the carbides Ln(2)FeC(4), Sc3CoC4, and Ln(3)MC(4) (M = Fe, Co, Ni, Ru, Rh, Os, Ir) may be considered to contain 1,1-mu(2)-C -2(4-), 1,2-mu(2)-C-2(4-), and mu(3)-C-2(4-) ligands respectively. Oth er ternary transition metal carbides contain C-2(2-) ligands in [M]=C= C units derived by complete deprotonation of a terminal vinylidene lig and, [M]=C=CH2. Such terminal deprotonated vinylidene ligands are isoe lectronic and isostructural with terminal metal carbonyl groups and ar e found in trigonal M(C-2)(3) units isoelectronic with M(CO)(3) units with the transition metal in very low formal oxidation states (e.g. Mn (-V) of Mn(C-2)(3)(11-) in Ln(3)Mn(2)C(6) isoelectronic with Mn(CO)(3) (5-) and Ni(-II) of Ni(C-2)(3)(8-) in Eu3.16NiC6 isoelectronic with Ni (CO)(3)(2-)). The carbide ScCrC2 contains a C-2 pair with a longer C-C distance of 1.60 Angstrom, suggesting a C-C single bond and the C-2(6 -) anion obtained by complete deprotonation of ethane. Examples of C-1 ligands found in the solid state ternary transition metal carbides in clude terminal C4- in Th2NiC2, Y2ReC2, and Ln(12)M(5)C(15) (M = Mn, Re ) and mu(2)(=C=)(4-) in YCoC and Y2ReC2. (C) 1997 Elsevier Science S.A .