Organometallic structural units in solid state ternary transition metal carbides

Abstract

Solid state ternary transition metal carbides containing carbon, a transition metal (Cr to Ni and their heavier congeners), and a highly electropositive multivalent metal such as a lanthanide (Ln), Sc, Y, or Th, exhibit a number of structural features resembling those in metal carbonyls and other transition metal derivatives of π-acceptor hydrocarbon ligands. Complete ionization of the electropositive metal to the stable ions Ln3+, Sc3+, Y3+, and Th4+ leads to a negatively charged transition metalȁcarbon subnetwork, which may be regarded as an organometallic net. Many such carbides contain discrete C2 units with C=C distances in the range 1.32 to 1.47 Å suggestive of double bonding and the C24ȡ tetra-anion obtained by the complete deprotonation of ethylene. Thus the carbides Ln2FeC4, Sc3CoC4, and Ln3MC4 (M = Fe, Co, Ni, Ru, Rh, Os, Ir) may be considered to contain 1,1-μ2-C24ȡ, 1,2-μ2-C24ȡ and μ3-C24ȡ ligands respectively. Other ternary transition metal carbides contain C22ȡ ligands in [M]=C=C units derived by complete deprotonation of a terminal vinylidene ligand, [M]=C=CH2. Such terminal deprotonated vinylidene ligands are isoelectronic and isostructural with terminal metal carbonyl groups and are found in trigonal M(C2)3 units isoelectronic with M(CO)3 units with the transition metal in very low formal oxidation states (e.g. Mn(ȁV) of Mn(C2)311ȡ in Ln3Mn2C6 isoelectronic with Mn(CO)35ȡ and Ni(ȁII) of Ni(C2)38ȡ in Eu3.16NiC6 isoelectronic with Ni(CO)32ȡ). The carbide ScCrC2 contains a C2 pair with a longer C-C distance of 1.60 Å, suggesting a C-C single bond and the C26ȡ anion obtained by complete deprotonation of ethane. Examples of C1 ligands found in the solid state ternary transition metal carbides include terminal C4ȡ in Th2NiC2, Y2ReC2, and Ln12M5C15 (M = Mn, Re) and μ2(=C=)4ȡ in YCoC and Y2ReC2.