THE ELECTRONIC-STRUCTURE OF 2 NOVEL CARBIDES, CA3CL2C3 AND SC3C4, CONTAINING C-3-UNITS

The triatomic C3 unit that is known to exist in Mg2C3 has recently been found in the new compounds Ca3Cl2C3 and Sc3C4. The electronic structure of these compounds is analyzed with the aid of extended Huckel calculations. A fragment molecular orbital analysis (FMO) is used to study the bonding characteristics of the C3 units in the ionic Ca3Cl2C3, and in Sc3C4, the latter containing C2 units and single C atoms as well. Sc3C4 contains partially filled Sc (d) and C2 bands leading to metallic conductivity and Pauli paramagnetism. The C-C bond distance in the diatomic C2 units is significantly increased (d(C-C) = 125 pm) relative to C(2)2- or acetylene, because antibonding pi(g)* orbitals are partially filled. The unusual bending of the C3 unit (d(C-C) = 134 pm) in Sc3C4 (175,8-degrees) and in Ca3Cl2C3 (169,0-degrees) is likely to be a result of the packing arrangement in these structures.