Bond dicarbides

The same reasoning may hold for the virtual absence of transition metal dicarbides with (C2) anions. On the other hand, stable multinary compounds in A-M-(BNx), AE-M-(BNx), or Ln-M-(BNx) systems can exist with an electron-rich transition metal (M), similar to the known ternary dicarbides AM(C2) with A=alkali and M = Pd, Pt [25]. In these cases the valence electrons provided from A or AE metals fill the bonding (BNx)" or (C2) levels, and the transition metal retains an approximate d ° configuration. 

A. The peripheral dicarbide cluster Co6(C2)(CO)14(S) (435) contains a boat array of Co atoms, of which the four basal ones form an essentially regular square. The two apical Co atoms are connected through a Co-C-C-Co link the C-C separation is 1.37(2) A. These two carbon atoms are also bonded to the four basal Co atoms. The core geometry of the cluster is illustrated in Fig. 40. 

Reduction of Ta(silox)3Cl2 with Na/Hg leads to a three-coordinate alkoxide complex Ta(silox)3. The coordinatively unsaturated tantalum complex is capable of cleaving H2 and O2 bonds resulting in the hydride and 0x0 complexes as illustrated in Scheme 7.14. Carbon monoxide is also split upon carbonylation of Ta(silox)3 generating the 0x0 and p-dicarbide complexes. This reaction models the C—O bond cleavage and C—C bond formation believed to occur in the Fischer-Tropsch reaction, and the ketenylidene complex Ta(silox)3(=C=C=0) was postulated as the key intermediate. On the other hand, when Ta(silox)3 was treated with pyridine and benzene, remarkable T -coordinated complexes were formed. 

Other interatomic distances in the rare earth dicarbides show unusual features. Pauling s bond number (Pauling 1960) for the C-R and R-R bonds in these dicarbides increases roughly with the atomic number of the rare earth atom 0.5 (C-La) to 0.9 (C Lu) (Atoji 1961) for the nearest C-R distance 0.2 to 0.35 for the next nearest C-4R distances 0.1 to 0.2 for the nearest R-R distances which are equal to the lattice parameters 3 to 5 and 3.5 to 4.5, respectively, for the total bond numbers of the carbon and rare earth atoms. The bond numbers for YC2 fall between Ho and Lu, indicating that Y in YC2 behaves as a heavy lanthanide (Atoji 1961, 1962). 

For a compound with higher carbon content than this compound, no indication that the additional occupation of the C2 pairs in the metal octahedra occurs was found although the octahedron at z 0 exhibits a relatively large Sc-Sc distance. The shortest interatomic distances, 2.99, 3.11 and 3.18 A for Sc-Sc, as well as 2.24 and 2.26 A for Sc-C, are related to the noticeable ionic portion of the bond. The C-C distance in the C2 pair, 1.25 A, is in the range of that for the dicarbides of Ca, Y and the rare earths. All the interatomic distances have been reported by Jedlicka et al. (1971), showing that the ScijCig compound has relatively large values, 3.30 and 2.55 A, for the Sc-Sc and Sc-C distances. 

The formation of C-C chemical bonds in a variety of solids, including some refractory dicarbides, has been considered by Li and Hoffman (1989) and Wijeyesekera and Hoffman (1984) based on EHT (extended Huckel theory) calculations. To our knowledge, these works are the only ones where the band analogues of bond populations, the so-called crystal orbital overlap populations (COOPs) have been calculated for refractory compounds. The most noticeable result is that, in spite of the evident crudeness of the nonself-consistent semiempirical EHT method, the calculations allow us to understand the nature of the phase transition from cubic to hexagonal structure which occurs in the ZrC, NbC, MoC,... series as the VEC increases. The increase of metal-to-metal bonding when going from cubic (NaCl-type) to hexagonal (WC-type) becomes evident. 

Containing rhodium and gold (atttioijgh the gold atom is not bonded to any carbonyl groiqw), the dicarbide-cluster anion [Rhj2C2(CO)23(AuPPh3)] has been prepared by Albano et al. ... 

---