Complex carbides

Complex Carbides. Complex carbides are ternary or quaternary intermetaUic phases containing carbon and two or more metals. One metal can be a refractory transition metal the second may be a metal from the iron or A-groups. Nonmetals can also be incorporated. 

Syntheses, crystallization, structural identification, and chemical characterization of high nuclearity clusters can be exceedingly difficult. Usually, several different clusters are formed in any given synthetic procedure, and each compound must be extracted and identified. The problem may be compounded by the instabiUty of a particular molecule. In 1962 the stmcture of the first high nuclearity carbide complex formulated as Fe (CO) C [11087-47-1] was characterized (40,41) see stmcture (12). This complex was originally prepared in an extremely low yield of 0.5%. This molecule was the first carbide complex isolated and became the foremnner of a whole family of carbide complexes of square pyramidal stmcture and a total of 74-valence electrons (see also Carbides, survey). 

Fig. 35. Skeletal structures of the rhodium-carbonyl carbide complexes Rh8(CO)i9C and [Rhis(CO)28C2]- (7).

Scheme 3.12 Formation of a naked carbide complex by complexation of K+ with benzo [15] crown-5 or [2.2.2]cryptand.

Figure 3.44 X-ray crystal structure of the molybdenum carbide complex. The coordinating K+ ion is sequestered by two benzo [15] crown-5 ligands.

There are two well-characterized examples of a naked carbon atom bound by a triple bond to a metal center (Fig 14.3.8). The molybdenum carbide anion [CMo N(R)Ar 3]- (R = C(CD3)2(CH3), Ar = C6H3Me2-3,5), an isoelectronic analog of NMo N(R)Ar 3, can be prepared in a multistep procedure via deprotonation of the d° methylidyne complex HCMo N(R)Ar 3. The Mo=C distance of 171.3(9) pm is at the low end of the known range for molybdenum-carbon multiple bonds. In the diamagnetic, air-stable terminal ruthenium carbide complex Ru(=C )C12(LL/)(L = L = PCy3, or L = PCy3 and L = l,3-dimesityl-4,5-dihydroimidazol-2-ylidene), the measured Ru-C distance of 165.0(2) pm is consistent with the existence of a very short Ru=C triple bond. 

The carbide-centered polynuclear transition-metal carbonyl clusters exhibit a rich variety of structures. A common feature to this class of carbide complexes is that the naked carbon is wholly or partially enclosed in a metal cage composed of homo/hetero metal atoms, and there is also a subclass that can be considered as tetra-metal-substituted methanes. The earliest known compound of this kind is FesC(CO)i5, in which the carbon atom is located at the center... 

R. G. Carlson, M. A. Gile, J. A. Heppert, M. H. Mason, D. R. Powell, D. Yander Velde, and J. M. Vilain, The Metathesis-Facilitated Synthesis of Terminal Ruthenium Carbide Complexes A Unique Carbon Atom Transfer Reaction, J. Am. Chem. Soc. 124, 1580-1581 (2002). 

For reasons discussed above, this section will not include metal cluster carbide complexes containing only one carbide atom, for which several reviews are available.35,37 38 40 485 486... 

The synthesis of various compounds, including oxides, phosphates, carbides, complexes, intermetallides, alloys, has been carried out by mechanical activation. The preparation of alloys has formed a separate direction, i.e., mechanical alloying [22-28],... 

Tungsten and Nitrogen, Phosphorus, Arsenic, Antimony, and Bismuth—Complex Salts containing Vanadium—Carbides—Complex Cyanogen Derivatives— Compounds with Silicon, Titanium, 2areonium, and Boron. 

The ultimate member of the series alkyl (-CR3), carbene (=CR2), and carbyne (=CR) is the carbide18 ligand, C. Although it is tempting to represent this, by extension, as quadruply bonded = C, this ligand, when attached to transition metals, is probably best considered =C. The first transition metal carbide complex to be reported was the anion [C=Mo N(C(CD3)2Me)(3,5-Me2(C6FI3) 3], shown in Figure 6-7.19... 

Subsequent research eventually led to the preparation of neutral carbide complexes. The first of these to be reported was the trigonal bipyramidal ruthenium complex illustrated in Figure 6-7, prepared from a carbene complex via the following reaction 20... 

The metal-carbon bond distance in carbide complexes is generally quite similar to the comparable distances in alkylidyne complexes. For example, the Ru—C distance in the ruthenium carbide complex in Figure 6-7, 165.0 pm, is nearly identical to that in [RuCl2(=CCH2Ph)(PR3)]+ (R = isopropyl), 166.0 pm, also shown in Figure 6-7. 

Carbide complexes themselves can act as a donors, with this function carried through the carbon atom. In such circumstances the carbido carbon functions as a two-electron donor, for example 21... 

One of the interesting characteristics of carbide complexes that can be useful in their characterization is their very large downfield 13C chemical shift, typically near 500 ppm. 

Theoretical calculations on transition metal carbide complexes suggest that, as expected, both s and p components exist in the metal-C bonds and the n bonding component of the bonds is stronger than the a bonding component.22... 

---