Molybdenum carbides bonding

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. 

FIGURE 4.34 Proposed electrochemical processes occurring on the MojC-derived carbon electrode at high positive polarizations in [EMIMJCBFJ electrolyte (a) oxidative dimerization of EMIM+ cations via N-N bond formation (b) possible complexation of in situ elec-trochemically formed BF3 with a free electron pair of a nitrogen atom in the EMIM cation and/or the substitution of an alkyl chain to the EMIM+ cation. (From Kruusma, J. et al. 2014. In situ XPS studies of electrochemically positively polarized molybdenum carbide derived carbon double layer capacitor electrode. Journal of the Electrochemical Society, 161 A1266-A1277. Reproduced by permission of The Electrochemical Society.)... 

Nitric Oxide. NO is a very active molecule because it has one electron in its relatively low lying LUMO orbital. As a result, it dissociates more readily than CO and it can more easily adopt tilted configurations. Thus, NO dissociates almost completely over a molybdenum carbide film like over pure molybdenum and ruthenium (78). The vibration spectra of NO on the M02C film showed two bands at 1800 and 1761 cm due to the on-top adsorption on two distinct Mo sites. The chemisorption bond between NO and M02C was a strong one since NO remained on the surface to 450 K. 

B.V. Cockeram, Flexural Strength and Shear Strength of Silicon Carbide to Sihcon Carbide Joints Fabricated by a Molybdenum Diffusion Bonding Technique, J. Am. Ceram. Soc., 88, 1892-99 (2005)... 

Slip-casting of technical ceramics has been steadily introduced over the past 60 years or so, and now it is standard practice to cast alumina crucibles and large tubes. The process has been successfully extended to include silica, beryllia, magnesia, zirconia, silicon (to make the preforms for reaction-bonded silicon nitride articles) and mixtures of silicon carbide and carbon (to make the preforms for a variety of self-bonded silicon carbide articles). Many metallics and intermetallics, including tungsten, molybdenum, chromium, WC, ZrC and MoSi2, have also been successfully slip-cast. 

This chapter is a review of the characteristics and properties of the interstitial carbides formed by the metals of Group VI chromium, molybdenum, and tungsten. These three carbide systems have similar atomic bonding, composition, and crystallography. Their properties and characteristics are also similar. 

Substrates. Suitable substrates for TiC deposition are the cemented carbides, such as tungsten carbide (WC) bonded with cobalt (Co), which are widely used as cutting-tool materials (see Ch. 16). Other substrates coated by TiC are molybdenum and graphite. 1 1 In the case of molydenum, it is essential to maintain the deposition temperature below 950"C, otherwise recrystallization of the metal and reduction in mechanical strength will occur. 

R. Messner and Y-M. Chiang, Liquid-Phase Reaction-Bonding of Silicon Carbide Using Alloyed Silicon-Molybdenum Melts, J. Am. Ceram. Soc., 73, [5], 1193-1200, (1990). 

PROCESSING AND CHARACTERIZATION OF DIFFUSION-BONDED SILICON CARBIDE JOINTS USING MOLYBDENUM AND TITANIUM INTERLAYERS... 

---