Mo-C bond

Insertion of CO into Mo—C bonds has been postulated in the reaction... 

Compounds containing M=C bonds can undergo [2+2] cycloadditions, and this reaction allows olefin metathesis to occur. The Mo=C bond [2+2] cycloadds to the C4=C5 bond to give a metallacyclobutane A retro [2+2] cycloaddition cleaves the C4=C5 bond and makes a Mo=C4 bond. This new bond cycloadds across another C4-C5 bond to make a new C4-C5 bond retro [2+2] cycloaddition cleaves the C4=C5 bond and completes the formation of the C4=C5 bond. The process repeats itself many times over to make the polymer. No change in Mo s oxidation state or d electron count occurs in any step. 

Olefin metathesis proceeds via reversible formation of metallacyclobutanes by [2 + 2] cycloaddition (Figure 1.7). The precise pathway for such a cycloaddition has been calculated for molybdenum complexes such as 1 (Figure 1.6) [9]. These calculations suggest that although Mo-C and C-C bond formation is concerted the Mo-C bond is formed more quickly than the C-C bond. It was also found, beautifully consistent with experimental results, that the activation barrier for [2 + 2] cycloaddition is lowered by increasingly electron-withdrawing alkoxy ligands. 

Similarly, [(Bu CH2)2Mo(=NAr)(=CHCMe2R)] reacts with the silanols group of a silica or a molecular silanol to yield [(=SiO)Mo(=NAr)(=CHCMe2R)(CH2Bu )] via direct electrophilic cleavage of the Mo-C bond, according to mass balance analysis, NMR and calculations (Scheme 2.27) [72, 73]. 

Here in general we exclude compounds with more than two Mo—C bonds as discussed earlier. 

Molybdenum(VI) compounds containing Mo—C bonds are a recent addition to the literature. These compounds include both oxo and non-oxo species. The latter have an alkylidyne (alternatively formulated as an alkylcarbido) group which is a bonding equivalent to the oxo group. 

The average Mo—C bond distance in K4Mo(CN)s 2HiO is 2.15 A,49 which corresponds to the value 1.38 A for the octacovalent radius of Mo(IV). The close approximation of this value to the trigonal-prism radius indicates that the bond orbitals are nearly the same for the two types of coordination. 

As already mentioned, no [6 + 4] cycloaddition is observed when tri-carbonyl-i 6-l,3,5-cycloheptatrienemolybdenum(0) is irradiated in presence of conjugated dienes. So an explanation of these experimental results has to be provided. One reason might be an unfavorable fit of the CuH14 n(CH3 ) and C, 4H18 - (CH3) hydrocarbons into the coordination sphere of molybdenum, arising from the increased Mo—C bond lengths. The other reason concerns the possible different stabilities of the intermediates of the photochemical [6 + 4] cycloaddition with molybdenum as the central metal. 

The extended Huckel calculations (45) start from an unbridged geometry where the OC—Mo—CO and Mo—Mo—C bond angles are both 90° while the Mo—Mo—Cp bond angle is set to 125.3°. This leads to a bond description including characteristic five below two frontier orbitals shown in Fig. 3. The five highest occupied molecular orbitals are 7r, 8, ir, 8, and a and allow a correlation with the triple bond assigned on the basis of the 18-electron rule. 

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