Cleavage of CO by Mo[N(R)Ar]₃ complexes

The reaction of MoL₃ [L = NH₂ and N(tBu)Ar] with CO was explored using DFT in order to rationalize why CO cleavage is not observed experimentally for this system in contrast to the corresponding N₂ reaction which results in spontaneous cleavage of the N-N bond. The binding of CO to MoL₃ was found to be both kinetically and thermodynamically favored over the binding of N₂, with the formation of the encounter complex, L₃Mo-CO, calculated to be without barrier and exothermic. While the overall reaction to form the C-MoL₃ and O-MoL₃ products was calculated to be energetically favorable, both the encounter complex and intermediate dimer, L₃Mo-CO-MoL₃, were found to be lower in energy than the products, with the final C-O cleavage step calculated to be endothermic by 169 kJ mol^-1 and 163 kJ mol ^-1 for L = NH₂ and N(tBu)Ar, respectively. The unfavorable CO cleavage step can be attributed to the fact that Mo does not possess the optimum d-electron configuration to sufficiently stabilise the carbide and oxide products relative to the CO-bridged intermediate dimer.