Reactivity of CO₂ towards Mo[N(R)Ph]₃

The Laplaza/Cummins L₃Mo (L = N(R)Ar) system is a very important complex for activating small molecules such as N₂. Previous experimental work has shown that CS₂ binds to the L₃Mo system and forms an Mo-CS-Mo intermediate, while the environmentally important CO₂ molecule is unreactive. The aim of this paper is to explain why there is this contrast in reactivity. We have used density functional methods to show that at first glance the reaction of 3L₃Mo + CO₂ should proceed smoothly to give L₃Mo-O + L₃Mo-CO-MoL ₃. However initial coordination of the CO₂ molecule to L₃Mo does not take place because of the bending of CO₂, the energy required to cross from the doublet to the quartet state, and the lower metal-CO₂ binding energy compared to metal-CS₂. The subsequent formation of the L₃Mo-CO-MoL₃ intermediate is similarly unfeasible due to steric and entropic effects. We have provided a molecular orbital rationalization for these effects and have also shown that it is important to take account of steric factors in order to get an accurate understanding of the energetic picture.