Breaking chemistry's strongest bond: Can three-coordinate [M{N(R)Ar} ₃] complexes cleave carbon monoxide?

The reaction pathway for the interaction of CO with three-coordinate Ta^III, W^III and Re^III complexes (modelled on the experimental [M{N-(tBu)Ar}₃] system) has been explored by using density functional methods. Calculations show that CO binds without a barrier to [Re(NH₂)₃], forming the encounter complex [OC-Re-(NH ₂)₃], which is stabilized by ≈ 280 kJ mol^-1 relative to the reactants. The binding of [Ta(NH₂)₃] to the oxygen terminus of CO is inhibited by a barrier of only 20 kJ mol ^-1 and is followed by spontaneous cleavage of the C-O bond to form the sproducts [C-Re(NH₂)₃] and [O-Ta(NH₂) ₃]. The salient features of the potential energy surface are more favourable to CO cleavage than the analogous N₂ cleavage by [Mo(NH₂)₃], which is less exothermic (335 vs. 467 kJ mol^-1) and is impeded by a significant barrier (66 kJ mol ^-1). The Re^III/Ta^III/CO system therefore appears to be an excellent candidate for cleaving the exceptionally strong C-O bond under mild laboratory conditions. The related W^III/Ta ^III dimer, which significantly weakens but does not cleave the CO bond, may be a suitable alternative when the chemistry is to be performed on activated CO rather than on the strongly bound oxide and carbide cleavage products.