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(NH2)(3)], which is stabilized by approximate to 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 products [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 N-2 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.