Investigating CN- cleavage by three-coordinate M[N(R)Ar](3) complexes

Three-coordinate Mo[N(tBu)Ar]3 binds cyanide to form the intermediate [Ar(tBu)N]3Mo-CN-Mo[N( tBu)Ar]3 but, unlike its N2 analogue which spontaneously cleaves dinitrogen, the C-N bond remains intact. DFT calculations on the model [NH2]3Mo/CN- system show that while the overall reaction is significantly exothermic, the final cleavage step is endothermic by at least 90 kJ mol-1, accounting for why C-N bond cleavage is not observed experimentally. The situation is improved for the [H2N]3W/CN- system where the intermediate and products are closer in energy but not enough for CN- cleavage to be facile at room temperature. Additional calculations were undertaken on the mixed-metal [H2N]3Re+/CN-/W[NH 2]3 and [H2N]3Re+/CN -/Ta[NH2]3- systems in which the metals ions were chosen to maximise the stability of the products on the basis of an earlier bond energy study. Although the reaction energetics for the [H2N]3Re+/CN-/W[NH2] 3 system are more favourable than those for the [H2N] 3W/CN- system, the final C-N cleavage step is still endothermic by 32 kJ mol-1 when symmetry constraints are relaxed. The resistance of these systems to C-N cleavage was examined by a bond decomposition analysis of [H2N]M-L1i=L2- M[NH2]3 intermediates for L1i=L2 = N2, CO and CN- which showed that backbonding from the metal into the L1i=L2 π* orbitals is significantly less for CN- than for N2 or CO due to the negative charge on CN- which results in a large energy gap between the metal dπ and the π* orbitals of CN-. This, combined with the very strong M-CN- σ interaction which stabilises the CN- intermediate, makes C-N bond cleavage in these systems unfavourable even though the C≡N triple bond is not as strong as the bond in N2 or CO.