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

Three-coordinate Mo[N(Bu-t)Ar](3) binds cyanide to form the intermediate [Ar(Bu-t)N](3)Mo-CN-Mo[N(Bu-t)Ar](3) but, unlike its N-2 analogue which spontaneously cleaves dinitrogen, the C-N bond remains intact. DFT calculations on the model [NH2](3)Mo/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](3)W/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](3)Re+/CN-/W[NH2](3) and [H2N](3)Re+/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](3)Re+/CN-/W[NH2](3) system are more favourable than those for the [H2N](3)W/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-L-1 L-2-M[NH2](3) intermediates for L-1 L-2 = N-2, CO and CN- which showed that backbonding from the metal into the L-1 = L-2 pi* orbitals is significantly less for CN- than for N-2 or CO due to the negative charge on CN- which results in a large energy gap between the metal d(pi) and the pi* orbitals of CN-. This, combined with the very strong M-CN- sigma 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 N-2 or CO.