Activation and cleavage of the N-O bond in dinuclear mixed-metal nitrosyl systems and comparative analysis of carbon monoxide, dinitrogen, and nitric oxide activation

The activation and scission of the N-O bond in nitric oxide using dinuclear mixed-metal species, comprising transition elements with d(3) and d(2) configurations and trisamide ligand systems, have been investigated by means of density functional calculations. The [Cr(III)-V(III)] system is analyzed in detail and, for comparative purposes, the [Mo(III)-Nb(III)], [W(III)-Ta(III)], and (mixed-row) [Mo(III)-V(III)] systems are also considered. The overall reaction and individual intermediate steps are favourable for all systems, including the case where first row (Cr and V) metals are exclusively involved, a result that has not been observed for the related dinitrogen and carbon monoxide systems. In contrast to the cleavage of dinitrogen by three-coordinate Mo amide complexes where the dinuclear intermediate possesses a linear [Mo-NN-Mo] core, the [M-NO-M'] core must undergo significant bending in order to stabilize the dinuclear species sufficiently for the reaction to proceed beyond the formation of the nitrosyl encounter complex. A comparative bonding analysis of nitric oxide, dinitrogen and carbon monoxide activation is also presented. The overall results indicate that the p interactions are the dominant factor in the bonding across the [M-(LL2)-L-1-M'] ((LL2)-L-1 = N-O, N-N, C-O) moiety and, consequently, the activation of the L-1-L-2 bond. These trends arise from the fact that the energy gaps between the pi orbitals on the metal and small molecule fragments are much more favourable than for the corresponding sigma orbitals. The pi energy gaps decrease in the order [NO < N-2 < CO] and consequently, for each individual pi orbital interaction, the back donation between the metal and small molecule increases in the order [CO < N-2 < NO]. These results are in accord with previous findings suggesting that optimization of the p interactions plays a central role in increasing the ability of these transition metal systems to activate and cleave small molecule bonds.