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

Abstract

<p>The activation and scission of the N–O bond in nitric oxide using dinuclear mixed-metal species, comprising transition elements with d³ and d² 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 π interactions are the dominant factor in the bonding across the [M–L¹L²–M′] (L¹L² = N–O, N–N, C–O) moiety and, consequently, the activation of the L¹–L² bond. These trends arise from the fact that the energy gaps between the π orbitals on the metal and small molecule fragments are much more favourable than for the corresponding σ orbitals. The π energy gaps decrease in the order [NO < N₂ < CO] and consequently, for each individual π orbital interaction, the back donation between the metal and small molecule increases in the order [CO < N₂ < NO]. These results are in accord with previous findings suggesting that optimization of the π interactions plays a central role in increasing the ability of these transition metal systems to activate and cleave small molecule bonds.</p>