Side-on mode

Many researchers have considered models for possible intermediates in the nitrogenase reaction. Two possible dinitrogen attachments to the FeMoco factor of MoFe-protein have been put forward. Symmetric, edge- or side-on modes discussed by Dance48 would lead to a reaction sequence such as is shown in reaction 6.11. In contrast, the asymmetric end-on terminal mode discussed in the work of Nicolai Lehnert50 may be favored thermodynamically and by molecular orbital calculations. Reaction sequence 6.13 below illustrates one scenario for the asymmetric model. 

Blaszkowski et al. (221) demonstrated that the methanol molecule is capable of adsorbing in a physisorbed state in two different modes, the end-on mode, shown in the first part of Fig. 12, and a side-on mode, shown in Fig. 13a. In this side-on mode, a C-H bond of the methanol CH3 group is directed toward the zeolitic basic oxygen site, while the acidic zeolite proton retains its strong hydrogen bond with the methanol oxygen. The authors used TST (4) to determine the equilibrium constants for the two modes of adsorption from the computed adsorption energies. The equilibrium constant for the side-on mode is a factor of 106 smaller than that for the end-on mode at 300 K. Thus, nearly all methanol molecules adsorb in an end-on manner, but the dehydration reaction necessitates conversion to the side-on form. 

The only X-ray crystal structure available is that of [Ni(CNBu )202] (11).29 In this complex nickel(O) has square planar coordination with oxygen bonded in the side-on mode. 

Unusual and complicated dinitrogen complexes have been reported to be formed by reactions (61)237,23S and (62).239 In both complexes (57) and (58) the N2 molecules are bound in a side-on mode to nickel atoms. The N—N bond distances (135 pm and 136 pm, respectively) are the longest ones found in any complex with coordinated dinitrogen. 

In the subsequent reactions of hydrazido(2-) complexes, the picture becomes much more woolly, and it is only relatively recently that authenticated complexes containing the hydrazidoi 1 -) ligands [formed formally by protonation of a hydrazido(2-) species] have been isolated. There is considerable structural evidence that hydrazidei 1 -) prefers a side-on mode of bonding to metals where this is feasible. 

Ruthenium has long been known to be an effective catalyst for ammonia synthesis. However, compared to the traditional iron-based catalysts, studies on ruthenium-based catalysts are limited. The rate determining step of ammonia synthesis, the dissociative adsorption of dinitrogen, has been shown to extremely structure sensitive on both iron and mthenium catalysts. To study this structure sensitivity on ruthenium, density functional theory calculations were performed on Ru(OOl) and Ru(llO) clusters. End-on, side-on, and dissociated adsorption states were investigated on both surfaces. While the Ru(llO) cluster could stabilize aJl three adsorption modes, a minimum energy structure for the side-on adsorption on Ru(OOl) could not be found. It is likely that this side-on mode can provide a low energy pathway to the dissociated state, thereby resulting in faster dissociative adsorption on Ru(llO). 

The most striking difference between the two surfaces was the inability of the Ru(OOl) surface to stabilize a side-on a state. All attempts either resulted in dissociation or desorption during geometry optimization. In contrast, the open structure of the Ru(llO) surface easily accommodated a side-on adsorption state as shown in Figure 3b the end-on adsorbate is only required to fall over and interact with the other rathenium atom. The inability to stabilize a side-on mode on the Ru(OOl) surface indicates that this simple falling over mechanism may not be available to end-on adsorbates on its surface. For the side-on state on the Ru(llO) surface, each nitrogen atom directly interacts with a surface mthenium atom, and both the N-N... 

Theoretical calculations show that the lower stability of the side-on mode with respect to the end-on mode can be attributed to the more antibonding character of the nitrogen-metal w-bond and the decrease in the total electronic population of the bond in the former (98). 

Therefore, it appears that this isomerization occiu via a side-on coordinated intermediate without ligand dissociation. The energetics of the isomerization and dissociation reactions indicate that the energy of activation of the dissociation of the nitrogen ligand in the side-on mode is only 7 kcal/mole as compared with 29 kcal/mole in the end-on mode (99). 

In the presence of a mixture of N2 and O2 the species Ni(N2) (02) ( = 1, 2) were identified. It has been proposed that O2 is coordinated to nickel in a side-on mode, while the dinitrogen is coordinated end-on. 

However, other known coordination modes, such as the side-on mode and the variety of bridging modes [8], shift to low fields the resonance of the CO2 ligand relative to the value found for free CO2. 

A C - N triple bond coordinates to metals in either an endo-on or a side-on mode, which kinetically facilitates the access of the metal center to the C-CN bond. Furthermore, the metal cyanide bond is generally thermodynamically stable. Consequently, various C-CN bonds underwent oxidative addition, particularly to zerovalent Group 10 metals (Scheme 1.19) [30]. The oxidative addition reaction was facilitated by addition of Lewis acids (Scheme 1.20). 

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