Coordinatively unsaturated complex or site

All mechanisms proposed in Scheme 7 start from the common hypotheses that the coordinatively unsaturated Cr(II) site initially adsorbs one, two, or three ethylene molecules via a coordinative d-7r bond (left column in Scheme 7). Supporting considerations about the possibility of coordinating up to three ethylene molecules come from Zecchina et al. [118], who recently showed that Cr(II) is able to adsorb and trimerize acetylene, giving benzene. Concerning the oxidation state of the active chromium sites, it is important to notice that, although the Cr(II) form of the catalyst can be considered as active , in all the proposed reactions the metal formally becomes Cr(IV) as it is converted into the active site. These hypotheses are supported by studies of the interaction of molecular transition metal complexes with ethylene [119,120]. Groppo et al. [66] have recently reported that the XANES feature at 5996 eV typical of Cr(II) species is progressively eroded upon in situ ethylene polymerization. 

Coordination requires vacant sites (coordinative unsaturation) (7) or facile ligand substitution. The trans and cis effect of ligands on substitution reactions has been discussed (2). Equilibria between free ligands and complexes have been studied, and information about the steric and electronic effects of ligands is available (5). 

The picture that emerges is that the bonding within the majority of thiophene molecules adsorbed on the catalyst surfaces is hardly perturbed, and this contrasts sharply with the situation in the thiophene complexes. The thiophene molecule parallel to the surface does not correspond to a metal f/ -bound thiophene. Rather, it is suggestive of a weakly chemisorbed precursor state of thiophene that lies parallel to the surface. In this state, the molecule interacts indiscriminately with the alumina, the basal or edge planes, or both. Moreover, the weakness of this binding enhances the surface mobility of thiophene and allows molecules to move across the surface to the catalytic site for reaction with hydrogen atoms. The few sulfur-bound thiophene molecules, no more than 5-10%, would then correspond to thiophene at the coordinatively unsaturated Mo (or Co) atoms. 

In some cases the ordering of the solvent around the solute goes one step further and coordination at a vacant site may occur, leading to a pronounced solvent effect, as the change in stereochemistry may alter the spin state of the metal and/or drastically change the (relative) energies of the d-orbitals. This leads to strong apparent solvatochromism in certain coordinatively unsaturated complexes, a number of examples of which are discussed below. Here the transitions are usually d-d in nature. 

An efficient catalytic cycle requires a facile entry and exit of the ligand. Both coordination and dissociation of hgands must occur with low-activation free energy. Labile metal complexes are therefore essential in catalytic cycles. Coordinatively unsaturated complexes containing an open or weakly coordinated site are labile. 

A transition metal complex is termed co-ordinatively unsaturated when it contains less ligands than normally observed for complexes of the metal s formal oxidation state. These complexes are often discussed in terms of containing one or more open sites or vacant coordination sites on the transition metal center. Real coordinatively unsaturated complexes generally only exist under very special conditions. In general, coordinatively unsaturated complexes are actually stabilized by weak intermolecular forces, such as interactions with the solvent and/or weak intramolecular forces between the ligand and the operf site. 

The important feature is the formation of a coordinatively unsaturated site (cus), permitting the reaction to occur in the coordinative sphere of the metal cation. The cus is a metal cationic site that is able to present at least three vacancies permitting, in the DeNOx process, to insert ligands such as NO, CO, H20, and any olefin or CxHyOz species that is able to behave like ligands in its coordinative environment. A cus can be located on kinks, ledges or corners of crystals [16] in such a location, they are unsaturated. This situation is quite comparable to an exchanged cation in a zeolite, as studied by Iizuka and Lundsford [17] or to a transition metal complex in solution, as studied by Hendriksen et al. [18] for NO reduction in the presence of CO. 

Monomeric sulfur diimides have an extensive coordination chemistry, as might be anticipated from the availability of three potential donor sites and two re-bonds.131 Under mild conditions with suitable coordinatively unsaturated metal complexes, sulfur diimides may coordinate without rupture of the -N=S=N- unit. Four modes of coordination have been identified or invoked as intermediates in fluxional processes (Scheme 8). 

Vacancies were later called coordinately unsaturated sites (cus). This is more in line with terminology used in organometallic chemistry. In view of the present understanding of the nature of the active sites, SBMS or Co(Ni)-Mo-S, the following discussion describes mechanisms in terms of catalysis by organometallic complexes. The references available on this topic are too numerous to mention, and the mechanisms are very well understood. A particularly useful reference is the book by Candlin, Taylor, and Thompson (90), although there are many others that can be consulted. 

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