Reactive intermediates coordinatively unsaturated

For Ru(OOOl) the corresponding reaction energy scheme is shown in Figure 1.7 [4]. The relative energies of the different reaction intermediates, Cjds or CHads, may strongly depend on the type of surface and metal. When for different surfaces or metals the relative interaction with Hads increases Cads may for instance become more stable than CH. This is found for more coordinative unsaturated surfaces or more reactive metals. 

In the course of the tempestuous development of organophosphorus chemistry, interest has only recently been focused on compounds of formally quinquevalent phosphorus having coordination number 3, such as 1, 2, or 3, although one of the other species of this kind has long been postulated as reactive intermediate of solvolysis of phosphorylation reactions. Definite evidence of even proof of the existence of such coordinatively unsaturated phosphorus compounds, however, has been obtained only recently in mechanistic studies, by trapping reactions with suitable cycloaddends, or actually by direct isolation. 

However, the pathways for these reactions, particularly in the gas phase, have been only -.rtially characterized. In a wide variety of these reactions, coordinatively unsaturated, highly reactive metal carbonyls are produced [1-18]. The products of many of these photochemical reactions act as efficient catalysts. For example, Fe(C0)5 can be used to generate an efficient photocatalyst for alkene isomerization, hydrogenation, and hydrosilation reactions [19-23]. Turnover numbers as high as 3000 have been observed for Fe(C0)5 induced photocatalysis [22]. However, in many catalytically active systems, the active intermediate has not been definitively determined. Indeed, it is only recently that significant progress has been made in this area [20-23]. 

High-pressure photochemistry has been used very successfully in studying the mechanisms of catalytic reactions. Irradiation of a suitable precursor permits in-situ preparation of reactive intermediates such as coordinatively unsaturated complexes or radicals. It is thus possible to check whether these species are involved in the catalytic cycle. 

The encapsulation of reactive organometallic complexes is not restricted to the anionic [Ga4(L13)6]12- cages. Thus, Fujita and coworkers were able to generate the coordinatively unsaturated complex [Cp Mn(CO)2] (Cp = C5H4Me) within a self-assembled [M Le] coordination cage 28 (Fig. 20) (132). Photoirradiation of solid 27 gave complex 28, the crystal structure of which confirms the presence of the unsaturated pyramidal [CpMn(CO)2] fragment. The direct observation of such intermediates is... 

A point of interest at this stop in our tour is that fragmentation of organometallic ions in ESI-MS often proceeds via ligand dissociation (e.g., phosphane loss) to generate coordinatively unsaturated organometallic ions [1-9]. One of the strengths of this technique is that such unsaturated ions are typically proposed as reactive intermediates in catalytic reactions carried out in solution (vide infra), allowing ESI-tandem-MS systems to study directly the gas-phase reactivity of such species. 

Species (A) and (B) constitute the main class of unsaturated carbenes and play important roles as reactive intermediates due to the very electron-deficient carbon Cl [1]. Once they are coordinated with an electron-rich transition metal, metal vinylidene (C) and allenylidene (D) complexes are formed (Scheme 4.1). Since the first example of mononuclear vinylidene complexes was reported by King and Saran in 1972 [2] and isolated and structurally characterized by Ibers and Kirchner in 1974 [3], transition metal vinylidene and allenylidene complexes have attracted considerable interest because of their role in carbon-heteroatom and carbon-carbon bond-forming reactions as well as alkene and enyne metathesis [4]. Over the last three decades, many reviews [4—18] have been contributed on various aspects of the chemistry of metal vinylidene and allenylidene complexes. A number of theoretical studies have also been carried out [19-43]. However, a review of the theoretical aspects of the metal vinylidene and allenylidene complexes is very limited [44]. This chapter will cover theoretical aspects of metal vinylidene and allenylidene complexes. The following aspects vdll be reviewed ... 

The kinetic data reveal a complex dependence on the anion concentration and the hydrogen-ion concentration and have been interpreted on the basis of ion-pair and ion-triplet formation. The uncatalyzed path ( 0)has been shown to involve (NH3)5Co(OH)Co(NH3)55+ (= M5+)and the ion pair M x Y4 +, and it was proposed that the ion pair M x Y4+ scavenges Y- from solution and not from the second coordination sphere (357). It was shown that the reactive intermediates are quite selective for anions (as well as being selective for the N terminus of NCS-, the ratio for N-bound S-bound being approximately 4), and this has been interpreted as arising from a genuine, coordinately unsaturated intermediate. The acid-catalyzed path has been interpreted in terms of the formation of protonated unaggregated reactant, MH6+, and small concentrations of the protonated ion pairs and ion triplets MH x Y5+ and MH x Y24+ (355, 356). 

Since mechanistic information for cluster expansions is scarce it cannot be excluded that all such reactions described so far proceed via the addition of coordinatively unsaturated mononuclear complex fragments, even though they can be formulated differently. It is likely that most uncontrolled reactions changing the cluster nuclearity do proceed this way. However, there are a small number of simple cluster expansions which can be understood best by assuming intermediate fragments. This was already taken into account in the previous paragraph. It holds for the following reactions between clusters and simple complexes which do not bear an obvious center of reactivity. 

The M(CO)6 (M = Mo, W) photoassisted interconversion of the linear pentenes, reaction (51), is an example of a situation where the role of the light, at least in part, is to generate a reactive intermediate which is responsible for the isomerization reactions.137 The key photoreaction is dissociative loss of CO from W(CO)s(alkene), reaction (52), to yield a coordinatively unsaturated intermediate which can lead to... 

The structural and spectroscopic studies provide a firm foundation upon which to postulate a catalytic mechanism, but unfortunately give little insight into the activation of dioxygen within the reactive quaternary complex. Two key intermediates are presented in Figure 18. The first is an adduct of the coordinatively unsaturated iron(II) center, O2 and the nearby C4a of the BH4 cofactor, which is best described as an iron(II)-peroxypterin complex (Figure Since no intermediate has been directly... 

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