Coordinative unsaturation transition metal complexes

The encapsulation of classical and organometallic transition-metal complexes to yield molecules of the type complex in a complex is a very attractive research area. A variety of inclusion complexes of this type has been reported. Of relevance for this review are reports on the encapsulation of coordinatively unsaturated transition-metal complexes inside self-assembled coordination cages (120), cyclodextrins (121,122), and cucurbiturils (123). 

Terminal alkynes readily react with coordinatively unsaturated transition metal complexes to yield vinylidene complexes. If the vinylidene complex is sufficiently electrophilic, nucleophiles such as amides, alcohols or water can add to the a-carbon atom to yield heteroatom-substituted carbene complexes (Figure 2.10) [129 -135]. If the nucleophile is bound to the alkyne, intramolecular addition to the intermediate vinylidene will lead to the formation of heterocyclic carbene complexes [136-141]. Vinylidene complexes can further undergo [2 -i- 2] cycloadditions with imines, forming azetidin-2-ylidene complexes [142,143]. Cycloaddition to azines leads to the formation of pyrazolidin-3-ylidene complexes [143] (Table 2.7). 

The most important synthetic access to acceptor-substituted carbene complexes is the reaction of ylides with electrophilic, coordinatively unsaturated transition metal complexes (Figure 4.1 see also Section 3.1.3). 

Compounds with a narrow HOMO-LUMO gap (Figure 5.5d) are kinetically reactive and subject to dimerization (e.g., cyclopentadiene) or reaction with Lewis acids or bases. Polyenes are the dominant organic examples of this group. The difficulty in isolation of cyclobutadiene lies not with any intrinsic instability of the molecule but with the self-reactivity which arises from an extremely narrow HOMO-LUMO gap. A second class of compounds also falls in this category, coordinatively unsaturated transition metal complexes. In transition metals, the atomic n d orbital set may be partially occupied and/or nearly degenerate with the partially occupied n + 1 spn set. Such a configuration permits exceptional reactivity, even toward C—H and C—C bonds. These systems are treated separately in Chapter 13. 

Germyl hydrides readily undergo oxidative addition to the low-valent coordinatively unsaturated transition-metal complexes. Optically active [Co(CO)4GeMe(l-Np)Ph] was... 

Strictly speaking, a catalyst is some species directly involved in the catalytic cycle and, in the reactions discussed here, these species are usually low-valent, coordinatively unsaturated transition metal complexes. Metal halides, e-.g., chloroplatinic acid, PdCl, etc., although often claimed as catalysts are more properly catalyst precursors, since in the presence of silyl hydrides the metal halides are reduced. If no stabilizing ligands, e.g., olefins, phosphines, etc. are present, the reduction normally proceeds to a finely divided form of the metal or to insoluble metal silyl/hydride clusters which may act as heterogeneous catalysts. 

If we pass gaseous nitric oxide through a solution of a coordinatively unsaturated transition metal complex, simple addition may occur. Since nitric oxide contains an unpaired electron, this method is most commonly applied to paramagnetic starting materials. Some examples of this reaction (16, 17) are... 

Another important reaction of alkenes with coordinatively unsaturated transition metal complexes is the alkene metathesis reaction ° [Eq. (6.158)] ... 

A typical method of formation of a transition metal-silicon bond is the oxidative addition of silane derivatives, involving either Si-H or Si-Si bond cleavage, to a low valent, coordinatively unsaturated transition metal complex. Various kinds of transition metal silyl or silylene complexes have so far been synthesised via this method. However, in contrast to the rich chemistry of the late transition metal complexes with an M-Si bond, a limited number of syntheses of Group 6 metal complexes with an M-Si bond have been reported. ... 

Coordinatively unsaturated transition metal complexes are known to bind to a variety of donor molecules. Qualitatively, the enthalpies of these interactions range from very strong dative bonds with ligands such as phosphines and CO to very weak interactions with saturated hydrocarbons. Despite the importance of these interactions in determining structure and reactivity, limited quantitative data exist for metal-ligand bond strengdis. 

Coordinatively unsaturated transition metal complexes can in general add neutral or anionic nucleophiles. Oxidative addition to coordinatively unsaturated transition metal compounds has opened up undreamt of synthetic possibilities [18]. This reaction and its reverse — reductive elimination — are formally described by the following equilibrium ... 

For Type lb sonochemical effects one may draw a parallel between the ability of sonication to create coordinatively unsatured transition metal complexes and the photochemical activation of resistant C-H bonds in the presence of selected transition metal complexes.22 The only problem in these cases is that one is dealing here with stoichiometric activation in most cases. In photochemistry, most of the corresponding reactions would be characterized by quantum yields smaller than 1. Examples of such reactions can be found in Ch. 2. Therefore, this type of sonochemical activation which creates active species (radicals, nitrenes, transition metal compounds with vacancies) unable to trigger a chain or a catalytic reaction should consume far more energy than Type la. 

Kawano M, et al. Direct crystallographic observation of a coordinatively unsaturated transition-metal complex in situ generated within a self-assembled cage. J Am Chem Soc... 

Stabilization of Coordinatively Unsaturated Transition-Metal Complex... 

While up to 1990 all attempts to isolate a stable N-heterocyclic carbene failed, metal complexes of unsaturated imidazol-2-ylidenes were known as early as 1968. The first complexes of this type were obtained by in situ deprotonation of imidazolium salts using mercury(ll) acetate or dimethylimidazolium hydridopentacarbonylchromate(-II) followed by coordination of the carbene to the metal center (Scheme 1.3). Shortly thereafter, the stabilization of the saturated imidazolin-2-ylidene in a metal complex was described by Lappert who treated electron-rich entetraamines of type 6=6 with coordinatively unsaturated transition metal complexes to obtain complexes with imidazolin-2-ylidene ligands (Scheme 1.3). ... 

The most general route to generate vinylidene complexes [M]=C=C(H)R is the direct activation of terminal alkynes HC=CR by a coordinatively unsaturated transition metal complex, via the generation of unstable 77 -alkyne or hydride-alkynyl intermediates which tautomerize into the thermodynamically more stable vinylidene isomers (Scheme 15) ia,ic,207 theoretical and kinetic studies on the metal-mediated alkyne-vinylidene isomerization... 

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