Metal alkoxides, multiple bonding

Alkoxide ligands play an important spectator role in the chemistry of metal-carbon multiple bonds. Schrock and coworkers have shown that niobium and tantalum alkylidene complexes are active toward the alkene metathesis reaction. One of the terminating steps involves a j8-hydrogen abstraction from either the intermediate metallacycle or the alkylidene ligand. In each case the -hydrogen elimination is followed by reductive elimination. The net effect is a [1,2] H-atom shift, as shown in equations (73) and (74), and a breakdown in the catalytic cycle. Replacing Cl by OR ligands suppresses these side reactions and improves the efficiency of the alkylidene catalysts. ... 

In some cases, alkoxide displacements at a metal center proceed sequentially and at different rates so that, if redistribution processes do not intervene, mixed diketonates can be obtained by adjusting the ratios of reactants.190,191 Recently, bimetallic / -diketonates of the type M2(diketonate)2(OR)4 (M = Mo, W R = alkyl) have been prepared from diketones and M2(OR)6. X-Ray studies show that one diketonate is attached to each metal atom so that unbridged M=M units are retained although rapid rotation about this multiple bond can occur.192 Diketonate complexes with metal-metal multiple bonds are rather uncommon, one other example being C2v CK-Mo2(Me-COCHCOMe), (02 CMe)2.193... 

The orthoesters of nonmetals E(OR) , E = B, Si, P, C - in contrast to metal alkoxides - are monomeric, the stabilization of their molecules being achieved via the increase in multiplicity of E-0 bonds due to p,-d, interaction of the filled orbitals of O-atoms and vacant orbitals of the E atoms. 

Intramolecular nucleophilic displacements are sometimes better suited to difficult cyclizations than additions to C-C multiple bonds, because nucleophilic substitutions are usually irreversible. Some metalated 4-halobutyl imines cydize to yield cydobutanes rather than six-membered cydic enamines (Scheme 9.22). If alkoxides are used as bases, however, exclusive N-alkylation is observed. No examples could be found of the cydization of 4-halobutyl ketones to cyclobutyl ketones, but 5-halopen-... 

The reactions and nature of multiple bonds between two metal atoms form a lively area of research and debate (1,2). Many examples of triple bonds between metals are known, including the extensively studied molybdenum or tungsten alkoxide, M2(OR)6, and amido, M2(NR2)6, complexes (3). Classic examples of quadruple bonds are epitomized by [Re2Cl8]2 (4) and molybdenum acetate, Mo2(OCOMe)4 (5). 

The metal-oxygen bond may be present in a metal alkoxide (e.g., BusSnOMe) or oxide (BusSnOSnBus) or peroxide (e.g., BujSnOOCMea). Rate and reversibility of the reactions vary with the components, and variations in the reactivities of different metallic derivatives can be exploited in synthesis. For example, reactions involving the Si—O bond are most important when it is formed in an elimination leading to the formation of a multiple bond by the reverse of equation (b). 

The groups capable of polymerization may be of different types, namely, vinyl, allyl, styryl, diene, allenic, acetylenic, acrylic, alkoxide, etc. Also, different rings bonded with metal such as epoxy, ethyleneimine, and others may be used. It goes without saying that multiple bond can be present not only as double bonds but also as triple, allene, diene bonds as well as their combinations. The correlation between the electronic structure and the... 

Since a discussion of the properties and structures of metal alkoxides is essential for an understanding of the chemical processes during hydrolytic polycondensation, a short introduction to the chemistry of metal alkoxides is given first. This overview is restricted to the alkoxides of the network forming elements relevant to sol-gel processing (i.e., alkali and alkaline earth alkoxides, organometallic alkoxides, or alkoxides with metal-metal multiple bonds are not treated) excluding silicon. Three comprehensive books on metal alkoxides have appeared. ... 

Unlike a simple alkyl, CO is an unsaturated ligand, by virtue of the C—O multiple bond. As we saw in Section 1.6, such ligands are soft because they are capable of accepting metal d, electrons by back bonding that is, these ligands are it acceptors. This contrasts to hard ligands, which are cr donors, and often ir donors, too (e.g., H2O, alkoxides). 

A specific class of dimeric alkoxides is represented by the cluster derivatives of Mo and W in lower oxidation states studied during the 2 last decades by Chisholm et al. The derivatives of Mo (II) and (HI) as well as W(TTT) with bulky or ramified radicals (sec-, tert-, neo-R, C6H3R2-2,6) form ethane-like molecules with multiple M-M bonds. In contrast to the abovementioned aggregates, they do not contain any bridging groups, for example, (RO)3Mo=Mo(OR)3,where R=CH2Bu . The length of a metal-metal bond is a... 

The carboxylate dimers resemble, in some ways, the dimeric alkoxides with multiple metal-metal bonds. This chemistry has been extensively developed by Chisholm and coworkers. These complexes react with CO to cleave the metal metal bond, as shown in equation (36). 

Special Reactions of Multiple Metal-Metal Bonded Alkoxides... 

The formation of multiple metal-oxo bonds by penta or hexa valent transition metals (d°) is not surprising but the more recent discovery that scandium, yttrium, and the tervalent lanthanides form oxo-isopropoxides rather than triisopropoxides was unexpected. Considerable efforts have been devoted to ensuring that the formation of these pentanuclear oxo-alkoxides [M5(/r5-0)(/r,3-0Pr )4(/r-0Pr )4(0Pr )5] was not due to hydrolysis, and there is evidence that elimination of ether is involved. An alternative mode of decomposition would be alkene elimination with concomitant formation of alcohol (Eq. 5.5). 

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