Stability of Metal-Olefin Complexes

The stability of olefin complexes is also sensitive to steric effects. The binding of ethylene is stronger than that of a-olefins in nearly all cases. However, the magnitude of the steric effect on the binding of an olefin depends on the other ligands. For example, the ethylene complex of bis(amine)PdMe in Equation 2.11b is 10-fold more stable than the propylene or hexene complex, while flie ethylene complex of PtClj is ortiy two-fold more stable than the corresponding propene complex.  

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The increased stability of ruthenium carbene complexes towards oxygen-containing compounds might be because later transition metals, having more d-electrons, are softer and hence react better with soft bases, e.g. olefins. The early transition metals, on the other hand, having few d-electrons, are generally harder and react preferentially with hard bases, such as water or carbonyl compounds. 

The first metal-olefin complex was reported in 1827 by Zeise, but, until a few years ago, only palladium(II), platinum(Il), copper(I), silver(I), and mercury(II) were known to form such complexes (67, 188) and the nature of the bonding was not satisfactorily explained until 1951. However, recent work has shown that complexes of unsaturated hydrocarbons with metals of the vanadium, chromium, manganese, iron, and cobalt subgroups can be prepared when the metals are stabilized in a low-valent state by ligands such as carbon monoxide and the cyclopentadienyl anion. The wide variety of hydrocarbons which form complexes includes olefins, conjugated and nonconjugated polyolefins, cyclic polyolefins, and acetylenes. 

Upon the addition of CO or H2 in the presence of appropriate stabilizers, the controlled chemical decomposition of zerovalent transition metal complexes yields isolable products in multigram amounts [49]. The growth of metallic Ru particles from Ru(COT) (COD) (COT = cyclooctatetraene, COD = cycloocta-1,5-diene) with low-pressure dihydrogen was first reported by Ciardelli et al. [49a]. This material was, however, not well characterized, and the colloidal aspect of the ill-defined material seems to have been neglected in this work. Bradley and Chaudret [49b-l] have demonstrated the use of low-valent transition metal olefin complexes as a very clean source for the preparation of nanostructured mono- and bimetallic colloids. 

The reaction of a-olefins with (CO)5W[C(p-CgH4Me)2] supports the preferential formation of the ot,a -metallacycle (see Table 3). Only trace amounts of the olefins coming from the a,jS-substituted metallacycle are formed. Competition studies demonstrate that the relative reactivity of olefins toward metathesis is 1-pentene > 2-methylpropene > cis-2-butene > > 2-methyl-2-hexene. This stability pattern parallels the stability of 7c-olefin-metal complexes. 

A possible explanation for the calculated trend in the metal ion binding energies to ethylene, which will be discussed for the triply bonded substrates, could lie in a consideration of the Dewar-Chatt-Duncanson donor-acceptor model for bridging-type metal-olefin complexes. Their proposed two-way interaction involves mixing of the olefin n electrons with a metal (n + l)sp a hybrid atomic orbital (L —> M, for short) and simultaneous back donation (M L) of metal nd electrons of appropriate symmetry into the olefin k molecular orbital MO. For the monocation metal ions the latter-type interaction should be less favourable due to stabilizaion of the nd electrons by the charge on the metal. L M should be favoured for the same reason stabilization of the (n + l)s and (n+ l)p orbitals by the + 1 charge. 

Chaudret and coworkers have demonstrated the use of low-valent transition metal olefin complexes as a very clean source for the preparation of nanostruc-tured mono- and bimetallic colloids (Co, Ni, Ru, Pd, Pt, CoPt, CoRh, and RuPt). Syntheses were carried out in the presence of suitable stabilizers using CO or Hj as reducing agents at room or slightly elevated temperature. A number of nanoparticulate metal oxide systems have also been successfully developed by this method. " Olefin complexes are similar to metal carbonyl complex, except the metal is in either low or zero oxidation state. The most commonly used ligands are 1,5-cyclooctadiene (COD), 1,3,5-cyclooctatriene (COT), dibenzylidene acetone (DBA), and cyclooctenyl (CgHjj). 

Many complexes that contain alkyl ligands bearing -hydrogens readily decompose to form olefins and metal-hydride complexes. Such p-hydrogen elimination is the most common process that limits the stability of met -alkyl complexes, although other elimination processes noted beloTv can occur. Kochi summarized early available information on the mechanism of such P-eliminations, and there is also considerable information in early reviews of the chemistry of alkyl ligands. More recent reviews survey all of the decomposition modes available to transition metal alkyl complexes, but emphasize the work of individual authors. ... 

The reactivity of metal-silylene complexes is more limited than the reactivity of carbene complexes. The cationic base-stabilized ruthenium-silylene complex in Equation 13.37 does not react with olefins or alkynes to undergo [2-1-2] addition reactions. However, a related complex did undergo [2-1-2] addition reactions with isocyanates, as shown in Equation 13.46. Other reactions of silylene complexes are distinct from those of carbene complexes or those of other conventional organometallic compounds. For example, the reaction of the silylene hydride with an acetylene generates a p-silylvinylarene complex... 

Table 6J1. Stability Constants of Metal Olefin and Acetylene Complexes...

Protonation of an olefin gives a carbonium ion that can be stabilized through metal 7r-complex formation. In reaction (6-94), the final 7r-cyclopen-tadienyl Tu-propylene iron dicarbonyl cation represents a stable d Fe octahedral d sp complex. 

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