Cyclooctatetraene-metal complexes

Cyclooctatetraene)metal complexes (13) were some of the first recognized fluxional organometallic complexes. The ( j4-cyclooctatetraene)Mn(CO)3 anion [13, ML3 =... 

The crystal structure of (ij4-cyclooctatetraene)(hexamethylbenzene)ruthenium (16) indicates bonding as a tetrahapto ligand60. For this complex and similar iron-, ruthenium- and osmium-(ij4-cyclooctatetraene)(arene) complexes, their XH and 13C NMR spectra exhibit only a single signal for the cyclooctatetraene ligand at temperatures as low as —145 °C. Using this temperature, the barrier-to-metal migration is estimated to be <6.6 kcal mol 1. 

Several lanthanide cyclooctatetraene complexes have been synthesized, including both the divalent metal complexes Eu(CgHg) and Yb(CgH8) 44), and the trivalent complexes [M(CgH8)2] (M =Ce, Pr, Nd, SmorTb) 35). The former were preprired by direct reaction of the metal with cyclooctatetraene in liquid ammonia. No structural data exist for these divalent compounds, but they probably involve some kind of bridging interaction since, even with coordinated solvent, one COT... 

According to the generalized Woodward-Hoffmann rule, the total number of (4q + 2)s and (4r)0 components must be odd for an orbitally allowed process. Thus, Eq. (14) is an allowed, and Eq. (13) a forbidden sigmatropic rearrangement. The different fluxional characteristics of tetrahapto cyclooctatetraene (52, 138) and substituted benzene (36, 43, 125) metal complexes may therefore be related to orbital symmetry effects. 

Tetrahaptocyclohexatriene- and cyclooctatetraene-metal-tricarbonyl complexes undergo some unusual cyclo-addition reactions with tetracy-anoethylene in which the dienophile undergoes a 1,3 addition across the coordinated ligand and causes a rearrangement of the metal-carbon bonds, for example (99),... 

The cationic dinitrosyl complexes (146) are apparently much more electrophilic than (143). Studies of the reactivity of (146) have been focused on attack of coordinated organic ligands of other transition metal complexes, such as the cyclooctatetraene ligand in L3M(cot) complexes (M = Fe, Ru) and Cp Co(cot) complexes (Cp = Cp, Cp ), the cyclopenta-dienyl ligand of CpRh(cot) complexes, and the cyanide ligand... 

Another important development in cyclooctatetraene-metal chemistry has concerned the protonation of C8H8-metal derivatives 59, 221). The complex (C8H8)Fe(CO)3 is readily protonated in strong acids to yield salts of the type [(C8H9)Fe(CO)3] X , where X = Cl, CIO4, or BF4. The proton NMR spectrum indicates the complex contains the bicyclo-[5.1.0]-octadienyl-iron tricarbonyl cation (XXXIX), and this formulation is supported by the observation that when the tetrafluoroborate salt of... 

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. 

Cyclooctatetraene, as a ligand in transition-metal complexes (especially iron carbonyls), reacts with electrophiles to form t -bicylo[5.1. Ojoctadienyl complexes such as 7. Reviews concerning early examples of this reaction type are available. 

A number of metal complexes catalyses specific alkyne polymerizations, giving rise to four-, six- or eight-membered carbocyclic rings. The first work in this area was the nickel-catalysed formation of cyclooctatetraene (40) from acetylene by the group of Reppe ", but since then formation of cyclic systems from acetylenes has been found to be also catalysed by molybdenum, cobalt, iridium and tantalum. ... 

The cyclotrimerization of (2,2-dimethylpropylidyne)phosphane (la) in the presence of the (cyclooctatetraene)hafnium complex 9 affords the hafnium complex 10 in 83% yield treatment of the latter with hexachloroethane then furnishes 2,4,6-tri-tert-butyl-l,3,5-triphosphinine (11a) through expulsion of the metal fragment [14]. 

Sections VI, A and VI, D). As very little information on the stabilities and modes of reaction of most cyclobutadiene-metal complexes is yet available, it is perhaps too early to be certain that under some conditions the tetra-merization of acetylenes to cyclooctatetraenes does not occur via cyclobutadiene-metal complexes. 

The following possibilities (Fig. 1) can therefore be considered for the bonding of cyclooctatetraene in metal complexes ... 

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