Metal atoms cyclooctatetraene

Nomenclature system . The prefix 17 indicates that all unsaturated carbon atoms of the ligand indicated are ir-bonded to the metal atom for complexes in which the metal atom is bonded only to particular carbon atom(s) of a ligand, the total (x) of such metal-bonded atoms is indicated by a superscript (tj ) or, for ambiguous cases, by preceding locant designators, e.g. (tj -allyl), (1-4-Tj-cyclooctatetraene). 

F. A. Cotton, Bonding of Cyclooctatetraene to Metal Atoms Simple Theoretical Considerations, J. Chem. Soc. 1960, 400-406. 

There has been considerable interest in the synthesis of polymers such as structure (6) in which aromatic rings and metal atoms alternate, due to the possibility of building extended electron-delocalized organometallic polymer chains. However, such a multiple decker polymer has not yet been synthesized, and only a few triple decker-sandwich compounds such as tris( 5-cyclopentadienyl)dinickel cation and tris-(cyclooctatetraene)dititanium dianion have been reported10. ... 

Fluxionality is characteristic of certain classes of organometallic compounds and is found occasionally in others. The phenomenon is seen characteristically in compounds containing conjugated cyclic polyolefins such as cyclopentadienyl, cyclohep-tatrienyl, or cyclooctatetraene, to name the three most common ones, attached to a metal atom through at least one but less than all of their carbon atoms, as illustrated by the following partial structures. 

Cyclooctatetraene is also of some interest in metal atom chemistry. Codeposited with Ti, Fe or Co, are formed uncharacterized polymers ". With Cr a Cr Cr bonded dimer, Cr2(C8Hg)3, is formed (see below) ". With Ti, a novel triple decker sandwich is obtained. ... 

Cocondensation of La, Ce, Nd, or Er metal atoms with cyclooctatetraene at — 196°C yielded dinuclear complexes of the formula [CgHgR(THF)2][R(CgHg)2] after extraction with tetrahydrofuran (De Kock et al., 1978). The structure of the neodymium complex (fig. 23, table 15) shows two cyclooctatetraene rings in the anion, which are neither equidistant from the neodymium atom, nor p allel. The neodymium atom in the cation is asymmetrically located with respect to the central cyclooctatetraene ring with neodymium-carbon distances between 2.68 and 4.63 A (Ely et al., 1976 De Kock et al., 1978). 

In order to study a bis[8]annulene compound with a divalent central metal atom we have synthesized and characterized the divalent ytterbium complex K2[Yb(CgHg)2]F K2[Yb(CgHg)2]r and its calcium analogue, K2[Ca(CgHg)2].51 These compounds were prepared by a reaction which utilizes the solubility of ytterbium and calcium metals in liquid ammonia. Reaction of cyclooctatetraene, potassium and either ytterbium or calcium, in liquid ammonia solution, gives the desired potassium salt of the complex dianion. 

A metal atom can lead to significant changes in the reaction pattern toward electrophilic reagents. For example, tricarbonyliron derivatives of cyclo-heptatriene and cyclooctatetraene undergo facile, unprecedented, 1,3-cycloaddition reactions with electrophiles such as hexafluoroacetone and tetra-cyanoethylene. Removal of the tricarbonyliron group can lead to useful products [Eq. (58) (Green et al., 1973)]. 

Other structurally characterized metal carbonyl complexes with bi- and trifurcation at a carbonyl oxygen atom include, for example, the cycloheptatrienyl and cyclooctatetraene cobalt clusters Co4 CO)6ir -C ii9) r -C-J ii ), 92, Co4(CO)e-(t/ -CsHsXlti-CsHg), 93, and Co4(CO)8(Ai3-C8H8), 94 [153]. 

Cycloheptatrienyl and cyclotatetraene complexes, like other sandwich compounds, and complexes [M(C H )L ] containing polyelectron ligands are thermally stable. Often, these compounds melt at higher temperatures without decomposition. Complexes with ligands which do not contain bulky substituents connected to the ring carbon atoms are usually very sensitive to oxidation and hydrolysis. The compounds of the type M(C0T)2 (M = U, Pa, Ce, etc.) are easily oxidized but are more resistant to water. Table 8.1 gives properties of some cycloheptatrienyl and cyclooctatetraene metal compounds. 

As was pointed out in Chapter 5, with reference to the conjugated ligand cyclooctatetraene, and as the structures of the cyClobutenyl nickel complex, 7.IS, and of the rhodium complex 7.18 show, it is not necessary for all the annular carbon atoms of a cyclic unsaturated hydrocarbon ligand to be involved in bonding to a metal. Further examples of this are shown in Figure 41, where in complexes derived from S-, 6- and 7-membered ring systems respectively, only 5 electrons are formally involved in the bonding in each case. 

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