Metal atoms cyclooctadienes

The reaction of metal atoms with 1,5-cyclooctadiene (1,5-COD) offer special scope for forming new naked metal systems [Wilke (142)] i.e., complexes in which the ligands are readily displaced. The desire to explore these reactions was the impetus to developing the rotary solution reactor (81) which facilitates the reaction of metal atoms with slightly volatile or involatile ligands and, most important, aids the isolation of thermally unstable compounds (Tdec -60°C) in a pure state. 

Cyclooctadiene yields stable complexes when codeposited with a variety of transition metal atoms. With Cr both 1,3- and 1,5-cyclooctadiene yield an intermediate complex that upon treatment with PF3 yields a 1,5-cyclooctadiene complex. Hydrogen migration to give isomerization to the 1,5-complex is facile. Hydrogen transfer is also... 

Nickel atoms codeposited with 1,5-cyclooctadiene yield the known bis(diene)Ni(0), and the analogous Pd and Pt complexes can be prepared by metal atom reactions " . However, these complexes can be more conveniently prepared on large scale by conventional methods . 

Scarcely more complicated, in principle, than simple monoolefin complexes are those in which two unconjugated double bonds form independent linkages to a metal atom. Two of the more important complex-forming non-conjugated diolefins are 1,5-cyclooctadiene and norbornadiene, representative complexes of which are shown as (23-1) and (23-11), respectively. An interesting case of three unconjugated double bonds coordinated to one metal atom is shown in (23-III). 

Transition Metal Compounds. - Macchi et al. performed the first experimental electron density study of a 7t-ligand -coordinated to a metal atom.156 They considered this work as an experimental test of the Dewar-Chatt-Duncanson (DCD) bonding formalism and expected that their work would provide information concerning the 7r-complex versus the metallacycle dichotomy. The authors claim that the successful application of AIM to the experimentally determined p has been the most important step in the coupling of X-ray studies and theoretical chemistry. Their paper reports on the determination of an accurate electron density of crystalline bis(l,5-cyclooctadiene)-nickel, Ni(COD)2 (Figure 11) by X-ray diffraction at 125 K. 

As Table 2.6 hints, complexes are known in which 1,5-cyclooctadiene (cod) bonds through both of its double bonds to a single metal atom. It may be that Fig. 2.2 can be applied to each double bond separately. Equally, it could be argued that although this approach may be valid for the n back-bonding, it is inadequate for the a donation from the cod. Suggest a reason for this reservation. 

In binding mode E, each sulfur atom binds to each metal atom. Only a few examples of this mode of binding have been found, and seem to be restricted to dirhodium complexes. In binuclear complexes, [Rh2L2(cod)(p-S2CNMePh)]" (L = CO L2 = cod), the dithiocarbamate binds in a p-ri, ri fashion as confirmed crystallographically (L2 = cod) (cod = cyclooctadiene) (33). However,... 

It is generally assumed that the Lewis acid in 3 decreases the charge on the metal, i.e., increases its electrophilicity. The removal of charge from the nickel allows additional electron donors to coordinate to the nickel atom, and reaction with, for example, 2 moles of carbon monoxide or 1 mole of 1,5-cyclooctadiene (COD) gives the insoluble, catalytically inactive and presumably ionic complexes 7 and 8. In contrast, 7r-allyl-nickel halides (1) add only 1 mole of carbon monoxide while they do not react with COD (52). 

The compounds Am[Co(COD)2] (27b) react with methanol according to Eq. (30) to give the known i73-cyclooctenyl(i74-l,5-cyclooctadiene)-cobalt (55-57). This complex reacts with lithium metal in a similar way to nickel(O)-olefin complexes. Two lithium atoms are added to give 28 (46). 

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