Dimers cyclopentadienides

At the beginning of the 19th century, Thiele [23, 24] reported that carbonation of sodium cyclopentadienide (37) affords a cyclopentadienecarboxylic acid dimer whose structure subsequently was firmly established as 38 ( Thiele s Acid, Scheme 12) via chemical [25] and spectroscopic [26] methods. 

Carbon-13 chemical shifts of transition metal carbonyls [471, 473] decrease as one proceeds down a given group of the periodic table, as demonstrated for the triad Cr(CO)6, Mo(CO)e, and W(CO)e in Table 4.71. Substitution of CO by cyclopentadienide and other organic ligands deshields the carbonyl carbon nuclei. This is exemplified by iron pentacarbonyl (211 ppm) in comparison to cyclopentadienylirondicarbonyl-dimer, which displays one avaraged carbonyl signal ( 240 ppm) at room temperature [488] due to rapid cis-trans isomerization and intramolecular bridged-terminal carbonyl interconversion ... 

It is generally quite pure however, since purity is directly dependent upon conversion which in turn depends upon flow rate and temperature, initial preparations could result in products contaminated with cyclopentadiene. If this should occur, the magnesium cyclopentadienide could be purified by sublimation in vacuo after the contaminant has dimerized. 

VI. OTHER SUPRAMOLECULAR DIMERS WITH LITHIUM A. Dimers of Lithium Isodicyclopentadienide and Cyclopentadienide... 

Two different synthetic routes leading to the iodo-bridged dimer [(CsHPr 4)Sm(/r-I)(THF)]2 have been reported.198 A mixture of 1,2- and 1,3-disubstituted isomers of potassium cyclopentadienide with two... 

The reaction of lithium cyclopentadienide 470 with ZrCh gave chelate 471 in 64% yield (Scheme 88). 471, which shows dynamic behavior in solution, was subjected to an X-ray structure analysis which showed the complex to be dimeric in the crystal. 471 was treated with sodium cyclopentadienide 469 to give unchelated 472 in 67% yield. 473 was prepared accordingly in 74% yield ° ° (Chart 30). 

C.N.-2. Phosphole anions (6) and 2H-phospholes (7). Very different species, the former is quite stable and possesses significant cyclic delocalization, while the latter readily undergoes dimerization. Phosphole anions can take the place of cyclopentadienide ion in metallocene complexes, forming... 

The cyclopentadienide ion is alkylated by alkyl halides to give dimeric products [25] but by reaction with methyl iodide in liquid ammonia, methylcyclopentadienes have been obtained. [26]. In the gas-phase, however, the cyclopentadienide ion is a poor nucleophile [8]. ... 

Tris-cyclopentadienyls of the lanthanides have been prepared from the trichlorides and sodium cyclopentadienide. They are isolated from tetrahydrofuran as adducts, Cp3Ln(THF), in which the metal atom is formally ten-coordinate (Cp formally occupies 3 coordination positions). These high coordination numbers (8-12) are characteristic of lanthanide chemistry. Cp3Ln(THF) are converted into dimeric halides by reacting with more LnCl3. These species are useful starting materials for preparing other derivatives. 

This method has the disadvantage that prior preparation of the cyclo-pentadienylmetal tricarbonyl dimers is necessary, either from the metal hexacarbonyl and cyclopentadiene or its dimer 128, 129) or in the cases of certain substituted derivatives from the metal hexacarbonyl and fulvene derivatives 44). Neither of these preparative methods works in the case of the chromium derivative [CsH5Cr(CO)3]2, which must be obtained either from the anion [C5HsCr(CO)3] or from (CsHs)2Cr (see below). This method has the advantage, however, that the solution of the sodium derivative Na[C5HsMo(CO)3] is free from sodium cyclopentadienide, a possible complicating impurity in certain reactions. 

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