Pentadienyl metal compounds

A special nomenclature for the cyclopentadienyl metal compounds has grown up because of the complexity of their chemical names. The dicyclo-pentadienyl metal compounds, (CsH5)2M, are called ferrocene, cobaltocene, vanadocene, ruthenocene, etc., and these names will be used in this review. Furthermore, the abbreviation Cp will be used to represent C5H5, the cyclopentadienyl radical. 

In the usual experimental procedure, an acidic salt is first complexed with base and then treated with cyclopentadiene which reacts to form the cyclo-pentadienyl metal compound. The by-product salt is separated from the cyclopentadienyl metal compound by extraction with water. 

Co (CO) 2, and (n -C5H5)(n -Ci4Phtj)Co. 35 As a consequence, alternate methods for the preparation of functionally substituted cyclo-pentadienyl metal compounds became an important goal of our research. Sodium cyclopentadienide containing aldehyde, ketone, and ester substituents can be made by the reactions of C5H5Na with ethyl formate, methyl acetate, methyl chloroformate, and dimethyl carbonate (Scheme l),36-39... 

From their analysis of the conformational energies of pentadienyl anion and the penta-dienyl metal compounds, Pratt and Streitwieser in 2000 pointed out that the stabilization of the planar forms of the organometallic structures results from both conjugation and electrostatic attraction between the negative carbons and the alkali metal cations. To determine the relative magnitude of these effects, the reaction energies were determined for hypothetical reaction, shown in Scheme 1 where M represents any alkali metal. 

Ernst RD (1984) Structure and Bonding in Metal-Pentadienyl and Related Compounds. 57 1-53... 

Polysulfides of several metals can be prepared by reaction of the metals with excess sulfur in liquid NH3 (group IA metals) or by heating sulfur with the molten metal sulfide. The polysulfide ion binds to metals to form coordination compounds in which it is attached to the metal by both sulfur atoms (as a so-called bidentate ligand). One example is an unusual titanium complex containing the S52-ion that is produced by the following reaction (the use of h to denote the bonding mode of the cyclo-pentadienyl ion is explained in Chapter 16) ... 

Such specific ligand systems as catenanes [968], calixarenes [969], fullerenes [970a,b], and cyclodextrines [971], and functionalized dendrimers [972a-d] are of great interest. Thus, on the basis of fullerenes C60 and C80 were obtained metal-cyclo-pentadienyl and metal-carbonylcyclopentadienyl compounds with rj1 - and r 5-coordination bonds [970b]. 

Ernst has recently reviewed structural and spectroscopic features of pentadienyl compounds. He discusses experimental and theoretical studies of pentadienyl anions, radicals, and cations and also the structures of transition metal pentadienyl complexes (21). He has also written an account of his own work in this field (22). 

Efficient methods for the preparation of pentadienyl compounds of the alkali metals have now been developed. Treatment of 1,4-dienes with butyllithium in the presence of tetrahydrofuran (thf) at —78° yields deep orange solutions which contain pentadienyllithiums (36,37). Any excess butyllithium may be destroyed by its reaction with thf by allowing the mixture to warm up briefly to room temperature (38). Similar results are obtained using potassium amide in liquid ammonia (39). 1,3-Dienes, however, do not yield pentadienyl anions under these conditions, unless the diene is conjugated with a phenyl (40), vinyl (41), trimethylsilyl (48), or similar stabilizing group. Unfortunately, 1,4-dienes are not very readily accessible. However, 1,3- as well as 1,4-dienes can be metallated using a 1 1 mixture of butyllithium and potassium tm-butoxide (49). Trimethylsilylmethylpotassium is also effective (44). 

Temperature variations of 1H- and 13C-NMR spectra of allyl and pentadienyl compounds of the alkali metals have given information about barriers to rotation about the C—C bonds. The endo and exo isomers of the allyl anions [Eq. (1)] are formed stereospecifically at low temperatures from Z-and E-alkenes, respectively (75,76). 

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