Mercury cyclopentadienyl compounds

Manganese cyclopentadienyl tricarbonyl (as Mn) Marble/calcium carbonate Mercury (alkyl compounds) (as Hg)... 

Cyclopentadienyl compounds of mercury are limited to (CsHj Hg, which is a molecular compound with linear C—Hg—C bonding and C5Me5HgCl, also having linear (C—Hg—Cl) bonds to Hg. (C5Me5)2Hg apparently cannot be made.102... 

The colorless zinc compound, Zn(CisH6)2, which sublimes at 160° under partial decomposition, is obtained in small yield from zinc chloride and cyclopentadienyl sodium in diethyl ether however, the less stable cadmium compound decomposes, with separation of cadmium, under these conditions (55). The mercury compound, Hg(CsH5)2, is produced in 20% yield by the action of the sodium derivative on mercuric chloride in tetrahydrofuran (215). The action of cyclopentadiene on the complex K2(HgI ) in aqueous alkaline solution results in the precipitation of a mixture of CsHsHgl and Hg(CsH6)2, from which the latter compound may be obtained in good yield by extraction with a mixture of tetrahydrofuran and petroleum ether (62). It forms pale yellow crystals which begin to decompose at about 60° and which melt at 83-85°. The compound is readily soluble in most solvents it decomposes slowly even when kept in the dark at room temperature it is insoluble in water and reacts with neither water nor bases. On the other hand, decomposition occurs in dilute hydrochloric acid. It converts ferric chloride to ferrocene quantitatively, and it yields an adduct with maleic anhydride (215). 

The cyclopentadienyltin compounds (and cyclopentadienyl derivatives of other metals such as mercury and lead) are very sensitive to photolysis, and on irradiation with UV light in an ESR spectrometer they show the spectrum of the cyclopentadienyl radical the stannyl radical is difficult to observe directly under these conditions, but its presence is established by the reactions which it shows with substrates such as alkyl halides. This provides a useful route to a variety of stannyl radicals (see Section 20.1.1), and a method of generating cyclopentadienyl radicals for study by ESR spectroscopy.125 It would be interesting to extend these ESR studies to the more complex compounds 9-6-9-9, which are known to be photosensitive. 

The X-ray structure analysis of the monohydrate, HgFc2 H2O, has confirmed that the cyclopentadienyl rings in the ferrocenyl substituents possess an eclipsed conformation [158]. The mercurial HgFcj can be used as ferrocenyl transfer reagent thus, rare-earth dihalides react with HgFc2 to give new diferrocenyl metal compounds such as YbFcj [159, 160], SmFcj and EuFc2 [160]. 

Mercury salts can react directly with hydrocarbons exchanging hydrogen for mercury. This reaction is an electrophilic substitution (equation 5) and hence can take place with arenes, cyclopentadienyls, terminal alkynes, and also with aliphatic hydrocarbons that contain activated carbon-hydrogen bonds (e.g. carbonyl or nitrile compounds). When the hydrocarbon contains several equivalent hydrogen atoms, polymercuration is often observed. 

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