Alkali-metal cyclopentadienides

Alkali metal boratabenzenes have a wide synthetic applicability just like alkali metal cyclopentadienides. Two syntheses have been developed Ashe s synthesis via organotin intermediates (23) and our cyanide degradation of bis (boratabenzene) cobalt complexes (61). 

Bis[(tris(isopropyl)cyclopentadienyl)]zinc (Zn C5(Pr1)3H2 2, 21) and bis[(tetrakis(isopropyl)cyclopentadienyl)]zinc (Zn C5(Pr1)4H 2, 22) were synthesized from the respective potassium cyclopentadienides and zinc iodide as shown in Scheme 18.50 The same slipped sandwich compounds were also isolated from zinc-reduced VC13 solutions when they were treated with these alkali metal cyclopentadienides at room temperature.51 The outcomes of these reactions suggest that zincocenes are likely intermediates in the syntheses of transition metal metallocenes, in which the metal halides have been pre-reduced with zinc. The solid-state structure of Zn G5(Pr1)4H 2 is shown in Figure 10. The sole... 

Since 1957, it has been known (41) that certain metallocenes react with alkali metals to give alkali metal cyclopentadienide and elemental transition metal as in Eq. (23). 

Reactions of the (Tj5-C5Hs)cobaIt-olefin complexes (26) prepared according to Eqs. (25) and (26) with alkali metals (Li, Na, K) in the presence of olefins lead to the elimination of the second C5H5 ligand from the cobalt (Scheme 5). Complexes 27a and 27b, or the mixed complex 27c are obtained in high yields [Eq. (28)]. The syntheses of the pure complexes 27a and 27b do not, of course, require the isolation of intermediates 26a and 26b. As mentioned previously, synthesis is readily achieved from cobaltocene (24) by reaction with either stoichiometric amounts or excess alkali metal in the presence of COD or ethylene [Eq. (24)]. The alkali metal cyclopentadienides which are formed are easily separated from the cobalt complexes and can be used for the synthesis of cobaltocene (51) [Scheme 5 Eq. (29)]. 

Wilkinson et al. 196) were the first to discover C5H5 ligand transfer from one transition metal to another. Thus a number of cyclopentadienyl compounds react with FeCl2 to give ferrocene. This is an important criterion for some ionic character in the metal-cyclopentadienyl bond. Only in solutions of the alkali metal cyclopentadienides is there a noticeable concentration of CjHj- ions 197). It is questionable whether even the so-called ionic magnesium and manganese dicyclopentadienyls produce CjHj" ions in solution 198). Thus it may be assumed that in all cases investigated... 

It stands to reason that, although not explicitly investigated so far, a large number of related ether-functionalized alkali-metal cyclopentadienides are chelates, too. 

As might be expected, alkali metals react with cyclopentadiene much more readily in a solvent in which the alkali metal cyclopentadienide has some solubility. Fischer and Jira (7-9) first used liquid ammonia as a solvent for this reaction and obtained high yields of the ionic cyclopentadienides of Li, Na, and K. It is interesting that the reaction of sodium with cyclopentadiene in liquid ammonia does not produce hydrogen, but yields cyclopentene according to the stoichiometry of the following equation 10,11)... 

Reaction of Alkali Metal Cyclopentadienides zvith Metal Salts Method 4)... 

In general, reactions between alkali metal cyclopentadienides and metal salts are carried out at the reflux temperature of the chosen solvent for a period of several hours. However, longer periods of reflux are required for the reaction of certain metal salts. Thus, the preparation of ruthenocene requires several days of refluxing in 1,2-dimethoxyethane to complete the reaction between RUCI3 and NaCp (59). 

The reaction of alkali metal cyclopentadienides with metal salts is generally applicable to the synthesis of substituted metalcyclopentadienyls. Thus, sodium methylcyclopentadienide readily reacts with metal halides to... 

Displacement of carbon monoxide from metal carbonyls by alkali metal cyclopentadienides is a convenient method for preparing the alkali metal salts of the mixed cyclopentadienyl metal carbonyls. This reaction of carbonyls of Group VIB transition metals is represented by the equation... 

Alkali metal cyclopentadienide salts are important reagents in organometallic chemistry, having been used to prepare innumerable cyclopentadienyl complexes. Potassium cyclopentadienide, KCp, is usually made by deprotonation of cyclo-pentadiene with potassium metal either in organic solvents such as THF and benzene or in liquid ammonia, or by deprotonation with KH" or KOH. The analogous sodium compound, NaCp, was reported by the groups of Fischer and... 

For the synthesis of these derivatives, just as other Cp-complexes of REM, the reaction of anhydrous halides LnX3 with alkali metal cyclopentadienide at the ratio 1 2 is utilized [269-271, 275] ... 

The ionic heterobimetallic compounds of the general formula [(RxC5H5.x)2LnCl2]M(B) (M - Li, Na, K) are formed as a major product in the synthesis of REM cyclopenta-dienyl complexes via the metathesis reaction of LnClj with alkali metals cyclopentadienides at the ratio 1 2 [215, 221, 234, 269, 276, 277, 278, 279]. 

The interaction of equimolar amounts of lithium or sodium alkylcyclopentadienides with LnClj in THF leads to the formation of adducts [(R, C5H5.JLnCl3]M [215, 234, 275, 277, 285]. Evidently, the same compounds are the first products in all reactions of alkali metals cyclopentadienides with salts of REM ... 

Many 7i-cyclopentadienyl complexes may be prepared from the reaction of alkali metal cyclopentadienides with ammonia-soluble transition metal salts such as nitrates and thiocyanates in liquid ammonia (2-30). The amine complexes, [M(NH3) ] (C5H5)2, lose ammonia when heated in vacuo, and the uncharged dicyclopentadienyl complexes of iron, cobalt, nickel, chromium, and manganese are obtained. 

Treatment of metal carbonyls with alkali metal cyclopentadienides in tetrahydrofuran can give u cyclopentadienyl metal complexes, e.g. 

The third class consists of the cyclopentadienides which behave chemically like the essentially ionic, alkali metal cyclopentadienides (see Vol. I). There is no abrupt transition between the ionic and covalent C5H5-metal bond. Indeed there is considerable ionic character in a number of complexes which are considered under the formal heading yr-cyclopenta-dienyl complexes . Even ferrocene, which is considerably covalent, has a charge separation between the metal atom and the tt-CsHs rings (see p. 102). Broadly speaking, the covalent character of the CsHs-metal bond increases steadily across the J-block transition series, i.e. with increasing atomic number, except in the case of manganocene which shows a sudden reversion to ionic character. This anomaly is discussed later. Transition metal cyclopentadienides are discussed on p. 110. 

Reaction of alkali metal cyclopentadienides with metal complexes. The most generally applicable and convenient method of preparation of jt-... 

Like the ionic alkali metal cyclopentadienides, manganocene hydrolyses easily, giving cyclopentadiene and the metal hydroxide and reacts with CO2 giving carboxylic acid salts. 

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