Dicyclopentadienyl compounds, transition

While dicyclopentadienyl compounds of transition metals (see Transition Metals) typically have a ferrocene-hke (see Ferrocene) sandwich structure, the electron-deficient cyclopentadienides of the main group metals tend to form polymers with bridging cyclopentadienyl (Cp) groups. Although exceptions are known, the tendency is rather consistent. The element zinc, owing to its borderline position between transition metals and main group metals, presents a variety of coordination (see Coordination Numbers Geometries) modes with the Cp anion. 

All neutral dicyclopentadienyl compounds of the first transition series except ferrocene are oxidized by air and hence must be carefully protected from air during storage. Ferrocene and nickelocene are oxidized to their unipositive cations by 3 A nitric acid, whereas vanadocene and cobaltocene are oxidized to their unipositive cations even by 3 A hydrochloric acid. 

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... 

Royo, P. Early transition-metal compounds with doubly sUyl-bridged dicyclopentadienyl ligands. NewJ. Chem. 1997, 21, 791-796. 

Direct synthesis of mixed cyclopentadienyl carbonyls of certain other transition metals may be accomplished by reaction of metal halide, sodium cyclopentadienide, and carbon monoxide under high pressure (50-300 atm). This reaction probably proceeds via the formation of dicyclopentadienyl metal compound followed by displacement by CO, rather than via formation of metal carbonyl and subsequent displacement by cyclopentadienide anion. An excess of sodium cyclopentadienide is normally used to provide a reducing medium, which is especially important for metals initially in the 4 + or 5 + valence state. Some of the compounds which have been synthesized using this approach are CpMn(CO)3 (74, 75, 76), CpRe(CO)3 (77), Cp2Ti(CO)2 (78), CpTc(CO)3 (79), and CpNb(CO)4 (80). 

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