Dicyclopentadienyl metal compounds

Although not so generally applicable for the preparation of dicyclopentadienyl metal compounds as the sodium cyclopenta-dienide method, the amine procedure represents the simplest preparation of ferrocene. The amine procedure can also be employed for dicyclopentadienylnickel (about 80% yield), using nickel bromide obtained by the action of bromine on nickel metal powder and 1,2-dimethoxyethane as the solvent. The method of preparation given here is a modified version of that originally described. ... 

The dicyclopentadienyl metal compounds undergo Friedel-Crafts alkylation and acylation, sulfonation, metalation, arylation, and formyla-tion in the case of ferrocene, dicyclopentadienyl ruthenium, and dicyclopentadienyl osmium, whereas the others are unstable to such reactions ( ). Competition experiments (128) gave the order of electrophilic reactivity as ferrocene > ruthenocene > osmocene and the reverse for nucleophilic substitution of the first two by n-butyl lithium. A similar rate sequence applies to the acid-catalysed cleavage of the cyclopentadienyl silicon bonds in trimethylsilylferrocene and related compounds (129), a process known to occur by electrophilic substitution for aryl-silicon bonds (130). 

Theoretical studies by E. Ruch (173-177) proved the correctness of this concept of the bonding, with which the retention of the aromatic character of the ring is in harmony. The nature of the bonding in the dicyclopentadienyl metal compounds was discussed by him in terms of molecular and equivalent orbitals, and on the basis of calculated overlap integrals for ferrocene an energy-level diagram was suggested which... 

X-ray work was particularly important in settling the problem of tne structure of dibenzene chromium, and showed that this compound, unlike the dicyclopentadienyl metal compounds studied earlier, possessed a... 

Dicyclopentadienyl metal compounds, see Cyciopentadienyl metal compounds... 

Cyclopentadiene reacts directly with metal carbonyls to form two classes of compounds, each with cyclopentadienyl-metal bonds. Under mild conditions cyclopentadienyl metal carbonyls, C5H5M(CO), are formed, and this is the preferred method for preparing many of them. Under more vigorous conditions dicyclopentadienyl metal compounds, (C5Hs)2M, are formed. Because, however, yields of dicyclopentadienyl metals are low, this is not an important method for their preparation except possibly in the case of ferrocene (40). This reaction under mild conditions, is carried out in the liquid phase in refluxing dicyclopentadiene. OccasionaUy, a solvent of high... 

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). 

Inner Vibrations of Linear Dicyclopentadienyl Metal Compounds... 

Of the many reviews of the dicyclopentadienyl metals, that of Pauson (8) gives a general discussion the substitution reactions have also been reviewed (9). The nature of the bonding in these compounds has been discussed by Richardson (10), and qualitative accounts are now available in standard texts (11, 12). 

Diarene metals generally decompose under conditions of normal electrophilic substitution, but dibenzene chromium can be metalated with amyl sodium in alkanes, and subsequent reaction with carbon dioxide and dimethyl sulfate results in a complex mixture of mono-, di-, and poly-substituted dibenzene chromium methyl esters. It is interesting that these polysubstituted compounds are homoannular in contrast to the corresponding ferrocene compounds. In view of the scanty evidence and ease of oxidation, it is impossible to conclude whether the diarene metals are less aromatic than the dicyclopentadienyl metals as predicted by simple theory (Sec. III.A). 

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

Reaction of metallic europium with cyclopentadiene in liquid ammonia yields [478] a yellow europous-dicyclopentadienyl (eq. 41) complex. Magnetic measurements on this compound (jn = 7.62 Bohr magneton) and its infrared spectrum definitely confirm the divalent state of europium. 

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. 

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... 

The dicyclopentadienyl derivatives of these elements which are known at the present time (those of magnesium and calcium) are in many respects similar to those of the alkali metals, being colorless, saltlike compounds. The magnesium compound was successfully prepared by the interaction of sodium cyclopentadienyl and magnesium bromide in tetra-hydrofuran 214), by thermal decomposition of cyclopentadienyl magnesium bromide 214) and, recently, by the action of cyclopentadiene... 

The nature of the bonding in the metal cyclopentadienyls and their derivatives is still the subject of much discussion, although the compounds have been known for some years. It is now quite certain that in the saltlike alkali, alkaline earth, and rare-earth metal cyclopentadienyls the bonding between the ring and the metal is essentially ionic. Compounds such as dicyclopentadienyl vanadium and chromium also show hydrolytic cleavage. On the other hand, however, cyclopentadienyl thallium, for which the recent investigations of Cotton and Reynolds (20) suggest an ionic bond, may be sublimed and is precipitated from an aqueous medium... 

ITie reaction of some dicyclopentadienyl lanthanide chlorides and some bis(meth-ylcyclopentadienyl) lanthanide chlorides with Na[WCp(CO)3] results in some complexes, stable up to 220°C, which are soluble in tetrahydrofuran and dimethylsulfo-xide. Other anionic metal carbonyl or metal nitrosyl compounds gave no satisfying results, mainly because of side reactions. Infrared spectra indicate polymeric structures containing isocarbonyl bridges (Crease and Legzdins, 1973b) ... 

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