Cyclopentadienyls bonding

B. Synthesis of Compounds with Silicon-Cyclopentadienyl Bonds. 2138... 

Fragmentation of compounds with metal-cyclopentadienyl bonds has been observed to give rise to very abundant CsHsSi ions (e.g., entries 9, 14, and 15). In Eq. (129), they correspond to the base peak... 

Substituted cyclopentadienyl with the pentamethyl group results in (i) increased covalent character of the metal-cyclopentadienyl bond, (ii) strong n donor and weak n acceptor properties, (iii) kinetic stabilization due to steric shielding of the metal ion, and (iv) increased thermal stability of the metal complexes, (v) decrease in tendency to form polymeric structures due to increase in intermolecular interactions, (vi) increased solubility in organic solvents, and (vii) increase in vapour pressure. 

The most generally applicable method of forming the transition- and inner transition-metal 7t-cyclopentadienyl bond involves the reaction of Na[Cp] with the appropriate anhydrous metal halide or complex halide in an ethereal solvent such as THF or glyme under a dry, inert atmosphere. The complexes are usually isolated by evaporation of the solvent and extraction into an aromatic or aliphatic hydrocarbon solvent. Purification is achieved by crystallization or sublimation. 

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... 

As the first transition series is ascended the metal—carbon bond becomes weaker and the alkyls of iron, cobalt and nickel are stable at ambient temperatures only when coordinated with strong it donors such as 1,1 bipyridyl [12], Organic groupings which form delocalized bonds with the metal are more stable and many tt complexes or 7r-allylic compounds of transition metals are known. The metal—cyclopentadienyl bond is too stable for these compounds to initiate polymerization, but some TT-allyl compounds will polymerize monomers such as butadiene where a comparable structure will be retained in the propagating chain. 

With the corresponding zirconium(IV) derivatives, Zr( j -C5H5)2(Me)2 treatment with liquid sulfur dioxide at dry-ice temperature yields the product resulting from insertion into the zirconium-methyl and the zirconium-cyclopentadienyl bonds ... 

Unfortunately, the complexes underwent facile epimerization under the condi tions of catalytic hydroamination via reversible protolytic cleavage of the metal cyclopentadienyl bond (Scheme 11.7) [36, 38 40]. Thus, the product enantioselec tivity was limited by the catalyst s epimeric ratio in solution and the absolute configuration of the hydroamination product was independent of the diastereomeric purity of the precatalyst. Complexes with a (+) neomenthyl substituent on the cyclopentadienyl ligand generally produced the R) ( ) pyrrolidines, whereas ( ) menthyl and ( ) phenylmenthyl substituted complexes yielded the (S) (+) pyrrolidines, which is in agreement with the proposed stereomodel and solution studies on the equilibrium epimer ratios in the presence of simple aliphatic amines. 

Acetylene also reacts with Fe3(CO)i.2 to yield a red complex melting at 140°C and an orange complex melting at 62°C, both of which have the composition (C2H.2)sFe2(CO)6 (47, 599). The red complex has the structure (102) involving a 7r-cyclopentadienyl bond (411), whereas the orange... 

Iodine cleaves one erbium-cyclopentadienyl bond in tricyclopentadienyl erbium with formation of pink dicyclopentadienyl erbium iodide (Maginn et al., 1963), and the tricyclopentadienyl complexes of neodymium and ytterbium are cleaved by hydrogen cyanide with formation of the corresponding dicyclopentadienyl lanthanide cyanide (Kanellakopulos et al., 1974). The colors and some physical data of the cyclopentadienyl rare earth halides and cyanides are given in table 4. 

The effect of such asymmetric bonding, which may be given a o -tt description or can, quite equivalently, be discussed in terms of a back-donation model, is to lead to an asymmetry in the metal-cyclopentadienyl bond, the result of which is that the cyclopentadienyl ligand loses its five-fold symmetry... 

---