Cyclopentadienyl bonding

Fig.5 Examples of simple metallocene structures a parallel sandwich, b multi-decker sandwich, c half-sandwich, d bent/tilted sandwich and e compounds with differently bonded cyclopentadienyl ligands...

The substituted cyclopentadienylmagnesium compounds 62a and 62b have been prepared from the corresponding fulvenes (equation 8) and were structurahy characterized in the solid state by X-ray crystallography . The structures are, as expected, (tj -bonded cyclopentadienyl groups) for bis(cyclopentadienyl)magnesium compounds. 

The structure of cyclopentadienyl(neopentyl)magnesium (65) in the gas phase has been determined by gas-phase electron diffraction . The Mg—C bond distances of the rf-bonded cyclopentadienyl group are 2.328(7) A while the Mg—C bond distance of the neopentyl group was found to be 2.12(2) A. 

The types of compounds formed by gold(I) and gold(III) often differ from those of other metals due to the constraints imposed by coordination number and electron count at the metal. Thus, for example, whereas 7r-bonded cyclopentadienyl complexes of palladium and platinum are numerous (336), and a copper(I) species of this type is known (337), cyclopentadienyl complexes of univalent (94, 96, 97) and trivalent (228) gold have invariably been found to be fluxional behavior, similar to that in dicyclopentadienylmercury, was involved (228). 

C5Hs)2Ti]2, and the unexpected diamagnetism was confirmed. A further unusual feature, deduced from infrared spectral data, was that in one case the compound was not of the 7r-sandwich type, but instead the dimer apparently contained both v- and rr-bonded cyclopentadienyl groups (8). 

In several theoretical papers at different degrees of sophistication attempts have been made to determine the most favorable structure for beryllocene (59-66). Ab initio molecular orbital calculations indicate that in the lowest energy form one pentahapto- and one monohapto-bonded cyclopentadienyl ring are present, as portrayed in structure H. Neither the slip sandwich F nor the off-center double well potential structure G suggested experimentally are indicated to be favorable. Even the symmetrical D5<) sandwich structure... 

An X-ray crystal structure analysis has shown that in the solid state LXIXa is a coordination polymer in which the tin atom is surrounded by one near chlorine atom, by two rather weakly bonded next-nearest-neighbor chlorine atoms, and by an asymmetrically pentahapto-bonded cyclopentadienyl ring, as indicated in Fig. 14. The perpendicular tin to ring distance (2.30 A) is significantly shorter than in stannocene the nearest tin-chlorine distance is even shorter in LXIXa (2.68 A) than in the solid state structure of SnCl2 (229). 

Although most alkali metal compounds have M—C a bonds, cyclopentadienyl 7r-complexes are known for Li, Na, and K. These are commonly polymeric but solvation can break the chains and even result in cation-anion separation.44... 

NMR satellites can be used to differentiate between fluxional tr-and TT-bonded cyclopentadienyl groups. It is found that for a 7r-bonded cyclopentadienyl group — H) is between 174 and 181 Hz, but for a fluxional bonded cyclopentadienyl group — H) is between 159... 

The structure of poly(ferrocenylene), 50, involved conformations where the directly bonded cyclopentadienyl rings were canted (rotated) to create sizable dihedral angles, which interfered with extended conjugation. We synthesized poly(ethynylferrocene), 54.86 This poly... 

---