Rr-cyclopentadienyls

D ibromo-di (rr-cyclopentadienyl) -titanium, AR28 1,A-Dibromohexane, AH87... 

Modification of the acceptor properties of the metal atom may be achieved by using complexes containing rr-cyclopentadienyl ligands. 

Hydrocarbon-Metal rr-Complexes, Other than rr-Cyclopentadienyl rr-Arene Complexes 331... 

BF4M0O3CBH, Molybdenum, tricarbonyl-(f -cyclopentadienyl) [tetrafluorobo-rato(l 26 96, 28 5 Bp4Mo03C oH9, Molybdenum(l+X tricarbonyl(rr -cyclopentadienyl) ti -ethene)-, tetrafluoroborate(l-), 26 102 BF4M0O4C11H111 Molybdenum(l+), (acetone)tricarbonyl(rr -cyclopenta-dienyl)-, tetrafluoroborate(l X 26 105 BF4N05WC,oH,o, Tungsten(l+),... 

TT-Cyclopentadienyl metal carbonyls are usually rath stable thermally, and in the solid state most resist oxidation by oxygen at room temperature. They can often be reduced by sodium amalgam in tetrahydrofuran to yield rr-cyclopentadienyl carbonyl anions. This reduction may proceed either by loss of carbon monoxide, e.g. 

Another example of rr-delocalized radical is cyclopentadienyl 43, which was reported by Sitzmann et al. to be rendered persistent and crystalline by derivatization with /io-propyl groups as in radical 44 (Fig. 7.19). ° "° An interesting feature in the X-ray structure of 44 is that the /io-propyl groups adopt a paddlewheel-like conformation, also showing disorder in the crystal. The bond distances show that the radical is not Dsh symmetric, but that it is slightly distorted (C1-C2 = 1.412 A, C2-C3 = 1.401 A). Radical 44 also packs in a columnar motif with rings eclipsing each other... 

The discovery in 1951 of the transition metal rr-complex, ferrocene or bis-cyclopentadienyl-iron, Fe(Cp)2, (1, 2) led to enormous interest being shown in the possible structures of such compounds and in the nature of the metal-ring bonding. Within a year the sandwich type structure (Fig. 1) had been proposed (2), and an outline treatment of the metal 3d-ligand 7r-orbital interaction by Jaffe (4) was soon followed by a more detailed molecular... 

This review deals with metal-hydrocarbon complexes under the following headings (1) the nature of the metal-olefin and -acetylene bond (2) olefin complexes (3) acetylene complexes (4) rr-allylic complexes and (5) complexes in which the ligand is not the original olefin or acetylene, but a molecule produced from it during complex formation. ir-Cyclopentadienyl complexes, formed by reaction of cyclopentadiene or its derivatives with metal salts or carbonyls (78, 217), are not discussed in this review, neither are complexes derived from aromatic systems, e.g., benzene, the cyclo-pentadienyl anion, and the cycloheptatrienyl cation (74, 78, 217), and from acetylides (169, 170), which have been reviewed elsewhere. 

A number of readily reversible cr-7r rearrangements have been observed wherein a labile ligand such as carbon monoxide is lost by pyrolysis or photolysis, producing a coordinatively unsaturated metal center, which then regains coordinative saturation by means of a tr-n rearrangement. For example, irradiation of o--alkyl-7r-cyclopentadienyl-molybdenum tricarbonyl (15) produces the rr-allene complex (16) (25). These... 

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). 

Among group 2 elements, only beryllium is known to have been inserted into a carborane polyhedron. Even though cyclopentadienyl rr complexes of the heavier alkaline earth meals are known, analogous carborane complexes have not been reported. 

In a recent study of the IR and NMR spectra of the cyclopentadienyl-copper and indenylcopper complexes with fcrf-butyl isocyanide, Saegusa and co-workers favored a rr-bonded structure for the former, but were uncertain of the latter 246). As the indenylcopper complex was deutera-ted at only positions 1 and 3 a preference was expressed for a o-bonded structure with 1,3-copper shifts. The structure of indenylcopper will doubtless be the subject of further research. 

In practice, both the cyclopentadienyl cation and the radical are highly reactive and difficult to prepare. Neither shows any sign of the stability expected for an aromatic system. The six-rr-electron cyclopentadienyl anion, by contrast, is easily prepared and remarkably stable. In fact, cyclopentadiene is one of the most acidic hydrocarbons known, with pKg = 16, a value comparable to tliat of water Cyclopentadiene is acidic because the anion formed by loss of H" is so stable (Mglire 15.5). 

Metal 77-cyclopentadienyls somewhat resemble the rr-allyl complexes. Initially, when the nature of the metal-allyl bond was not sufficiently clear, the similarity was emphasized many times [see the review by E. O. Fischer (425)]. The similarity shows itself, for example, in the equal antisymmetric C—C stretching frequencies (1640 cm ), which indicate that the force constants, hence the bond orders, are close. The central rr-allyl proton absorbs in the same NMR region as do the protons of coordinated cyclopentadienyl. Both ligands display the symmetrical sandwichlike bond with their metals. Today, however, it is clear that the complexes differ significantly in type, the difference being associated first of all with the fact that TT-allyl complexes are much more efficient than 77-cyclopentadienyls at transforming to o-allyl or 77-olefin compounds. This may be due to the difference between the delocalization energies, 2.472 and 0.828 eV for cyclopentadienyl and allyl anions, respectively (426). 

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