Pentadienyl groups

In addition to the mixed Cp/carborane complexes, there has been one report on the lanthanacarboranes with the open pentadienyl ligand, R7C5 (R = H or an alkyl derivative). This is an alternate 6-7T electron donor that has led to the syntheses of number of so-called open metallocenes where the pentadienyl group replaces one or more Cp ligands. This lanthanacarborane report covered the syntheses and structural determinations of two open... 

The allyl group can be extended to create a pentadienyl group, a five-carbon system with two double bonds, C U—CHCH=CH—CH,. Its many modes of bonding (t7J, tj3. i)1) have led to a rich chemistry, including the synthesis of mctallabenzene complexes which are thought to be aromatic [Pg.346]

Pentadienyl group Metal Medium Configuration Method Refs. 

The zinc compound 1 begins to decompose at 10°C to zinc and hydrocarbons, but the magnesium and beryllium complexes are more stable. Crystalline adducts 2 can be isolated on addition of TMEDA. The 1H-NMR spectra of 2 (M = Mg, Zn) show averaged AB2X patterns on account of rapid fluxional processes. The spectra can be frozen out at low temperatures, indicating terminally bonded r l-pentadienyl groups (85-87). 

The crystal and molecular structures of Pl Mg-TMEDA1 (3) (55) and of PlZnCl"TMEDA (4) (87) have been determined by X-ray diffraction. The dimethylpentadienyl groups in 3 retain a U shape, and the two double bonds are twisted relative to each other through 23°. The W-shaped group in 4 is, however, essentially planar. Bond lengths alternate as expected for t] -bonded pentadienyl groups. 

CoCl2 and Pl K yield 13 in which the pentadienyl groups have dimerized. The monomer CoPl 2 is a probable intermediate. In the decatetraene ligand the two butadiene units are conjugated (133) in contrast to the dimer formed on reduction of pentadienyltricarbonyliron cations (Section IV,D) in which the two units remain unconjugated (134). 

In principle pentadienyls can bond to transition elements in at least three basic ways, tj3, and tjs (Fig. 1). These can be further subdivided when geometrical factors are considered. If r 5 coordination could be converted to rj3 orr/1, one or two coordination sites could become available at the metal center, and perhaps coordinate substrate molecules in catalytic processes. Little is known about the ability of pentadienyl complexes to act as catalysts. Bis(pentadienyl)iron derivatives apparently show naked iron activity in the oligomerization of olefins (144), resembling that exhibited by naked nickel (13). The pentadienyl groups are displaced from acyclic ferrocenes by PF3 to give Fe(PF3)5 in a way reminiscent of the formation of Ni(PF3)4 from bis(allyl)nickel (144). 

Anions of the type (16-XXXIII) can interact with a metal atom (or atoms) to serve as (2 +l)-electron donors. The two cases of most importance are allyl and pentadienyl groups. The allyl ligand can, of course, be monodentate, (16-XXXIV(a)) but more interestingly can be bonded in the rf fashion, (16-XXXIV(b)). 

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