Cyclopentadienyl cation, properties

Iridium nanoparticles, preparation, 12, 82 Iridium(III) O-ligated complexes, preparation, 7, 315 Iridium polyhydrides, preparation and characteristics, 7, 405 Iridium pyrrolyl derivatives, reactivity, 7, 282 Iridium tetrahydrides, characteristics, 7, 407-408 Iridium trihydrides, preparation, 7, 405 Iridium vinylidenes, synthesis and characteristics, 7, 352 Iridium xyliphos complexes, properties, 7, 442 Iridoids, via Pauson-Khand reaction, 11, 360 Iron(arene) (cyclopentadienyl) cations, preparation and reactivity, 6, 166... 

It is interesting to compare the properties of the above homotropylium ions, in which the breaking of the cyclopropane ring results in a homoaromatic system, with those of the bicyclo[3,l,0]hexenyl cations in these a similar breaking would result in a 4 71-electron antihomoaromatic system similar to the cyclopentadienyl cation (cf. >). 

The reactivity ratio rij/r of 6.85 0.30 was heavily in favor of the 1,3-disubstituted monomer it is obvious that 1,3-disubstitution enhances the stability of the cyclopentadienyl cation. These products displayed remarkable properties, totally different from those of the homopolymers. For instance, (1) they can be dissolved in benzene and the solution can be cast to yield transparent films, (2) the softening points of 150-155°C are somewhat higher than those of the homopolymers, and (3) the rate of oxidation and oxygen absorption upon exposure to air are found to be slower. These particular properties were attributed to the decreasing number of tertiary allylic structures in the copolymer chain as compared to the respective homopolymers. 

While benzene was the first aromatic system studied, the formulation of HtickePs rule and the theory behind it created an impetus to prepare non-benzenoid species such as the tropylium cation and cyclopentadienyl anion that also obeyed Huckel s rule to see if these species were also aromatic. This required that the properties of aromatic compounds be defined. 

A further group of nonbenzenoid aromatics is the series of odd-membered cations and anions such as cycloprope-nium (14) and tropylium cations (15) as well as cyclopentadienyl (16) and cyclononatetracenyl anions (17). Regarding the arguments for the properties of Hiickel-like 4 + 2 jr-systems, all these molecules should be energetically stabilized. Obviously, this is not fulfilled in all cases. The tropylium cation (15) can be reduced in a one-electron step to the tropyl radical even at A = +0.06 V vs. SCE [85, 86]. The radical is unstable and rapidly dimerizes to bitropyl. The hep-taphenyl tropylium radical is stable on the voltammetric timescale, but decays... 

Enantioselective conjugate addition [40] has become truly useful with the aid of dialkylzinc, cationic copper catalyst, and a chiral ligand (Eq. 1, see also Chapt. 7) [41]. Magnesium-based reagents have found use in quantitative fivefold arylation of Cgo (Eq. 10.2) [42] and threefold arylation of C70 [43], paving ways to new classes of cyclopentadienyl and indenyl ligands with unusual chemical properties. 

The chemistry of rhenium(I) is dominated by organometallic compounds which are not covered by this review. Thus, cyclopentadienyl and related compounds, where the organometallic part of the molecule dominate the properties will generally not be considered. Nevertheless, compounds with carbonyl or isocyanide co-ligands will be treated when they can be regarded as constituents of a typical coordination compound or the compounds are of fundamental interest in a radiopharmaceutical context such as the hexakis(isocyanide)rhenium(I) cations. For the same reason a separate section has been included which gives a brief summary of recent attempts to develop synthetic routes to tiicarbonylrhenium(I) complexes for nuclear medical applications. 

The compound Rh( 5116)2 has not yet been obtained, but the action of cyclopentadienyl magnesium bromide on rhodium (III) acetylacetonate yields a derivative of the cation [Rh( 6H6)2], which forms a yellow solution in water (25). It gives precipitates with the usual reagents and is generally similar in properties to the [Co(CsHs) o] + ion. 

In addition to the neutral molecules that we have discussed so far, there are a number of monocyclic species that bear either a positive or a negative charge. Some of these ions show unexpected stabilities that surest that they are aromatic ions. Hiickel s rule is helpful in accounting for the properties of these ions as well. We shall consider two examples the cyclopentadienyl anion and the cycloheptatrienyl cation. 

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