Cyclopentadienyl anion formation from cyclopentadiene

Anions are similarly examined, and the cyclopentadienyl anion (118) has six 7t-electrons and meets all criteria for aromaticity. It is aromatic, very easy to form, and quite stable. Formation of 118 from cyclopentadiene (120) is an acid-base reaction. It is known that 120 has a relatively low pK that reflects the special aromatic stability of the aromatic conjugate base. The pK of cyclo-pentadiene is 14-15 (compare that with a pK of 15.8 for water).This contrasts sharply to the cycloheptatrienyl anion (119), which has 4n 7i-electrons, is not aromatic, and is particularly unstable and difficult to form. As with 118, formation of 119 is an acid-base reaction from cycloheptatriene, 121. The pKg of 121 is about 36,1 however, which reflects the great difficulty in forming the antiaromatic conjugate base. 

Synthesis of cyclopentadienyl compounds of metals from cyclopentadiene depends on the strongly acidic properties of cyclopentadiene. This acidity may be attributed to the ready formation of the highly resonance-stabilized cyclopentadienide anion, CsHs , in which there are six delocalized ir electrons. This method is limited primarily to the active metals of Groups lA and IIA and to their strongly basic compounds. The compounds produced are principally the ionically bonded metal cyclopentadienides. 

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. 

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