Cyclopentadienyl-Like Compounds

An excellent comprehensive review of heterocyclopentadienyl complexes of the group 3 and lanthanide metals was published in 2001 by Nief.715 It covers complexes of monocyclic pyrrolyl, phospholyl, and arsolyl ligands, ring-fused 

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Cyclopentadienyl-ligated complexes, with chromium, 5, 380 Cyclopentadienyl-like compounds, lanthanide and Group 3 complexes, 4, 105... 

Other kinds of substances besides benzene-like compounds can also be aromatic. For example, the cyclopentadienyl anion and the cycloheptatrienyl cation are aromatic ions. Pyridine, a six-membered, nitrogen-containing heterocycle, is aromatic and resembles benzene electronically. Pyrrole, a hve-membered heterocycle, resembles the cyclopentadienyl anion. 

Other kinds of molecules besides benzene-like compounds can also be aromatic. The cyclopentadienyl anion and cycloheptatrienyl cation, for instance, are aromatic ions. Pyridine and pyrimidine are srx-memhered, nitrogen-containing, aromatic heterocycles. Pyrrole and imidazole are five-membered, nitrogen-containing heterocycles. Naphthalene, quinoline, indole, and many others are polycyclic aromatic compounds. 

Bis[(tris(isopropyl)cyclopentadienyl)]zinc (Zn C5(Pr1)3H2 2, 21) and bis[(tetrakis(isopropyl)cyclopentadienyl)]zinc (Zn C5(Pr1)4H 2, 22) were synthesized from the respective potassium cyclopentadienides and zinc iodide as shown in Scheme 18.50 The same slipped sandwich compounds were also isolated from zinc-reduced VC13 solutions when they were treated with these alkali metal cyclopentadienides at room temperature.51 The outcomes of these reactions suggest that zincocenes are likely intermediates in the syntheses of transition metal metallocenes, in which the metal halides have been pre-reduced with zinc. The solid-state structure of Zn G5(Pr1)4H 2 is shown in Figure 10. The sole... 

Figures 5 and 6 show one of the first examples of organo-metallic compounds adsorbed on the alumina of a metal-insulator-metal junction. The carboxyl groups on the benzene ring and cyclopentadienyl rings are most likely coordinated to the alumina. The arrangement of the carbonyl groups with respect to the surface would then be as shown below ...

A comprehensive study of 15 zirconecene monomethyl monochloro compounds and their exchange of chloride for methyl with AlMe3 indicates that the exchange process is a function of the electron deficiency at the metal.62 With low electron densities, the zirconium is more likely to exchange chloride for methyl. The study revealed low electron densities at the metal for the indenyl compounds by comparison to the cyclopentadienyl compounds. The study also revealed that steric effects are minor compared to the electronic effects of the ligands on the zirconium. 

In a similar way Cp(Me)Mg(OEt2) is capable of deprotonating iV,Ai -bis(2,4,6-tri-methylphenyl)(ierr-butyl)amidine to form the corresponding cyclopentadienylmagnesium amidinate complex (226) (equation 17). An X-ray crystal-structure determination of 226, of which the structure is shown schematically (equation 17), showed that this compound also exists as a monomer in the solid state. Like in 225 the cyclopentadienyl group is /7 -bonded to magnesium while the amidinate anion is Ai,A -chelate bonded with almost equal Mg—N bond distances [Mg-N 2.090(2) and 2.097(2) A]. Furthermore, an additional... 

Cp2Mo2(CO)4] is an important member of a group of cyclopentadienyl compounds and, like its chromium and tungsten analogues, this species is considered to involve a metal-metal triple bond.7,220 These and related compounds manifest a rich and novel chemistry,221 but this falls outside the field of this review. 

The five-membered aromatic heterocycles pyrrole (5), furan (6) and thiophene (7) are formally derived from cyclopentadienyl anion by replacement of one CH group with NH, O or S, each of which can contribute two p-electrons to the aromatic ir-electron sextet. Heteroatoms of this type have in classical structures only single bonds and are called pyrrole-like . Other five-membered aromatic heterocycles are derived from compounds (5), (6) and (7) by further replacement of CH groups with N, 0+ or S+. 

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