Organometallic Compounds with 7r-Bonding

7T-allylnickel(I) bromides which are prepared readily from allyl bromides and nickel carbonyl are examples of compounds with 7r-allyl ligands.Bis(allylnickel) can be prepared by reaction of allyl magnesium bromide and nickel bromide. 

One of the best known of the Ti-organometallic compounds is ferrocene. This compound is a neutral molecule that is derived from two cyclopentadienide anions and iron(II). Two convenient methods of preparation h ve been described. 

Related compounds in which other metals are present have been prepared. Many of these have ligands in addition to cyclopentadienide attached to the metal. The total number of electrons contributed by the ligands plus the valence electrons of the metal atom (or ion) usually is 18, to satisfy the effective atomic number rule.  

Tsutsui, M. N. Levy, A. Nakamura, M. Ichikawa, and K. Mori, Introduction to Metal ir-Complex Chemistry, Plenum Press, New York (1970), pp. 44 and 45. 

Numerous chemical reactions have been carried out on ferrocene and its derivatives. The molecule behaves as an electron-rich aromatic system, and many of the typical aromatic substitutions occur readily. Reagents such as the halogens that are relatively strong oxidizing agents, however, react by oxidation-reduction, with a change in the oxidation state and coordination sphere of the iron atom. 

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