Metallocene Compounds - Structure and Properties
On the Nature of Metallocene Sandwich Compounds
Understanding the Cyclopentadienyl Anion - First Cyclopentane
The simplest organic molecule is the hydrocarbon methane, CH4. Now if two of its hydrogen atoms are stripped off, a methylene group results, -CH2-. Five methylene groups strung together like beads form the structure, -CH2-CH2-CH2-CH2-CH2-. If the two end methylene units are attached (as in putting on a necklace), the result is a ring structure, cyclopentane, chemical formula C5H10.
Understanding the Cyclopentadienyl Anion - Second Cyclopentadiene
Cyclopentadiene has the chemical formula, C5H6. This structure is formed by removing hydrogen atoms from the string drawn above to change it to -CH2-CH=CH-CH=CH-. Of special note are the double bonds, which are shorter and help flatten the ring. The species involved in metallocenes is not cyclopentadiene, however, but the cyclopentadienyl anion. The two differ in a small but important way.
Behold the Cyclopentadienyl Anion!
The lone carbon in the ring that has the two hydrogen atoms must lose one of them to also become a -CH- group. There is a special feature in this instance. The leaving hydrogen atom does not take its electron with it. Rather, it leaves it behind on the carbon atom, providing the cyclopentadienyl structure a single negative charge. This process is accomplished using a chemical called a base.
Properties of Metallocenes
The end result is a cyclopentadienyl "anion" (negative ion) formula C5H5-. This ion possesses perfect flatness. Coupled with a Huckel's Rule number of "pi-electrons," this imparts the special chemical stability known as aromaticity.1 Aromaticity reduces the reactivity of the double bonds, making them much less subject to addition reactions. Substitution reactions, instead, predominate. In substitution reactions, one or more of the hydrogen atoms on the rings may be replaced by a different atom.
The Example of Ferrocene
To form a compound, the negative charge of each cyclopentadienyl anion requires a balancing metallic valence of plus two. In order to meet the strictest International Union of Pure and Applied Chemistry (IUPAC) definition, the positively charge ion must be that of a d-block (transition) metal atom. The best known metallocene is also the one first discovered-ferrocene. The metal ion in this case is the plus two ferrous iron-written Fe II or Fe+2.
The Discovery of Ferrocene
First accidentally prepared in 1951 during a reaction of cyclopentadienyl magnesium bromide (C5H5-MgBr) with ferric chloride (FeCl3), the recognition of ferrocene and the accurate understanding of its structure were not deduced until later. For their role in this elucidation and additional organometallic research, Geoffrey Wilkinson and Ernst Otto Fischer shared a Nobel Prize in 1973.2
A more practical method of producing ferrocene in the laboratory involves the use of cyclopentadiene, ferrous chloride, and metallic sodium.3
Most Favored Configuration
The most energetically favorable configuration has the cyclopentadienyl anions staggered. Going back to the sandwich illustration, ordinary bread is square. Grasping the bottom slice of bread with the left and the upper slice with the right hand. Now twist the sandwich 45 degrees, and the bread is staggered. The corners of the upper slice of bread are over top the middle of the sides of the lower slice of bread.
In the case of cyclopentadienyl anions, because they have five vertices, twist them 72 degrees per vertex divided by two, or 36 degrees. It should be noted, however, that the rings are not rigidly held in this position, but may freely rotate.
1 Pearson Education Organic Chemistry Fifth Edition by L.G. Wade - Chapter 16 Aromatic Compounds
2 Nobelprize.org - Press Release October 23, 1973
3 Organic Syntheses, Coll. Vol. 4, p.473 (1963); Vol. 36, p.31 (1956).
Additional Resources:
University of Bristol, School of Chemistry: Ferrocene - Molecule of the Month
Connexions - Friedel-Crafts Reaction: Acetylation of Ferrocene
Published by Vincent Summers
My secular expertise includes 23 years of experience at the National Radio Astronomy Observatory, with a share in NASA's extended Voyager 2 effort. I formerly wrote for Demand Studios, Bukisa, Suite 101, Exa... View profile
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11 Comments
Post a CommentGood article. Makes me to think of couple terms while reading the article: Grignard-Reaction (after seeing Mg-Br), conformation, and planar (in relation to aromaticity and Huckel's Rule).
You never cease to amaze me with your in-depth knowledge of so many subjects!
Never much good at Chemistry, sorry. You're one smart guy, we need you.
So an aroma sandwich...huh? Cool
Thanks Vincent!
That went way above my head. lol Thanks for sharing.
Thanks great lesson:0)
In the next to the last paragraph, the word "grasping" should read "grasp."
You'll make a scientist out of me yet!
Great explanation. I found this piece enjoyable to read.