In the years following Kekulé brilliant proposal for the structure of benzene, organic chemistry underwent a tremendous expansion, and in the process a wide variety of paradigms or working hypotheses were developed about what kinds of compounds could "exist" and what kinds of reactions could occur. In many cases, acceptance of these hypotheses appeared to stifle many possible lines of investigation and caused contrary evidence to be pigeonholed as "interesting but not conclusive." As one example, the paradigm of angle strain was believed to wholly preclude substances that we know now are either stable or important reaction intermediates, such as cubane ( Section 12-10 ), cyclopropanone ( Section 17-11 ), and benzyne ( Sections 14-6C and 23-8 ). No paradigm did more to retard the development of organic chemistry than the notion that, with a "few" exceptions, compounds with bonds between carbon and transition metals (Fe, Co, Ni, Ti, and so on) are inherently unstable. This idea was swept away in 1951 with the discovery of ferrocene (\(C_5H_5)_2Fe\)) by P. L. Pauson. Ferrocene has unheard of properties for an organoiron compound, stable to more than 500 ° and able to be dissolved in, and recovered from, concentrated sulfuric acid! Pauson's work started an avalanche of research on transition metals in the general area between organic and inorganic chemistry, which has flourished ever since and has led to an improved understanding of important biochemical processes. 31.1: Metallocenes A metallocene is a compound typically consisting of two cyclopentadienyl anions bound to a metal center (M) in the oxidation state II. Certain metallocenes and their derivatives exhibit catalytic properties, although metallocenes are rarely used industrially. The first metallocene to be classified was ferrocene. 31.2: Other Organometallic Compounds of Transition Metals Not all organometallic compounds of transition metals date from the discovery of ferrocene. Many have been known for a long time but their structures were not understood. 31.3: Transition-Metal Compounds as Reagents for Organic Syntheses Transition-Metal Compounds can act as reagents for organic syntheses. For example, some transition-metal hydrides show promise as synthetic reagents and sodium reacts with iron pentacarbonyl to produce a salt known as sodium tetracarbonylferrate(-II)2, which has been shown to have considerable potential as a reagent for organic synthesis. 31.4: Some Homogeneous Catalytic Reactions Involving Transition-Metal Complexes Some transition-metal complexes can catalyze homogeneous reactions such as hydrogenation, Hydroformylation of Alkenes, Carbonylation of Methanol and alkene metathesis. 31.5: π-Propenyl Complexes of Nickel A considerable body of highly useful chemistry based on nickel has been developed. Many of these reactions involve what are called π -propenyl complexes and their formation has a close analogy in the formation of ferrocene from cyclopentadienylmagnesium compounds and ferric chloride. 31.6: Vitamin B₁₂ as an Organometallic Compound The structure of vitamin B12 with a cyanide ion coordinated with cobalt is not the active form of the vitamin but is a particularly stable form, convenient to isolate and handle. The active form is a coenzyme that is remarkable in having a carbon-cobalt bond to an essentially alkyl-type carbon. 31.E: Transition Metal Organic Compounds (Exercises) These are the homework exercises to accompany Chapter 31 of the Textmap for Basic Principles of Organic Chemistry (Roberts and Caserio). Thumbnail: Ball-and-stick model of a metallocene molecule where the cyclopentadienyl anions are in a staggered conformation. The purple ball in the middle represents the metal cation. (Public Domain; Ben Mills).