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|Section2= |Section3= |Section8= }} Rhodocene, formally known as bis(η5-cyclopentadienyl)rhodium(II), is a chemical compound with the formula (). Each molecule contains an atom of rhodium bound between two planar systems of five carbon atoms known as cyclopentadienyl rings in a sandwich arrangement. It is an organometallic compound as it has (haptic) covalent rhodium–carbon bonds.〔 The () radical is found above 150 °C or when trapped by cooling to liquid nitrogen temperatures (−196 °C). At room temperature, pairs of these radicals combine to form a dimer, a yellow solid in which two of these cyclopentadienyl rings are joined.〔〔〔 The history of organometallic chemistry includes the 19th century discoveries of Zeise's salt〔〔 and Ludwig Mond's discovery of nickel tetracarbonyl.〔 These compounds posed a challenge to chemists as the compounds did not fit with chemical bonding models as they were then understood. A further challenge arose with the discovery of ferrocene,〔 the iron analogue of rhodocene and the first of the class of compounds now known as metallocenes.〔 Ferrocene was found to be unusually chemically stable,〔 as were analogous chemical structures including rhodocenium, the unipositive cation of rhodocene and its cobalt and iridium counterparts.〔 The study of organometallic species including these ultimately led to the development of new bonding models that explained both their formation and their stability.〔〔 Work on sandwich compounds, including the rhodocenium / rhodocene system, earned Geoffrey Wilkinson and Ernst Otto Fischer the 1973 Nobel Prize for Chemistry.〔 Owing to their stability and relative ease of preparation, rhodocenium salts are the usual starting material for preparing rhodocene and substituted rhodocenes, all of which are unstable. The original synthesis used a cyclopentadienyl anion and tris(acetylacetonato)rhodium(III);〔 numerous other approaches have since been reported, including gas-phase redox transmetalation〔 and using half-sandwich precursors.〔 Octaphenylrhodocene (a derivative with eight phenyl groups attached) was the first substituted rhodocene to be isolated at room temperature, though even it decomposes rapidly in air. X-ray crystallography confirmed that octaphenylrhodocene has a sandwich structure with a staggered conformation.〔 Unlike cobaltocene, which has become a useful one-electron reducing agent in the research laboratory setting,〔 no rhodocene derivative yet discovered has sufficient stability for such applications. Biomedical researchers have examined the applications of rhodium compounds and their derivatives in medicine〔 and reported one potential application for a rhodocene derivative as a radiopharmaceutical to treat small cancers.〔〔 Rhodocene derivatives are also used to synthesise linked metallocenes so that metal–metal interactions can be studied;〔 potential applications of these derivatives include molecular electronics and research into the mechanisms of catalysis.〔 The value of rhodocenes tends to be in the insights they provide into the bonding and dynamics of novel chemical systems, rather than their direct use in applications. ==History== Discoveries in organometallic chemistry have led to important insights into chemical bonding. Zeise's salt, K()·H2O, was reported in 1831〔 and Mond's discovery of Ni(CO)4 occurred in 1888.〔 Each contained a bond between a metal centre and small molecule, ethylene in the case of Zeise's salt and carbon monoxide in the case of nickel tetracarbonyl.〔 The space-filling model of the anion of Zeise's salt (image at left)〔 shows direct bonding between the platinum metal centre (shown in blue) and the carbon atoms (shown in black) of the ethylene ligand; such metal–carbon bonds are the defining characteristic of organometallic species. However, bonding models were unable to explain the nature of such metal–alkene bonds until the Dewar-Chatt-Duncanson model was proposed in the 1950s.〔 The original formulation covered only metal–alkene bonds〔 but the model was expanded over time to cover systems like metal carbonyls (including ()) where π backbonding is important.〔 Ferrocene, (), was first synthesised in 1951 during an attempt to prepare the fulvalene (C10H8) by oxidative dimerization of cyclopentadiene; the resultant product was found to have molecular formula C10H10Fe and reported to exhibit "remarkable stability".〔 The discovery sparked substantial interest in the field of organometallic chemistry,〔〔 in part because the structure proposed by Pauson and Kealy (shown at right) was inconsistent with existing bonding models and did not explain its unexpected stability. Consequently, the initial challenge was to definitively determine the structure of ferrocene in the hope that its bonding and properties would then be understood. The sandwich structure was deduced and reported independently by three groups in 1952: Robert Burns Woodward and Geoffrey Wilkinson investigated the reactivity in order to determine the structure〔 and demonstrated that ferrocene undergoes similar reactions to a typical aromatic molecule (such as benzene),〔 Ernst Otto Fischer not only deduced the sandwich structure but also began synthesising other metallocenes including cobaltocene,〔 whilst Eiland and Pepinsky provided X-ray crystallographic confirmation of the sandwich structure.〔 Applying valence bond theory to ferrocene by considering an Fe2+ centre and two cyclopentadienide anions (C5H5−), which are known to be aromatic according to Hückel's rule and hence highly stable, allowed correct prediction of the geometry of the molecule; however, it was only once molecular orbital theory was successfully applied that the reasons for ferrocene's remarkable stability became clear.〔 The properties of cobaltocene reported by Wilkinson and Fischer demonstrated that the unipositive cobalticinium cation ()+ exhibited stability similar to that of ferrocene itself. This observation is not unexpected given that the cobalticinium cation and ferrocene are isoelectronic, although the bonding was not understood at the time. Nevertheless, the observation led Wilkinson and F. Albert Cotton to attempt the synthesis of rhodocenium〔 and iridocenium salts.〔 They reported the synthesis of numerous rhodocenium salts, including those containing the tribromide (()Br3), perchlorate (()ClO4), and reineckate (() ()·H2O) anions, and found that the addition of dipicrylamine produced a compound of composition () ().〔 In each case, the rhodocenium cation was found to possess high stability. Wilkinson and Fischer went on to share the 1973 Nobel Prize for Chemistry "for their pioneering work, performed independently, on the chemistry of the organometallic, so called sandwich compounds".〔 The stability of metallocenes can be directly compared by looking at the reduction potentials of the one-electron reduction of the unipositive cation. The following data are presented relative to the saturated calomel electrode (SCE) in acetonitrile: :()+ / () +0.38 V〔 :()+ / () −0.94 V〔 :()+ / () −1.41 V〔 These data clearly indicate the stability of neutral ferrocene and the cobaltocenium and rhodocenium cations. Rhodocene is ca. 500 mV more reducing than cobaltocene, indicating that it is more readily oxidised and hence less stable.〔 An earlier polarographic investigation of rhodocenium perchlorate at neutral pH showed a cathodic wave peak at −1.53 V (versus SCE) at the dropping mercury electrode, corresponding to the formation rhodocene in solution; however, the researchers were unable to isolate the neutral product from solution. In the same study, attempts to detect iridocene by exposing iridocenium salts to oxidising conditions were unsuccessful even at elevated pH.〔 These data are consistent with rhodocene being highly unstable and may indicate that iridocene is even more unstable still. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Rhodocene」の詳細全文を読む スポンサード リンク
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