Odd-electron chemistry

Nitric oxide is the simplest thermally stable odd-electron molecule known and, accordingly, its electronic structure and reaction chemistry have been very extensively studied. The compound is an intermediate in the production of nitric acid and is prepared industrially by the catalytic oxidation of ammonia (p. 466). On the laboratory scale it can be synthesized from aqueous solution by the mild reduction of acidified nitrites with iodide or ferrocyanide or by the disproportionation of nitrous acid in the presence of dilute sulfuric acid ... 

Reasons for interest in the catalyzed reactions of NO, CO, and COz are many and varied. Nitric oxide, for example, is an odd electron, hetero-nuclear diatomic which is the parent member of the environmentally hazardous oxides of nitrogen. Its decomposition and reduction reactions, which occur only in the presence of catalysts, provide a stimulus to research in nitrosyl chemistry. From a different perspective, the catalyzed reactions of CO and COz have attracted attention because of the need to develop hydrocarbon sources that are alternatives to petroleum. Carbon dioxide is one of the most abundant sources of carbon available, but its utilization will require a cheap and plentiful source of hydrogen for reduction, and the development of catalysts that will permit reduction to take place under mild conditions. The use of carbon monoxide in the development of alternative hydrocarbon sources is better defined at this time, being directly linked to coal utilization. The conversion of coal to substitute natural gas (SNG), hydrocarbons, and organic chemicals is based on the hydrogen reduction of CO via methanation and the Fischer-Tropsch synthesis. Notable successes using heterogeneous catalysts have been achieved in this area, but most mechanistic proposals remain unproven, and overall efficiencies can still be improved. 

One of the relatively few simple odd electron species, nitric oxide is an intriguing heteronuclear diatomic and the parent member of the oxides of nitrogen. Like carbon monoxide, nitric oxide has a long and distinguished coordination chemistry, but unlike CO, it forms very few binary metal... 

Armstrong D, Sun Q, Schuler RH (1996) Reduction potentials and kinetics of electron transfer reactions of phenylthiyl radicals comparisons with phenoxyl radicals. J Phys Chem 100 9892-9899 Asmus K-D (1979) Stabilization of oxidized sulfur centers in organic sulfides. Radical cations and odd-electron sulfur-sulfur bonds. Acc Chem Res 12 436-442 Asmus K-D (1990a) Sulfur-centered free radicals. Methods Enzymol 186 168-180 Asmus K-D (1990b) Sulfur-centered three-electron bonded radical species. In Chatgilialoglu C, Asmus K-D (eds) Sulfur-centered reactive intermediates in chemistry and biology. Plenum, New York, pp 155-172... 

Transition metal (TM) chemistry stands in contrast to this. Many compounds involve metal centres with partially filled d shells, and/or with one or several unpaired electrons. Therefore, it is not always straightforward to predict the orbital occupation pattern of a given stable compound. For intermediates on a reactive pathway, this is an even greater problem. This is also true for organometallic chemistry, despite the fact that many compounds obey the 18-electron rule and have closed-shell singlet ground states. Thus, there are many 16- or even 14-electron intermediates, odd-electron species [1], and polymetallic clusters and complexes for which the spin state is not readily predicted. 

Alongside electron-pair bonds, odd-electron bonds play an important role in chemistry, and constitute therefore a compulsory test case for any computational method. Odd-electron bonds can be represented as two resonating Lewis structures that are mutually related by charge transfer, as shown in (13) for two-center, one-electron (2c,le) bonds and in (14) and (15) for typical two-center, three-electron (2c,3e) bonds. 

These are important questions, for the answers apply not only to this simple radical but to any free radical, however complicated, that we shall encounter. The shape, naturally, underlies the three-dimensional chemistry—the stereochemistry—of free radicals. The location of the odd electron is intimately involved with the stabilization of free radicals by substituent groups. 

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