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The existence of the orange oxygen-bonded (nitrito) and yellow nitrogen-bonded (nitro) linkage isomers of [Co(NH3)5N02]2+ was recognized as early as 1894 (1). They were the first such pair of isomers isolated and therefore represent the classic example in coordination chemistry (2). These complexes, commonly synthesized as their chloride salts in undergraduate laboratories, illustrate a number of principles taught usually at second-and third-year university levels. Experiments describing their synthesis and the application of spectroscopic techniques to detect the nitrite bonding mode are to be found in several standard texts (3-7), as well as in this Journal (8, 9).It is well known that the nitrito isomer is unstable and isomerizes slowly to the nitro form. The reaction goes to completion in solution and to essential completion (10) in the solid state. The kinetics of the solid state isomerization studied by IR spectroscopy has been described in this Journal (9). The reaction occurs somewhat faster in solution (11) (~50fold) and both the solid state and solution reactions are accelerated appreciably by light (12):

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