Naturally-occurring Organometallics and Synthetic Models

This possibility led to the development of the CMIA method to assay antiepileptic medications [101,112-114] and cortisol [115], a method that offers the additional potential of the simultaneous triple immunoassay, thanks to the distinct and nonoverlapping infrared bands of carefully-selected metal carbonyl probes [116]. It is also possible to displace the Vqq bands in the infrared by modifying the electron density at the metal, via substitutions of L or Cp ligands [117]. In addition, solvation and environmental phenomena can cause the Vqq vibrations to shift. The harnessing of these phenomena is described in Chapter 7. 

The relationship between organometallics and Nature is very old, and the last few decades have revealed in it new levels of complexity. 

The elucidation of the structure of vitamin Bj2 was a tour de force of the early days of X-ray diffraction and was recognized in 1964 by the award of the Nobel Prize to Dorothy Hodgkin. The coenzyme Bj2 acts in concert with a variety of enzymes that catalyze reactions of three main groups. In the first, two substituents on adjacent carbon atoms, -X and -H, are permuted, this is called the isomerase reaction. In the second type, methylcobalamin methylates a substrate, as in the conversion of homocysteine to methionine, for example. Finally, B j2 is also involved 

The coen2yme is required only in small amounts 2-5 mg is present in the average human, for example, and one of the first signs of deficiency is the failure to form red blood cells. 

Nitrogenase, because of its facUity for producing NHj in mild conditions (Eq. 1.2), was a long-standing chaUenge for organometaUic chemists. 

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