Molybdenum complexes electronic spectroscopy

It is obvious that x-ray cyrstallographic methods will be the final arbiter of the structural features of molybdoproteins, but until such structures are obtained, and even afterwards as far as dynamic features are concerned, spectroscopic methods must be used to gain insight into the nature of these catalysts. Electronic spectroscopy so far has been of little use here since molybdenum complexes in general appear to exhibit broad weak absorptions. In proteins these are always buried under absorptions from hemes, flavins, and iron-sulfur centers. Massey et al., (15) discovered that pyrazolo [3,4-d] pyrimidines will bind Mo (IV) in milk xanthine oxidase that had been reduced with xanthine... 

As yet, no X-ray crystal structures are available for any of the molybdenum enzymes in Table I. Therefore, present descriptions of the coordination environment of the molybdenum centers of the enzymes rest primarily upon comparisons of the spectra of the enzymes with the spectra of well-characterized molybdenum complexes. The two most powerful techniques for directly probing the molybdenum centers of enzymes are electron paramagnetic resonance (EPR) spectroscopy and X-ray absorption spectroscopy (XAS), especially the extended X-ray absorption fine structure (EXAFS) from experiments at the Mo K-absorption edge. Brief summaries of techniques are presented in this section, followed by specific results for sulfite oxidase (Section III.B), xanthine oxidase (Section III.C), and model compounds (Section IV). 

With chromium, molybdenum, and tungsten carbonyls, 1 and 2 displace two CO molecules and form the trans complexes, (l)2M(CO)4 and (2)2M(CO)4 (M = Mo, Cr and W). As an example, the structure of (l>2Cr(CO)4 is shown in Fig. 6. To provide a measure of the strength of these ligands as electron donors, the C=0 stretching frequencies for these complexes were determined by IR spectroscopy [9]. The frequencies for the molybdenum complexes, and for a number of isostmctural Mo compounds, are shown in Table 1. The data indicate that toward Mo(CO)4 as a reference acid, 1 is about equal to triphenylphosphine in donor ability, whUe 2 is slightly weaker, more resembling a trialkoxyphosphine. The stable carbene isostmctural with 1 is an extremely powerful Lewis base toward molybdenum, however, surpassing even trialkylphosphines. 

The third principal application of the electron spin resonance technique is to the study of paramagnetic transition metal ions in biochemical systems. Most examples are complexes of copper, iron, manganese, chromium, cobalt and molybdenum. Other metals such as titanium, vanadium and nickel are sometimes employed as structural probes. Only four of these ions, Cu ", Mn, Gd " and VO ", are seen in ESR spectroscopy at room temperature under virtually all conditions. Therefore, they are of special importance. 

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