Coordination chemistry studies

Mechanistic speculations about the molybdoenzymes must be considered to be in their infancy with the possible exception of those for xanthine oxidase. Although the detailed structural nature of the molybdenum site is unknown, there is sufficient information from biochemical and coordination chemistry studies to allow informed arguments to be drawn. Here we first discuss evidence for the nuclearity of the molybdenum site and then discuss both oxo-transfer and proton-electron transfer mechanisms for molybdenum enzymes. A final discussion considers the unique aspects of nitrogenase and the possible reasons for the use of molybdenum in enzymes. 

Figure 2.31. Schematic representation of some coordination chemistry studies performed on bis-porphyrin-stoppered [3]-rotaxane 86. The black disk is Cu(I) black and hatched diamonds represent Au(III) and Zn(II) porphyrins, respectively. The thick lines represent chelate (phenan-throline) fragments.

Most reactions occurring in solution are in some respect connected with complex-formation reactions. Even solution itself (whether the solvent acts as a donor or as an acceptor) is a complexing reaction [Gu 68, Gu 78, Ma 75,79]. Accordingly, coordination chemistry studies, and primarily complex equilibrium studies, have contributed significantly to a deeper understanding of both the structures of solutions and the processes taking place in solution. 

A new route to phosphaalkenes is provided by treatment of dichloro-phenylphosphine with two equivalents of carbonyl-functional carbenes, resulting in a two-electron reduction of the phosphorus centre coupled with carbene oxidation. A range of amido-functional phosphaalkenes, e.g., (112), (alternatively viewed as carbene-phosphinidene adducts), was prepared using this approach, followed by full spectroscopic and structural characterisation. A related diamino-functional phosphaalkene has also been described and its coordination chemistry studied. Among other new phosphaalkene systems reported is a series of phosphaalkene-phospholes, e.g., (113), the monoanionic phos-phaalkenyl-phosphido ligand (114), alkali metal salts of the phosphaalkene radical anion (115), ° and new 1,3-diphosphacyclobutadiene... 

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