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The electrochemistry of nine monomeric molybdenum(V)-oxo complexes in dimethylformamide has been investigated by cyclic voltammetry and controlled-potential coulometry at a platinum electrode. MoOC13L (L = o-phenanthroline, ol,a'-bipyridyl), MoOClL2 (L = 8-hydroxyquinoline, 8-mercaptoquinoline), and MoOClL (L = disalicylaldehyde ophenylenediimine, N,N'-dimethyl-N,N'-bis(2-mercaptoethyl)ethylenediamine, N,N'-bis(2-mercapto-2-methylpropyl)-ethylenediamine) are facilely reduced by one-electron reductions to Mo(1V) species. (C2H5)4NMoOC12(salicylaldehyde o-hydroxyanil) is reduced in a two-electron step to an Mo(1II) species. None of the complexes are oxidizable in the voltage range used (+OS0 to -2.50 V vs. SCE) to Mo(V1) complexes. Comparison with reduction peaks for molybdenum(V1)-dioxo complexes indicates the Mo(V) monomers are not obtainable by electrochemical reduction of the Mo(V1) complexes, and are, in some cases, thermodynamically unstable to disproportionation into Mo(1V) and Mo(V1). Implications for redox states in molybdenum enzymes are discussed. IntroductionMolybdenum is now well established as a necessary cofactor for a number of redox and there is considerable current interest in the properties and reactions of its complexes as possible models for these During catalysis, Mo(V) has been identified by electron spin resonance (ESR) spectrometry for the molybdenum enzymes xanthine oxidase, aldehyde oxidase, sulfite oxidase, and nitrate redu~tase.',~*~ The ESR signal appears to arise from a monomeric Mo(V) center as a result of electron transfer to or from the ~u b s t r a t e . '~~~~ Clearly, a knowledge of the structures, properties, and reactions of monomeric Mo(V) complexes is of importance for an understanding of these enzymes.The aqueous chemistry of Mo(V) is dominated by ESR

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The copper protein mavicyanin has been isolated and purified from the green squash Cucurbita pepo medullosa. Mavicyanin contains one type‐1 copper/18000 Mr, which can be characterized by: intense absorption maximum at 600 nm (ɛ= 5000 M−1 cm−1/Cu, A280/A600= 8.0 ± 0.5, A600/A403= 7.0 ± 0.25, maximum of fluorescence emission at 335 nm. In the oxidized state the copper of mavicyanin is 100% detectable by electron paramagnetic resonance (EPR). Computer simulation of the rhombic EPR signal gives gz= 2.287, gy= 2.077, gx= 2.025, Az= 3.5 mT, Ay= 2.9 mT and Ax= 5.7 mT. Like other simple type‐1 copper proteins, such as stellacyanin, azurin or plastocyanin, mavicy‐anin is readily reduced by hydroquinone or L‐ascorbic acid. Its midpoint potential E′m was determined to be + 285 mV. The reduced protein reacts rather slowly with dioxygen, but is rapidly reoxidized by ferricyanide.

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