Dioxo unit

Many osmyl complexes with group VI ligands have been reported. The purple diamagnetic potassium osmate K2[0s(0H)4(0)2] is the best known and is a useful starting material for the preparation of other osmyl or osmium complexes. It is best prepared from the reaction of 0s04 with excess KOH. The X-ray crystal structure of K2[0s(0H)4(0)2] shows that the complex has the trans-dioxo unit, with a d(0s=0) of 1.77 A and a 0=0s=0 angle of 180° (238,239). The acid dissociation constants of H2[0s02(0H)4] have been determined. 

There are only a few marginally stable Npvn and Puvn compounds, which formally contain the dioxo unit An02+ in alkaline solution. 

The majority of the chemistry that has been investigated for the actinide elements has been in aqueous solutions. For the light actinides in acidic solutions, four types of cations persist trivalent, tetravalent, pentavalent, and hexavalent. The later two ions are always found to have trans oxo ligands, making up a linear dioxo unit. Actinide ions of this type are typically referred to yls and have the names, uranyl (1, U02+/ +), neptunyl (2, Np02+ +), plutonyl (3, Pu02+ +), (4, Am02+/ ) and so on. 

The electrochemistry of the light actinides is impacted by the stability of the linear dioxo unit. The redox reactions in which there is no making or breaking of an An=0 bond are fast and reversible, for example, reducing the tetravalent to... 

A wide range of d Os complexes has been prepared and characterized. These complexes are usnally diamagnetic see Diamagnetism) and often incorporate the osmyl trans-0=0s=0 dioxo unit or the monooxo 0s=0 nnit, often with halide, N-, or O-donor ligands. They are more stable and less oxidizing than their Ru analogs, and are often prepared by rednction of OSO4 or, in certain cases, by controlled oxidation of Os and Os precursors. 

The first question to. ask concerning the mechanism in Figure 4 is whether the crystal structure is compatible with adjacent surface dioxo units. Figure 5 shows the (010) surface of a— Bi2(Mo04)3 [corresponding to the parent Scheelite structure, CaWOj (11). Here we see that there are adjacent dioxo centers along the crystal surface. 

With these adjacent dioxo units, we would expect allyl transfer steps such as... 

Summarizing, we agree with Grasselli et al. (B.) that Mo dioxo units... 

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