Carboxylato complexes, osmium

In this section we consider the complexes of mono-, di- and poly-carboxylato complexes. There appear to be marked differences from ruthenium carboxylato chemistry particularly in respect of the II/III and III states. A number of osmyl complexes are known with monodentate or bidentate carboxylates (p. 582) but apparently none of ruthenium(VT). On the other hand, the binuclear ruthenium species [Ru30(OCOR)6L3]"+, in which OCOR functions as a bidentate ligand, does not appear to have an osmium counterpart both elements form lantem -like species containing [M2(OCOR)4]"+ cores, but again with marked differences between ruthenium and osmium. This is clearly an interesting and rewarding field for further study. 

Schemes 6-17 Reaction of hydrido(carboxylato)osmium(IV) complexes with alkynes...

We have already alluded to the diversity of oxidation states, the dominance of oxo chemistry and the cluster carbonyls. Brief mention should be made too of the tendency of osmium (shared also by ruthenium and, to some extent, rhodium and iridium) to form polymeric species, often with oxo, nitrido or carboxylato bridges. Although it does have some activity in homogeneous catalysis (e.g. of m-hydroxylation, hydroxyamination or animation of alkenes, see p. 558, and occasionally for isomerization or hydrogenation of alkenes, see p. 571), osmium complexes are perhaps too substitution-inert for homogeneous catalysis to become a major feature of the chemistry of the element. The spectroscopic properties of some of the substituted heterocyclic nitrogen-donor complexes may yet make osmium an important element for photodissociation energy research. 

The 0x0 (0 ) ligand is dominant in the coordination chemistry of osmium, participating in the VIII to IV oxidation states inclusive. The tetroxide OSO4 is the single most important compound of osmium OSO4 and the recently discovered [0s04] ion arc tetrahedral. The /ra 5-[0=0s —O] osmyl moiety displays an extensive chemistry, comparable with that of the uranyl 0=U=0 unit, and there is an extensive and unique cyclic oxo-ester chemistry (p. 584). There is surprisingly little information on hydroxo, aqua, sulfato, nitrato or phosphato complexes, but much recent work has been carried out on carboxylato species, and clearly much work remains to be done on the O-donor chemistry of the element. There are a reasonable number of sulfur-donor complexes but few with selenium or tellurium. 

The first osmium Tp complex was prepared by Singleton et al in 1990 via the reaction of the -carboxylato dimer 0s2(jU-02CMe)2(C0)6 with KTp in refluxing methanol to give Tp20s2(C0)4 Os-Os). Treatment of this dinuclear compound with Br2 affords the mononuclear Os(II) complex TpOsBr(CO)2 via oxidative cleavage of the Os-Os bond (Scheme 35). 

Griffith WP, Joliffe JM (1991) Ruthenium and osmium carboxylato 0x0 complexes as organic oxidants. Stud Surf Sci Catal 66 395 100... 

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