Osmium osmyl complexes

The most important members of this class are the osmium nitrido, and the osmyl complexes. The reddish-purple K2[OsNCl5] mentioned above is the result of reducing the osmiamate. The anion has a distorted octahedral structure with a formal triple bond Os=N (161pm) and a pronounced /ram-influence (pp. 1163-4), i.e. the Os-Cl distance trans to Os-N is much longer than the Os-Cl distances cis to Os-N (261 and 236 pm respectively). The anion [OsNCls] also shows a rram-effect in that the Cl opposite the N is more labile than the others, leading, for instance, to the formation of [Os NCl4] , which has a square-pyramidal structure with the N occupying the apical position. 

Many ruthenyl and osmyl complexes with O donor ligands are known. The ruthenium compounds are usually prepared from [RuOJ, which can be generated by the oxidation of RuCl3 xH20 or [RUO2] with [104] . The osmium compounds are usually prepared from [OSO4] or K2[0s(0)2(0H)4]. 

The [2+2] Mechanism Already in 1977 Sharpless proposed a stepwise [2+2] mechanism for the osmylation of olefins in analogy to related oxidative processes with d°-metals such as alkene oxidations with CrO,Cl2 [23, 24], Metallaoxetanes [25] were suggested to be formed by suprafacial addition of the oxygens to the olefinic double bond. In the case of osmylation the intermediate osmaoxetane would be derived from an olefm-osmium(VIII) complex that subsequently would rearrange to the stable osmium(VI) ester. 

Osmyl complexes of phthalocyanine have been claimed. Reaction of 0s04 and 1,2-dicyanobenzene at 180°C gives the dark blue Os02-Pc C6H4(CN)2, which is paramagnetic (/u,eff = 1.4 BM at 20°C) so this could well be an osmium(IV) or even an osmium(III) species (231). Another osmyl species claimed is 0s02Pc-3PhNH2, made from... 

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. 

The same features are observed in the osmylation of arene donors. Thus, osmium tetra-oxide spontaneously forms complexes with arenes, and the systematic spectral shift in the CT bands parallels the decrease in the arene IP [59]. The same osmylated adducts are obtained thermally on leaving mixtures to stand in the dark or upon irradiation of the CT bands at low temperature. Time-resolved spectroscopy establishes that irradiation of the CT band of the anthracene/osmium tetraoxide complex leads directly to the radical-ion pair ANT+, 0s04, which then collapses to the osmium adduct (with a rate constant fc 109 s 1) in competition with back ET [59]. 

The osmyl complex ra j -[0s02(en)2]2+ is discussed on p, 583 (see also Table 21). The only other species in this category is the deep red crystalline l[Os(en)3I2j4 , obtained by treatment of [Os(en)(en — H)2]Br2 with HI.79 Although analytical data fit the formulation reasonably well it seems unlikely that this really is an eight-coordinate osmium(VI) species an alternative possibility is [Os(en)(en — H)2](I3)2. 

Since, like terpyridyl (p. 542), these ligands are good n acceptors by virtue of their considerable ring conjugation, it is the osmium(II) (d6) state which is the commonest and generally the most stable thus [Os(LL)3]3+ species are good one-electron oxidants. Nevertheless there are also examples of osmium(IV) and even a few of osmium(V) and osmium(VI) (the osmyl osmium(VI) complexes are considered on p. 581). 

In the area of osmium oxo chemistry there are three main topics of interest the tetroxide itself, the osmyl complexes which contain the irons 0=0svi=0 unit, and the cyclic oxo ester species which contain the OsVI moiety. This is perhaps the most appropriate point at which to comment briefly on the reasons for the stability of these three systems. 

As might be expected for chelating 0,0 donor ligands it is the intermediate trivalent state of osmium which is stabilized by /J-diketonates (as is the case for catechols, p. 597, and tropolonato complexes, p. 597). In the presence of rc-acceptor chelates such as bipy, phen and terpy the II state is favoured, while with halides the IV state is preferred. The apparent absence of an osmyl complex, e.g. 0s02acac2, is surprising. 

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. 

Two reaction mechanisms have been proposed for these dihydroxylations (pathway a or b, Figure 7.23), either a concerted [3+2] cycloaddition of the olefins on osmium-diamine complex 7.33 or a stepwise reversible [2+2] cycloaddition followed by a rearrangement [559,1350], An X-ray crystal structure of the resulting osmic ester 2.89A shows its symmetrical structure. Houk s calculations [1351] are in favor of a concerted reaction, and his transition state model is reactant-like, with steric interactions dictating the face selectivity of osmylation. 

Whereas OSO4 and the 0x0 esters considered below possess certain features unique to osmium, analogues of the osmyl complexes are found for other elements capable of attaining configura-... 

We have already mentioned that the easily attained configuration for osmium favours the formation of these species (just as d°, for other elements but apparently not for osmium, favours the formation of cis dioxo species ). The realization in 1960 that the long-known and supposedly tetrahedral K2[0s04]-2H2 0 was diamagnetic and its consequent reformulation as tra j-K2[0s02(0H)4] (subsequently confirmed by X-ray studies ) led to a rationalization of the osmyl complexes. 

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