Trinuclear osmium clusters bonding

The various modes of bonding that have been observed for alkenes to the trinuclear osmium clusters are shown in Fig. 7 [see (88)]. The simple 77-bonded structure (a) is relatively unstable and readily converts to (c) the vinyl intermediate (b) is obtained by interaction of alkene with H2Os3(CO)10 and also readily converts to (c) on warming. Direct reaction of ethylene with Os3(CO)12 produces (c), which is considered to be formed via the sequence (a) — (b) — (c) and (d). Both isomers (c) and (d) are observed and involve metal-hydrogen and metal-carbon bond formation at the expense of carbon-hydrogen bonds. In the reaction of Os3(CO)12 with C2H4, the complex 112088(00)902112, (c), is formed in preference to (d). Acyclic internal olefins also react with the carbonyl, with isomerization, to yield a structure related to (c). Structure (c) is... 

The most extensive studies of the chemistiy of cluster complexes have been associated with the trinuclear cluster unit, as may be anticipated. A wide range of substitution reactions has been demonstrated for both Ru3(CO)i2 and Os3(CO)i2, with the full range of ligands normally employed in the study of metal carbonyl chemistry. In genera 1, the trinuclear osmium cluster is more readily maintained, ruthenium often giving rise to cluster breakdown, yielding mononuclear and binu-clear adducts. This reflects the increased bond enei of the metal-metal bond on descending the triad (see Table X later in this section). 

Trinuclear osmium clusters have received considerable attention recently. The green isomer of [Os3(//-H)2(CO)9(CNBu )], in contrast to the previously known red isomers, contains the CNBu ligand bonded in an axial site to the Os atom not associated with bridging hydrides. Fluxionality is associated with 3-fold rotation of the carbonyls of the Os(CO)3 units, the hydride ligands and carbonyls of the Os(CO)3(CNBu ) remaining unaffected. In the complex [ (OC)4(Bu NC)Os Os3(CO)u] there is evidence of restricted rotation about one of the Os—Os bonds at... 

Carbon monoxide insertion into an osmium-carbon bond of a trinuclear cluster has been reported to give a cluster containing a t-ketene ligand ... 

Photochemical Activation. Coordinative unsaturated fragments may also be produced by photolytic reactions. In presence of UV-irradiation metal carbonyl compounds lose sequentially CO-ligands. Electron-deficient, solvent coordinated species produced in this way may combine with inactivated metal complexes via the formation of donor-acceptor metal-metal bonds. Iron, ruthenium, and osmium trinuclear carbonyl clusters may be prepared by this way ... 

So far the discussion has concentrated on the reactions of symmetric 1,3-diynes with trinuclear ruthenium and osmium clusters. Related reactions occur with asymmetric diynes. For example, the reaction of [Ru3(CO)8(/tt-dppm)2] with HC CC CSiMes in thf gives [Ru3(/t-H)(CO)s(/r-dppm)2(/t3-7 --L-C2C GSiMe3)] 263 via the oxidative addition of the terminal C CH component of the diynyl ligand across one of the dppm-bridged Ru-Ru bonds. 

This observation may well explain the considerable difference between metal-olefin and metal-acetylene chemistry observed for the trinuclear metal carbonyl compounds of this group. As with iron, ruthenium and osmium have an extensive and rich chemistry, with acetylenic complexes involving in many instances polymerization reactions, and, as noted above for both ruthenium and osmium trinuclear carbonyl derivatives, olefin addition normally occurs with interaction at one olefin center. The main metal-ligand framework is often the same for both acetylene and olefin adducts, and differs in that, for the olefin complexes, two metal-hydrogen bonds are formed by transfer of hydrogen from the olefin. The steric requirements of these two edgebridging hydrogen atoms appear to be considerable and may reduce the tendency for the addition of the second olefin molecule to the metal cluster unit and hence restrict the equivalent chemistry to that observed for the acetylene derivatives. 

There are few reports of reactions between alkynes and trinuclear clusters of metals other than iron, ruthenium, or osmium. Some rhodium, platinum, and mixed-metal clusters undergo metal-metal bond rupture in reactions with alkynes (54-56), while in other cases the alkyne coordinates to the trinuclear unit without causing any major changes in framework geometry (56-59), as illustrated in Eq. (3). 

Fe3(CO)i2] was expected, but whereas reactions of the iron cluster normally lead to cleavage of the trimeric unit, the ruthenium analog seems to give stable trinuclear species. This was correlated with an increase in the stability of metal-metal bonds on descending the transition metal triad. For instance, when R = Et, Bu" or -in the case of osmium, Ph, the reaction in benzene or toluene under reflux affords [M3(CO)io( -H)(//-SR)] (M = Ru, Os) complexes. The structure of [Os3(CO)io(//-H)(/i-SEt)], synthesized in octane under reflux, has been reported. ... 

Following the isolation of these complexes, all of the mechanistic studies on the carbonylation and reduction reactions of nitroarenes catalysed by Ru3(CO)i2, even in the presence of several promoters, have focused on the reactivity of these or related clusters [157-164]. Moreover, many studies have been also conducted on analogous osmium [165-172] and iron (see paragraph 6.6.) clusters, including insertion reactions of isocyanates, which yield potential intermediates in the carbonylation reaction (Insertion reaction of other cumulenes into the Ru-N bond will not be discussed here. However, see the paragraph of the synthesis of heterocycles later in this chapter). Although not all of the previously mentioned studies were intended to be a basis for a mechanistic understanding of the reactions here discussed, they still contain a lot of information on the possible transformations of amido or imido moieties on a trinuclear cluster. 

The simplest bonding mode found in trinuclear hydrocarbon-substituted clusters of osmium and ruthenium is the 7 -vinyl coordination in which one carbon center is formally cr-bound to one metal atom in the triangular core and the alkene/alkyne unit is formally vr-bound to an adjacent metal, so that the ligand donates three electrons to the cluster. Vinyl complexes are generally prepared by alkyne insertion into [Os3(/U-H)2(GO)io] or by the oxidative addition of an alkene to [Os3(GO)io(NGMe)2l or [Os3(GO)i2], and may be considered to be intermediates in reactions to other hydrocarbon-containing cluster products. A list of reported 77 -vinyl- and the related 77 -acetylide-substituted complexes is presented in Table 1. The related 77 -vinylidene-substituted clusters, in which one carbon atom of the ligand is cr-bonded to two metal centers and the alkene unit is formally vr-bound to the third metal center, can be prepared by the thermal conversion of an 77 -vinyl cluster (Scheme 3). The 77 -vinylidene formally donates four electrons to the cluster core. 

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